Marine Chemistry 48 (1995) 157-182
Iron chemistry in seawater and its relationship to phytoplankton:
a workshop report
Mark L. Wells”, Neil M. Priceb, Kenneth W. Bruland”
ahWute
of Marine Sciences, University of California, Santa Cruz, Santa Cruz, CA 95064, UsA
bBiology Department, McGill University, Montreal, Canada
Received 26 July 1994; revision accepted 6 October 1994
1. Introduction
The ongoing debate about iron limitation of
phytoplankton in the open ocean has highlighted
how little we know about the marine chemistry of
iron and its relationship to iron uptake by microorganisms. The idea that iron may be limiting
phytoplankton was suggested and investigated by
Gran (1931) but the topic suffered from analytical
problems for almost sixty years (de Baar, 1994).
The concept is now treated more seriously since
the advent of trace metal clean procedures which
permit the accurate measurement of dissolved and
particulate iron (Bruland et al., 1979; Gordon et
al., 1982; Landing and Bruland, 1987). These
measurements demonstrate that in some ocean
habitats dissolved iron concentrations in surface
waters are as low as 20-30 pM (Martin et al.,
1991), concentrations that are unlikely to support
high phytoplankton biomass. The controversy over
the role of iron in ocean productivity has stimulated the development of new methodologies for
rapidly analyzing low-level iron concentrations in
seawater (Elrod et al., 1991; O’Sullivan et al., 1991;
Yokoi and van den Berg, 1992; Obata et al., 1993).
Unfortunately, most of these novel methods provide little information
about
the ambient
speciation of iron and hence its accessibility to
phytoplankton.
Critical assessment of the inter0304-4203/95/$07.00
0
active relationship between iron and marine phytoplankton assemblages is hindered greatly by our
current analytical resources.
Most of the iron debate centers on the remote
HNLC regions of the subarctic Pacific, the equatorial Pacific and the Southern Ocean. These
regions are characterized by a persistence of excess
major nutrients (N, P) and low biomass relative to
coastal systems having similar major nutrient concentrations. Because the root cause(s) for this situation may have important ramifications to the
ocean-atmosphere
exchange of CO2 and global
climate cycles (Martin, 1990), a special symposium
was held (February 1991) to address what controls
phytoplankton production in nutrient-rich areas of
the open sea (see Limnology and Oceanography,
Vol. 36). John Martin and his colleagues argued
that an inadequate iron supply was the major factor (Martin et al., 1991). Results from bottle
enrichment studies of theirs and others support
this hypothesis (Martin and Fitzwater, 1988; de
Baar et al., 1990; Martin et al., 1990a, 1994; Helbling et al., 1991; Greene et al., 1994; Kolber et al.,
1994; Price et al., 1994), though alternate interpretations have suggested that other factors also
play an important role (Dugdale and Wilkerson,
1990; Banse, 1991; Buma et al., 1991; Mitchell et
al., 1991; Nelson and Smith, 1991) [see the reviews
of Cullen (1991) and de Baar (1994)]. The recent
1995 Elsevier Science B.V. All rights reserved
SSDZ 0304-4203(94)00055-7
158
M.L. Wells et aLlMarine Chemistry 48 (1995) 157-182
mesoscale iron fertilization experiment south of the
Galapagos Islands demonstrated that enhanced
iron input indeed increases phytoplankton growth
rates and biomass (Kolber et al., 1994; Martin et
al., 1994). Even so, the ecosystem response to the
transient iron addition was far less dramatic than
seen in bottle experiments, which illustrates our
incomplete understanding of iron:plankton interactions in HNLC systems.
But iron also may play an important role in the
other oceanic domains, namely the subtropical
shelves, coastal upwelling
gyres, continental
zones, and the transitional waters separating
these regions. For example, colonies of the oceanic
nitrogen-fixing
cyanobacterium
Trichodesmium
observed in sub-tropical gyres may “capture” and
extract iron from freshly deposited mineral aerosols, thus contributing “new” iron and nitrogen to
the ecosystem (Rueter, 1988, abstract). In contrast,
shelf and coastal regions have much higher iron
concentrations and supply rates but phytoplankton iron requirements also are higher and differ
greatly among species (Brand et al., 1983; Murphy
et al., 1984). Iron may thus exert selective control
over species dominance within algal assemblages,
thereby regulating the food web structure of these
ecosystems (e.g. Hansen et al., 1994). Clearly, the
relationship between iron and phytoplankton may
have significance beyond its role in HNLC regions.
2. Workshop summary
Improving our understanding of the relationship
between iron and phytoplankton in the oceans has
been hampered by the complexity of the biological
and chemical aspects of the problem. There was a
strong need to identify what currently is known
with certainty about iron speciation in seawater,
the metabolic requirements of phytoplankton for
iron, the various strategies phytoplankton employ
to obtain iron, and how they adapt to low iron
conditions. This critical assessment could then
serve as a springboard for focusing efforts on the
research issues of greatest importance.
A major challenge facing biological and
chemical oceanographers is to develop techniques
to quantify those fractions of iron that are
accessible to phytoplankton and how this accessibility influences marine ecosystems. Traditionally,
however, there has been little direct interaction
among theoretical marine chemists, analytical
marine chemists and algal physiologists and ecologists working on this problem. To overcome
this obstacle, we (Mark Wells and Ken Bruland)
were funded by NSF, in cooperation with ONR,
NASA, NOAA, and DOE, to organize a workshop
(May I-5, 1994) composed of such individuals (see
Table 1) to address these central issues. This report
is a summation of the workshop findings.
The workshop did not address the role of iron in
limiting regional or global scale primary production. The intent instead was to facilitate crossdisciplinary interaction to identify productive
research avenues for developing the tools needed
to better examine these and other questions about
the marine biogeochemistry of iron.
To help establish a conceptual framework of
iron biogeochemistry, each participant prepared
an extended abstract of their newest research and
viewpoints. The titles of these abstracts are listed in
Appendix 1; full abstracts are available by contacting the Institute of Marine Sciences, University of
California, Santa Cruz. After two days of individual presentations, the participants split into working groups to identify important
unresolved
problems
which could
produce
significant
advances in understanding the marine biogeochemistry of iron. Afterwards, the working group findings were discussed in a series of plenary sessions.
This report is derived largely from the working
notes of these group discussions and thus is a
summed reflection of the ideas and insights of
many individuals. In addition to this summary
and the available abstracts, a series of articles
based on presentations at the workshop will
appear shortly in a special issue of Marine
Chemistry.
2.1. Dejining iron availability
A key aim of the workshop was to better define
biological availability of iron in seawater to help
focus the development of analytical tools to
quantify this fraction.
However, it became
apparent during the course of discussions that
M.L. Wells et al.lkfarine Chemistry 48 (1995) 157-182
159
Table 1
List of participants
Anderson, Donald, M. - Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
Barber, Richard - Duke University, Marine Laboratory, Beaufort, NC 28516, USA
Brand, Larry, E. - Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami,
FL 33149, USA
Bruland, Kenneth, W. - Institute of Marine Sciences, University of California, Santa Cruz, Santa Cruz, CA 95064, USA
Butler, Alison - Department of Chemistry, University of California, Santa Barbara, Santa Barbara, CA 93106, USA
Coale, Kenneth, H. - Moss Landing Marine Laboratories, P.O. Box 450, Landing, CA 95039-0450, USA
Crumbliss, Alvin, L. - Chemistry Department, P.M. Gross Chemistry Laboratory, Duke University, Durham, Box 90346, NC 277080346, USA
de Baar, Hein, J.W. - Netherlands Institute for Sea Research (NIOZ), P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands
Ditullio, Giacome, R. - Ecology Department, University of Tennessee, 108 Hoslins, Knoxville, TN 37996-l 191, USA
Geider, Richard, J. - Cannon Laboratory, University of Delaware, Lewes, DE 19958-1298, USA
Gledhill, Martha - Department of Oceanography, University of Liverpool, Liverpool, Great Britain L69 3BX
Goldberg, Edward, D. - Marine Research Division, 0220, Scripps Institution of Oceanography, La Jolla, CA 92093-0220, USA
Greene, Richard, M. - Biology Department, Brookhaven National Laboratory, Upton, NY 11973, USA
Haygood, Margo, G. - Marine Biology Division, 0202, Scripps Institution of Oceanography, La Jolla, CA 92093-0202, USA
Hudson, Robert, J. M - Institute of Marine Sciences, University of California, Santa Cruz, Santa Crux, CA 95064, USA
Hutchins, David, A. - Marine Sciences Research Center, State University of New York, Stony Brook, NY 11794, USA
Johnson, Kenneth, S. - Moss Landing Marine Laboratories, P.O. Box 450, Moss Landing, CA 95039-0450, USA
Kester, Dana, R. - University of Rhode Island, Graduate School of Oceanography, Narragansett, RI 02882, USA
King D., Whitney - Colby College, Department of Chemistry, Waterville, ME 04901, USA
Landing, William, M. - Department of Oceanography, Florida State University, Tallahassee, FL 32306, USA
Luther, George, W. - College of Marine Science, University of Delaware, Lewes, DE 19958, USA
Matsunaga, Katsuhiko - Department of Chemistry, Faculty of Fisheries, Hokkaido University, Hakodate 041, Japan
Measures, Chris, I. - Department of Oceanography, University of Hawaii, 1000 Pope Rd., Honolulu, HI 96822, USA
Millero, Frank, J. - University of Miami, Rosenstiel School of Marine and Atmospheric Science, 4600 Rickenbacker Causeway, Miami,
FL 28516, USA
Morel, Fragois, F.M.M. - Ralph M. Parsons Laboratory, Massachusetts Institute of Technology, Building 48-423, Cambridge, MA
02139, USA
Price, Neil, M. - Biology Department, McGill University, Montreal, Canada
Rue, Eden, L. - Department of Chemistry, University of California, Santa Cruz, Santa Cruz, CA 95064, USA
Rueter, John - Department of Biology, Portland State University, P.O. Box 751, Portland, OR 97207, USA
Sulzberger, Barbara - Swiss Federal Institute for Water Resources and Water Pollution Control (EAWAG), Dtibendorf, Switzerland
Sunda, William, G. - National Marine Fisheries Service, Southeast Fisheries Center, Beaufort Laboratory, Beaufort, NC 28516, USA
Takeda, Shigenobu - Central Research Institute of Electric Power Industry, Biology Department, Abiko Research Laboratory, 1646,
Abiko, Abiko-city, Chiba, 270- 11, Japan
Trick, Charles, G. - University of Western Ontario, Department of Plant Sciences, London, Ont., Canada
van den Berg, Constant, M.G. - Department of Oceanography, University of Liverpool, Liverpool, Great Britain L69 3BX
Voelker, Bettina, M. - Swiss Federal Institute for Water Resources and Water Pollution Control (EAWAG), CH-8600, Diibendorf,
Switzerland
Waite, T., David - Department of Water Engineering, University of New South Wales, Sydney, NSW 2052, Australia
Wells, Mark, L. - Institute of Marine Sciences, University of California, Santa Cruz, Santa Cruz, CA 95064, USA
Zhuang, Guoshun - Environmental Sciences Program, Univ. of Massachusetts at Boston, 100 Morrissey Blvd., Boston, MA 02125,
USA
this task presently is largely beyond our capability.
The perspective that developed during the workshop is that iron availability will be a function of:
(1) the various chemical forms of iron in seawater,
(2) the preference of the uptake mechanism of each
organism for one or another of these forms, and (3)
the balance between the reaction kinetics of iron
exchange among chemical species, the iron uptake
kinetics of a given organism, and the iron demand
of each member population within the phytoplankton assemblage. Clearly, the complexity of the
natural seawater system frustrates attempts to
define a general “biological availability” of iron
based on a singular analytical measurement of
160
M.L. Wells et aLlMarine
Chemistry 48 (1995) 157-182
Biological Component
l
l
l
Concentrations of Fe
species
Mechanisms and kinetics of
exchange among species
Thermodynamic and kinetic
modelllng
Chemical Component
l
l
l
Ecosystem structure
Fe and C exports
External and internal Fe
inputs
EcologicaVBiogeochemical
Component
Fig. 1. Conceptual diagram of the interaction among the biological, chemical and ecological/biogeochemical
biogeochemistry of iron.
“reactive” metal. Moreover, our present ignorance
of many aspects of iron chemistry in seawater
makes it unlikely that we can accomplish this
task within the near future.
2.2. Conceptual diagram
A measure of our limited understanding of the
inter-relationship between iron chemistry and the
biota was the difficulty participants experienced in
formulating a simplified conceptual framework
within which to identify the most pressing
questions. Eventually, a conceptual diagram comprising three main components was chosen: (1) a
biological component encompassing the biochemical and physiological aspects of single-celled organisms, (2) a chemistry component comprising
various forms and chemical species of iron in seawater along with the abiotic processes regulating
the interchange
among them, and (3) an
ecological/biogeochemical
component
reflecting
both ecosystem structure and transfer of matter
into and out of the upper ocean (Fig. 1).
The considerable overlap among these com-
components of the marine
ponents is illustrated by the processes interlinking
them. It is expected that biology will influence iron
speciation by releasing organic molecules that
chelate or complex iron, by mediating redox transformations at the cell surface, and by selectively
taking up various iron species. The resultant
“availability” of iron can influence the size structure and species composition of the algal assemblage and hence the community food web. The
ecology of the system, in turn, will control both
the form and magnitude of regenerated iron in
surface waters as well as influence the cycling and
removal of other elements.
A vital but external component to this conceptual framework is the input of energy as light.
Variations in the intensity and spectral qualities of
light not only influence photosynthetic rates, and
hence the overall biological demand for iron, but
also will modify the solution, colloidal, and particulate chemistry of iron via various photochemical
transformations.
Of the many questions surrounding this simplified conceptual model, five emerged as the most
pressing during the workshop discussions:
M.L. Wells et al./Marine Chemistry 48 (1995) 157-182
(1) What are the sources and sinks of new and
regenerated soluble iron in the upper ocean?
What
is the chemical speciation of iron among
(2)
the soluble, colloidal and particulate fractions,
including the rates and mechanisms of transformations among these forms?
between the
(3) What are the inter-relationships
chemical speciation of iron and the microbial
community (e.g. how the chemical speciation
influences iron uptake by the biotic community and how the community influences
iron speciation)?
(4) How do microbes adapt biochemically and
physiologically to variations in iron availability in the ocean?
(5) What ecological and biogeochemical implications arise from temporally and spatially
changing iron supply and availability in the
oceans?
Participants sought to provide guidance for
answering these broad questions by dissecting
each into a set of more specific questions, some of
which already have answers and others useful for
posing testable hypotheses. Wherever possible,
recommendations
were made regarding specific
methods or new analytical approaches that would
be promising tools for attacking these problems.
(1) What are the sources and sinks of new and
regenerated soluble iron in the upper ocean?
Iron is a particle reactive trace metal existing at
extremely low concentrations in the oceans. Its
nutrient-type characteristics are inferred from vertical profiles [low surface values, higher deepwater
values (e.g. Martin and Gordon, 1988)] and from
rapid biological recycling (Hutchins et al., 1993,
abstract). Iron also exhibits particle-sorption
(scavenging) characteristics (decreasing concentrations with increasing age of deepwaters) with
relatively short residence times both in surface
waters (weeks, months, to a year depending upon
the regime) and in the oceans as a whole (less than a
century). Much of the iron remineralized in the
intermediate and deep waters is scavenged and
removed to the sediments and is not mixed back
161
into the surface waters, leading to a continuous
depletion of iron relative to the major nutrients
nitrate and phosphate. Thus, it is particularly relevant to examine new external sources of iron.
A. What are the sources of iron?
Sources of dissolved iron to the photic zone
include both “new” sources from external inputs
as well as “regenerated” sources recycled in-situ
from various particulate phases. These sources
include:
. wet and dry deposition
of atmospheric aerosols,
- vertical mixing and upwelling,
. inputs from rivers and bottom sediments, and
. biogenic recycling of cellular iron in surface
waters
In remote oceanic regions, wet and dry
deposition of aerosols is an important external
source of new iron. The aerosol iron deposited is
a mixed assemblage of terrestrially weathered
particles, solid forms reworked by photochemical/
reprecipitation
processes, and dissolved metal
species. The delivery of wet and dry deposition to
the surface ocean varies markedly both spatially
and temporally (Moore et al., 1984; Duce, 1986).
A consensus of the workshop was that we need a
better understanding of the precise chemical nature
of deposited iron and the spatial and temporal
variability of the atmospheric source. In addition,
the various processes that act to solubilize iron
from the mineral aerosols, both in the atmosphere
and within the surface ocean, need to be quantified
(see belowj.
Another important source of iron to surface
waters is vertical mixing and upwelling, which
also varies markedly on spatial and temporal
scales.
Atmospheric deposition of iron generally has
been considered to be the dominant source of
iron to surface waters of the open ocean. For
example, the subarctic Pacific lies in the path of
an extended aerosol plume originating in China
(Young and group, 1991). Martin et al. (1989)
estimated that atmospheric deposition accounted
for 84-93% of the external iron input to these
162
M.L. Wells et aLlMarine
surface waters. A similar predominance of atmospheric iron input occurs in the major subtropical
gyres, where a high degree of stratification strongly
limits vertical mixing of iron from below the
thermocline. However, in the HNLC regions of
the equatorial Pacific and Southern Ocean, iron
input from upwelling and vertical mixing overwhelms the meager aerosol deposition in these
remote areas (Bruland, this meeting; de Baar,
abstract).
In coastal and shelf waters, substantial external
inputs of iron come from riverine sources and
bottom sediments, leading to markedly higher dissolved and particulate iron concentrations. Much
of the particulate iron in nearshore waters is
inorganic and processes that solubilize this reservoir, making it accessible to phytoplankton, are
especially relevant (see below). The greater iron
inputs to coastal and shelf regions compared to
the open ocean are accompanied by high iron
requirements of neritic phytoplankton
species
(Brand et al., 1983; Sunda et al., 1991). These systems therefore might be strongly influenced as the
comparatively rich iron resource is diminished by
phytoplankton blooms.
The single largest reservoir of iron in the surface
waters of HNLC regions may be the biota itself.
New evidence indicates that this biological pool of
iron is recycled on the time scale of days, much like
N and P (Hutchins et al., 1993). This “input” of
regenerated iron to surface waters is estimated to
be more than an order of magnitude greater than
the external supply rate of iron (Bruland, this meeting; Morel, this meeting) and may largely satisfy
the iron-demand of phytoplankton in these systems. This view is supported by recent results of
Price et al. (1994) showing that iron uptake rates
of plankton in the equatorial Pacific are sufficient
to entirely turn over the dissolved iron pool within
half a day or less. Presently, there is no indication
of the chemical forms of this regenerated iron or
whether these forms are directly reassimilated by
phytoplankton.
B. What are the sinks of iron?
The removal of iron from surface waters is fairly
Chemistry 48 (1995) 157-182
well constrained within a geochemical (i.e. mean
residence time) perspective, however, the mechanisms and dynamics of this removal is not well
understood. Mechanisms for removing iron from
surface waters include:
* sorption and precipitation,
0 biological assimilation,
- aggregation of inorganic or organic colloids,
and
. sinking of mineral and biogenic particles.
While much of the particulate iron introduced
via rivers, sediment resuspension, or as mineral
aerosols will be removed by settling, ascertaining
the underlying basis for the removal of “dissolved”
iron forms is much more difficult. In regions with a
high sinking flux of inorganic mineral particles (e.g.
in some coastal and well mixed shelf waters), dissolved iron may be removed abiotically by sorption
to surfaces of these particles. Similarly, sinking
organic particles also can scavenge soluble iron
from surface waters (Morel and Hudson, 1985).
Dissolved iron also is “removed” via direct assimilation by phytoplankton. The subsequent sinking
of live cells or fecal matter will transport a portion
of this biogenic iron from surface waters.
In addition to direct assimilation and sorption
onto sinking (mineral and biogenic) particles, iron
may sorb to colloidal organic matter which is abundant in surface waters (Wells and Goldberg, 1992,
1994). The stability of this colloidal phase is a topic
of much dispute (Honeyman and Santschi, 1989;
Bauer et al., 1992; Moran and Buesseler, 1992;
Wells and Goldberg, 1993), but the extremely
large colloidal surface area combined with the
particle reactive nature of iron suggests that aggregation of organic colloids could be important for
removing iron.
Significant unresolved issues regarding iron
removal include (1) identifying the specific mechanisms of iron sorption to abiotic and biotic sinking
particles, which will shed light on how changes in
iron speciation may affect this removal pathway,
(2) changes in the iron “export efficiency” of the
assimilation pathway with shifts in primary productivity or species assemblage, and (3) the abundance and reactivity of iron in the colloidal
reservoir. The relative importance of abiotic, bio-
M.L. Wells et al.iMarine Chemistry 48 (199.5) 157-182
tic and colloid aggregation removal processes will
vary from regime to regime and with season.
(2) What are the chemical speciation and forms of
iron among the soluble, colloidal and particulate
fractions, including the rates and mechanisms of
transformations among these forms?
There are some large gaps in our knowledge of
iron chemistry in seawater. The development of
trace metal clean techniques for seawater collection and analysis over the past decade
(Bruland et al., 1979; Gordon et al., 1982; Landing
and Bruland, 1987) has given us reliable profiles for
iron in the traditional categories of particulate
(> 0.4 pm) and “dissolved” (< 0.4 pm) fractions;
however, the chemical forms of iron within these
fractions
has been largely speculative.
For
example, there now is evidence that iron exists, at
least partially, in the small colloidal phase (Wells
and Goldberg, 1991; Wu and Luther, 1994; Powell
and Landing, abstract) which is included in
operationally defined “dissolved” fractions. Determining how iron is partitioned among these phases,
and among various chemical forms within each
phase, is central to understanding iron speciation
in seawater.
A. Soluble iron
What is the solubility and speciation of Fe(III) in
seawater?
The major issues that remain unresolved include:
- the disputed existence of the soluble Fe(OH)i
species,
. the degree of organic complexation
of Fe(III),
and
. the source, supply, and chemical nature of
Fe(III)-binding organic ligands.
It is an embarrassing observation that despite
decades of studies on the marine chemistry of
iron, and despite increasing evidence that iron
may play a singularly significant role in ocean ecosystems, there is no clear consensus among marine
chemists on the thermodynamic solubility of inorganic Fe(II1) in oxic seawater. Solubility estimates
163
for Fe(II1)’ (the sum of dissolved inorganic species)
vary from N 10e8 M to N lo-” M depending on
the disputed presence of the dissolved Fe(OH)i
species (Byrne and Kester, 1976; Morel, 1983).
Experimental
evidence from filtration studies
suggest that solubility lies near the upper end of
this range, but these findings are equivocal
because small colloidal forms of iron may pass
through even small pore-size filters. It is crucial
that we ascertain the upper limit of iron solubility
in seawater because natural “dissolved” concentrations of iron span the disputed solubility
range, i.e. colloidal iron precipitates are expected
or not depending on the solubility estimate chosen.
Better quantification of the thermodynamic formation constants of the Fe(OH)i3-‘) species as a
function of ionic strength, pH, temperature and
pressure would bring us closer to this goal.
But the relationship between iron chemistry and
its uptake by phytoplankton is more than just a
question of solubility; the coordination exchange
kinetics of the various species exert a primary control over the rate of iron uptake (Morel et al., 199 1,
Crumbliss,
abstract;
Hudson
and Bruland,
abstract). For example, while iron uptake generally correlates with “free metal ion” activity in
well defined, chelate buffered media of laboratory
studies, this correlation results from the rapid interequilibration of Fe3+ with the more abundant and
kinetically labile hydrolysis species and the constant proportionality
between concentrations of
these species, Fe(II1)’ and Fe3+, in constant pH
seawater (Hudson et al., 1992). In natural waters,
the concentration of Fe(II1)’ is the relevant factor
to consider with respect to the uptake of inorganic
iron.
One of the most intriguing new aspects of iron
chemistry presented at the workshop were very
recent data indicating that up to 99.9% of the dissolved iron in surface waters is bound within
organic complexes in both nearshore and open
ocean environments (Gledhill and van den Berg,
1994; Rue and Bruland, submitted; van den Berg,
submitted; Wu and Luther, submitted), resulting in
extremely low (subpicomolar) concentrations of
dissolved Fe(II1)‘. A similar degree of complexation has been shown for both Cu and Zn in surface
waters, though complexation
of Cu and Zn
164
M.L. Wells et al./Marine
decreases significantly in deep waters (Coale and
Bruland, 1988; Bruland, 1989) while preliminary
results indicate iron complexation remains high
(Rue and Bruland, submitted; van den Berg, submitted). Both the high stability of this ligand class
for iron, and its significant concentration in seawater appear to be remarkably consistent in the
North Pacific, North Sea and western Mediterranean. The source of these ligands is unknown,
though preliminary results suggest that they may
originate from phytoplankton (van den Berg, submitted). Strong iron-binding proteins also have
been isolated and identified from the marine
bivalve Mytilis edulis (Taylor et al., 1994), though
the extent that these compounds are released to
coastal waters is unknown.
It is crucial that preliminary measurements of
strong Fe(II1) complexing organic ligands in seawater be verified. If these ligands indeed are in
ubiquitous excess over dissolved iron concentrations in the oceans, it will profoundly alter our
perception of iron speciation in seawater and raise
more questions than provide answers. What is the
composition and source of this ligand class? Do
they exist within the soluble or colloidal organic
phase? Do these organic ligands serve to “buffer”
dissolved iron concentrations and thereby limit its
rapid removal from seawater (Rue and Bruland,
are
submitted)?
Most
importantly,
they
an assemblage of siderophores
released to
facilitate iron uptake (see below) or do they
comprise general cell exudation and degradation
products? If a major fraction of these organic
ligands are siderophores, then iron speciation
in seawater would reflect a vigorous competition among various autotrophic and heterotrophic microorganisms for control of a scarce
resource.
Insight into the potential role of these newly
found organic ligands will come from better
characterization
of their sources, sinks (algal
uptake, microbial degradation, photolysis, colloid
aggregation), residence times, and their complexing
characteristics
with iron and other metals.
Competitive
ligand
exchange
voltammetry
techniques combined with cell culture studies
offer one the more promising directions for future
research.
Chemistry 48 (199.5) 157-182
What factors injkence the redox state of iron in
seawater?
Though Fe(II1) is the thermodynamically stable
oxidation state of iron in oxic seawater, a substantial body of evidence now suggests that a significant fraction of dissolved iron in surface waters
exists as Fe(I1). Inorganic Fe(I1) oxidation pathways are well described for inorganic seawater
and are rapid (Miller0 and Sotolongo, 1989) but
considerably less is known about rates of Fe(II1)
reduction. Some reductive processes responsible
for maintaining measurable Fe(I1) concentrations
in oxic surface waters may include:
- photochemical reduction,
- thermal reduction (i.e. by organics),
. enzymatic reduction,
- reduction within microenvironments, and
- retardation of Fe(I1) oxidation rates by for-
mation of Fe(I1) organic complexes.
Photoreduction of Fe(III)
The deposition of photochemically altered aerosols may be a significant source of Fe(I1) to surface
waters. Both direct and indirect photoprocesses
can potentially transform iron redox states during
aerosol transport. For example, soluble Fe(I1) has
been measured in fog and cloud aerosols (Behra
and Sigg, 1990; Erel et al., 1993; Zhu et al., 1993)
and in aerosols from remote ocean regions
(Zhuang et al., 1992; Zhu et al., 1993). In addition, the photoreduction of soluble and synthetic
colloidal Fe(II1) has been measured in seawater
under laboratory and field conditions (Waite and
Morel, 1984a; Rich and Morel, 1990; Wells et al.,
1991a; Waite and Szymczak, 1993; Miller and
Kester, 1994; Zhang et al., submitted; Kester,
abstract; King, abstract; Gledhill and van den
Berg, abstract) and diurnal variations in surface
water concentrations
of Fe(I1) and reactive
Fe(II1) species have been observed
(Hong
and Kester, 1986; O’Sullivan et al., 1991; Johnson
et al., 1994), though at somewhat higher iron
concentrations
than expected for open ocean
environments.
While photoreduction
of Fe(II1) inorganic
ligand complexes occurs readily at short UV wavelengths (King et al., 1993, abstract; Miller et al.,
M.L. Wells et al.lMarine Chemistry 48 (1995) 157-182
submitted), natural organic chromophores
can
greatly enhance iron reduction under earth-surface light regimes (Erel et al., 1993; Waite and
Morel, 1984b; Wells et al., 1991a). Reduction of
Fe(III)-organic ligand complexes may occur by
direct ligand-to-metal charge transfer or from reaction with secondary photolysis products. Workshop participants heard the first results from
laboratory
studies showing that the photoproduced superoxide radical (02) rapidly reduces
Fe(III)-inorganic
ligand
complexes
soluble
(Voelker and Sedlak, submitted) but apparently
does not react significantly with colloidal iron oxyhydroxides (Voelker, this meeting). In HNLC
waters, where iron concentrations are below solubility estimates, reduction by superoxide was
argued to maintain up to 30-75% of dissolved
iron as Fe(I1). However, complexation of Fe(II1)
by natural organic ligands may sharply curtail,
though not eliminate, iron reduction by 0,.
There is a pressing need to identify and characterize: (1) the nature and source of natural Fe(II1)
reducing chromophores, (2) the relative importance of direct and indirect photoprocesses to
iron chemistry in surface waters, and (3) the spectral dependence and kinetics of these reactions at
ambient concentrations of iron and chromophores.
Thermal reduction of Fe(III)
Several organic ligands can reduce Fe(II1) upon
complexation (e.g. tannic acid) and it is plausible
that similar redox reactions might occur in seawater. Though thermal reduction of iron by
organic ligands in surface seawater is thought to
be of much less consequence than direct or indirect
photolysis, the significance of this reduction pathway below the upper photic zone is presently
unknown.
Enzymatic reduction of Fe(III)
Fe(II1) reduction also may be enzymaticallymediated either by plasmalemma (membrane
bound) redox proteins or by intracellular enzymes
secreted as a result of cell damage. Both mechanisms may enhance iron acquisition by phytoplankton and yet have distinct implications for biological
effects on iron speciation. On one hand, release of
intracellular enzymes should be linked to grazing
165
activity and thus correlated
positively with
phytoplankton
growth rates [i.e. extracellular
Fe(II1) reduction rates would increase as cell
growth rates and grazing rates increase]. Conversely, if reduction at the cell surface by electron
pumping is important, Fe(II1) reduction rates
should be an inverse function of growth rates.
Cells continue to produce reducing power (as
NADPH) by photosynthesis even when nutrient
limitation inhibits cell division, and this nonutilized reducing capacity may be eliminated from
the cell by pumping electrons across the membrane
(i.e. via plasmalemma redox proteins). Rates of
extracellular Fe(II1) reduction increase as ironlimited growth rates decrease in Thalassiosira
oceanica, suggesting that the reductant pumping
mechanism may be related to iron-deficiency
(Maldonado and Price, unpubl. data).
Fe (III) reduction in microenvironments
Reducing microenvironments are more common
in oxic waters than is generally recognized (Alldredge and Cohen, 1987). Detrital aggregates
(“marine snow”), diatom mats, the feeding structures of larvaceans, fecal pellets and Trichodesmium colonies all can harbor sub-oxic internal
environments which could facilitate Fe(I1) production. One acidic, sub-oxic environment often
not considered is the digestive systems of planktonic grazers. Protozoan feeding vacuoles may
drop to pH N 3 and copepod guts can reach pH
N 5 during the initial stages of digestion. Because
cell turnover rates in open ocean and nearshore
waters are on the order of days or less, grazers
could have a major effect on iron speciation.
An additional reducing microenvironment that
needs to be explored is that within organic colloidal matrices (Goldberg, abstract). There is preliminary evidence that marine colloidal organic
matter can harbor thermodynamically
unstable
species such as iodide and arsenite, suggesting
that higher oxidation states of elements are
reduced within the electron rich colloids (Goldberg, abstract).
Fe(II) oxidation in seawater
The oxidation of inorganic Fe(I1) species in surface seawater is rapid and well quantified, with a
166
M.L.
Wells et al./kfarine
reaction half-time of a few minutes. The dominant
inorganic oxidation pathway for Fe(I1)’ in surface
seawater is by reaction with photo-produced H202
(Moffet and Zika, 1987) which is N 4-40 times
faster than oxidation
by O2 (Miller0 and
Sotolongo, 1989). In deep waters, where H202 concentrations are much lower, oxidation of Fe(I1)’ is
dominated by reactions with Oz. Because H202
forms in surface waters from disproportionation
of photochemically-generated
superoxide radical
(02) it is expected that sunlight enhances rates of
iron reduction and oxidation leading to rapid
photoredox
cycling in surface seawater. Such
photoredox cycling should enhance ambient concentrations of kinetically labile inorganic species of
Fe(I1)’ and Fe(II1)’ (Morel et al., 1991; Wells et al.,
1991a; Sunda, 1989; Kester, abstract) and thereby
increase cellular iron uptake by membrane transport ligands (see below).
Though the inorganic speciation of Fe(I1)’ in
seawater is better characterized than that for
Fe(III)‘, recent measurements of significant concentrations of Fe(I1) in surface seawater implies
that it may be considerably more stable than predicted. It is possible that Fe(I1) species are stabilized by complexation with organic ligands that
impede oxidation (Kester, abstract; Gledhill and
van den Berg, submitted). The apparent stability
of Fe(I1) in surface waters is of particular importance because Fe(I1)’ species are substantially more
soluble in seawater than Fe(II1)’ and are kinetically
more labile. Thus, a well buffered Fe(I1)’ system
might increase iron availability markedly. One methodological avenue which shows much promise for
elucidating Fe(I1) speciation in seawater is competitive equilibrium/adsorptive cathodic stripping voltammetry using Fe(I1) selective added ligands.
B. Colloidal iron
The marine colloidal phase may serve as a reservoir of iron in seawater, though its significance
within the different oceanographic domains has
yet to be established. The central issues needing
attention are:
- the abundance
and distribution
iron in ocean waters,
of colloidal
Chemistry
48 (1995) 157-182
. the forms of iron occurring in the colloidal
phase,
- rates of exchange between colloidal iron and
soluble species,
- photochemical reactivity of colloidal iron, and
- the dynamics of colloid behavior.
Without precise knowledge about the inorganic
solubility of Fe(III)‘, the degree of organic complexation of soluble Fe(III), and the fraction of
total “dissolved” iron existing as Fe(I1) species, it
is unclear where and when iron is likely to precipitate in seawater. While colloidal oxyhydroxides
occur in nearshore surface waters (Wells and Goldberg, 1992) and in hydrothermal plumes (Feely et
al., 1990; Cowen, 1992) most colloidal matter in
seawater has a largely organic matrix (Koike et al.,
1989; Longhurst et al., 1992; Wells and Goldberg,
1992; Wells and Goldberg, 1994). Pure forms of
colloidal iron oxyhydroxides,
such as those
typically used in experimental studies, are rare in
surface seawaters (Wells and Goldberg, 1994).
Even so, preliminary results indicate that 20-40%
of the filterable iron in surface waters of the North
Atlantic and Greenland Sea occurs in the colloidal
phase (Wu and Luther, 1994; Powell and Landing,
abstract), presumably associated with colloidal
organic matter.
There is a need to better characterize the oceanographic distributions of marine colloids and the
concentrations of iron within this phase. Ultrafiltration studies with systems specially modified
to minimize contamination would be a good step
towards this goal. Verification of preliminary
findings showing iron associated with marine
colloidal matter must be followed by determination
of the chemical nature
of this
association. Are inorganic precipitates of iron
agglomerated into the organic matrix, or do
soluble Fe(II1)’ hydrolysis
species sorb or
become complexed at the organic colloid surface?
Do soluble organic-Fe(II1) complexes become
associated with colloidal surfaces, or partition
into the colloid matrix? Exploring the mechanisms
for iron-colloid association will require development of new clever methods, however, these
results will be crucial for predicting the subsequent
reactivity of natural “colloidal” iron with respect
M.L. Wells et aLlMarine Chemistry 48 (1995) 157-182
to solution phase
chemical processes.
thermochemical
and photo-
161
(3) What are the inter-relationships between the
chemical speciation of iron and the microbial
community?
C. Particulate iron
Particulate iron is generally ignored when contemplating iron “availability”.
Though some
phytoplankton
are known to eat bacteria (Bird
and Kalff, 1986) there is presently no evidence
that phytoplankton
directly engulf inorganic
particles. Nevertheless, as noted above particulate
substances may become accessible by facilitated
dissolution in specialized microenvironments (e.g.
within Trichodesmium colonies). But even outside
these microenvironments,
particles may rapidly
equilibrate thermochemically with solution species
to provide a ready source of iron to phytoplankton
(Wells et al., 1983; Rich and Morel, 1990). For
example, measurements of labile iron in coastal
and shelf seawaters by reaction with a chelating
agent showed the particulate fraction to contain
the majority of easily exchangeable iron in these
waters (Wells and Mayer, 1991b). The source, distribution, and input rates of particulate iron may
therefore have important consequences for the supply of iron to phytoplankton.
Unfortunately, while reliable data now exist for
particulate iron distributions in seawater, very few
data exist on the specific composition and chemical
reactivity of this size fraction. For example, though
Fe/Al ratios in atmospheric dusts often are similar
to average crustal values calculated from silicate
mineral abundance, more iron occurs as oxyhydroxides in these mineral aerosols than within
the parent (crustal) silicates due to weathering
(Murad and Fischer, 1988; Goldberg, this meetspatially distinct terrestrial
ing). Moreover,
weathering patterns, combined with apparently
extensive photochemical and acidic reprocessing
of mineral aerosol particles (Behra and Sigg,
1990; Zhuang et al., 1992; Erel et al., 1993; Zhu
et al., 1993) makes it likely that the chemistry of
these solid phases varies as a function of source
point and atmospheric residence time. Better
characterization
of the chemistry of particulate
inputs is essential to understanding the interaction
between particulate and soluble iron forms in
seawater.
As discussed previously, preliminary evidence
suggests that a major fraction of soluble iron is
chelated with organic ligands. The microbial community may therefore play a key role in controlling
the speciation and redox state of iron in seawater.
But in addition to indirect effects arising from the
inadvertent release of cell metabolites and cell surmay actively
face reactions, microorganisms
attempt to regulate iron speciation to facilitate its
acquisition. The form such manipulation might
take will depend on the various iron uptake
strategies employed by marine microorganisms.
Strategies for iron acquisition; how do these AifSer
among organisms and across environments?
Phytoplankton may utilize a variety of strategies
for extracting iron from the surrounding environments, including:
. uptake of Fe’ by membrane bound porter sites,
- uptake of Fe-siderophore
chelates,
- extracellular reduction,
* excess (or luxury) uptake and storage, and
. solid-phase Fe acquisition.
Oceanic microorganisms have evolved at least
two different mechanisms for acquiring iron from
seawater at extremely low concentrations. Diatoms
and other eukaryotic phototrophs are thought to
primarily utilize dissolved inorganic Fe(II1)’ and
Fe(I1)’ (Morel et al., 1991) by a transport ligand
process involving cell surface complexation and
internalization. This conceptual model is founded
on a very limited study of 4 species and thus may
not be applicable to all organisms. In this model,
dissolved labile inorganic Fe(I1) and Fe(II1) species
diffuse to the cell surface where they undergo
ligand exchange reactions with membrane porter
sites whose nature is unknown (Hudson and
Morel, 1990, 1993). The iron is then transported
into the cell. If high levels of iron chelation
(Gledhill and van den Berg, 1994; Rue and
Bruland, submitted; van den Berg, submitted; Wu
and Luther, submitted) are characteristic of all
ocean surface waters, iron uptake strategies based
168
M.L. Wells et aLlMarine Chemistry 48 (1995)
solely on Fe(II1)’ acquisition may be inefficient
because Fe(II1)’ concentrations (- 0.1 PM) would
be too low for diffusion to support the typical
growth rates of many oceanic phytoplankton
(Sunda and Huntsman, submitted).
But strong organic complexation of iron in seawater and Fe’-based uptake strategies are not
necessarily
exclusive.
Cell
surface-mediated
reduction of these organic-iron complexes by
reduced biochemicals such as thiols or plasmalemma redox proteins (Price and Maldonado,
abstract) may convert them to transportable labile
Fe(I1)’ and Fe(II1)’ species. In addition, there now
is ample evidence for a dynamic iron redox cycling
in seawater (Waite and Morel, 1984b; O’Sullivan et
al., 1991; Wells et al., 1991a; King et al., 1993;
Waite and Szymczak, 1993; Miller and Kester,
1994; Miller et al., submitted; King et al.,
abstract). Organisms may promote this process
by releasing extracellular organic compounds
which thermally
or photochemically
reduce
Fe(II1) (Waite et al., abstract). Photochemical
redox cycling enhances steady state concentrations of both Fe(I1)’ and Fe(II1)’ and thus would
facilitate iron uptake via membrane-bound,
Fe’based transport systems (Sunda, 1989; Morel et
al., 1991; Wells and Mayer, 1991a).
The extent of photochemical enhancement of Fe’
concentrations is currently unknown. However, a
mere 5 % steady-state photoconversion of organically bound Fe(II1) species to Fe’ at a filterable iron
concentration of 50 pM would increase Fe’ concentrations by 20-30 fold to a value of 2.5 pM (Sunda,
unpubl. data). An Fe(II1)’ concentration of 2.5 pM
would be sufficient to support specific growth rates
of 0.5-0.7 day-’ which are typical for oceanic
phytoplankton (Sunda and Huntsman, in prep.).
Thus, photoredox cycling may well buffer Fe’ at
sufficiently high levels to support observed algal
growth rates, even if high levels of Fe(II1) chelation greatly diminishes Fe’ in the dark under equilibrium conditions.
Redox cycling notwithstanding, preliminary evidence suggests that a substantial portion of tilterable (i.e. “dissolved”) iron is inaccessible to
phytoplankton in equatorial Pacific waters. Using
a novel approach, Fe’ in this surface seawater was
manipulated by titration with a terrestrial fungal
157-182
siderophore (Wells et al., 1994). The rapidly
formed siderophore-Fe
complex was unavailable
to the marine microorganisms and thus could be
used to test the availability of ambient iron species.
Less than 40% of the N 25 pM filterable iron concentration appeared to be available to increase
picoplankton biomass, suggesting that Fe’ species
were not the dominant form of iron in these waters,
and that the remaining forms were resistant against
conversion to biologically accessible species.
Production of siderophores represents a second
strategy for microbial iron acquisition and is common in soil and enteric habitats subject to iron
deficiency (Neilands, 1974; Winkelmann et al.,
1987; Matzanke et al., 1989; Crumbliss, 1991). In
this mechanism, microbes produce strong Fe(II1)
chelating ligands, primarily with multiple catechol
and hydroxamate functional groups, which effectively out compete other ligands for Fe(II1). These
Fe-siderophore chelates are then taken up into the
cell by membrane transport proteins and the iron is
released internally for metabolism. Iron release
involves either degradation of the siderophore,
reduction of Fe(II1) to Fe(I1) or both. Following
iron release, the siderophore typically is excreted to
complex more iron. Permeations on this siderophore mechanism include release of the siderophore bound iron at the cell surface (often by
reduction; Crumbliss, abstract; Trick, abstract)
and transport of a new internal Fe-chelate.
Such ligands may play similar roles in seawater
(Butler, abstract; Crumbliss, abstract; Trick,
abstract). Indeed, microorganisms isolated from
coastal and oceanic habitats produce siderophores
in iron deficient culture media (Trick et al., 1983;
Trick, 1989; Reid and Butler, 1991; Haygood et al.,
1993) and aqueous extracts of coastal blue green
algal mats demonstrate the presence of hydroxamate siderophores (Estep et al., 1975). In no
cases, however, have soluble siderophore complexes been identified in field water samples,
although little effort has been directed toward
their detection. The preliminary findings of strong
iron-binding
ligands in the North Atlantic
(Gledhill and van den Berg, 1994), North Pacific
(Rue and Bruland, submitted), and Mediterranean
(van den Berg, submitted) should rekindle interest
in marine siderophore research.
169
M.L. Wells et aLlMarine Chemistry 48 (1995) 157-182
In soils, siderophores facilitate iron uptake by
the siderophore producer but limit access to iron
by other organisms. However, many terrestrial
microorganisms possess the capability of utilizing
Fe-siderophore
complexes produced by other
organisms, gaining the benefits without incurring
the considerable metabolic costs involved in siderophore synthesis. Interspecific utilization of Fesiderophore complexes may entail “copying” siderophore transport sites or involve surface reductive
dissociation of Fe(lII)-siderophore
complexes by
catechols at the cell membrane followed by uptake
of the released iron (Trick, abstract). The prevalence of such Fe-siderophore
“pirates” in seawater is entirely unknown, but verification of
their presence would greatly complicate attempts
to quantify the availability of iron to specific members of the phytoplankton assemblage. As a further
complication, terrestrial bacteria often have multiple uptake systems for iron that can operate simultaneously; marine microorganisms likely also have
this capability.
Because iron input can be sporadic, the ability of
microorganisms to store iron may be an important
adaptive strategy. Sunda and Huntsman (in prep.)
found that 5 out of 6 algal species tested had at
least some capability to store excess iron. The capacity for “luxury uptake” differed among species, in
accordance with the higher, more variable iron
levels in neritic environments. Luxury uptake was
highest for coastal diatoms (Thalussiosira pseudonana and T. weissjlogii), which were able to take up
a 20-40 fold excess of cellular iron over levels
needed for maximum growth (i.e. enough iron to
fuel 5 cell divisions). The ability to store large levels
of excess iron may be an important strategy for
organisms which bloom episodically.
Several lines of circumstantial evidence suggest
that the nitrogen-fixing cyanobacterium Trichodesmium can help meet its high iron requirement by
the adsorption and leaching of atmospheric dust
particles. In the North Tropical Atlantic, Saharan
dust may support the population of Trichodesmium
(Rueter et al., 1990) and a Trichodesmium bloom
was observed at the Hawaii ocean times series
station in the North Pacific following a storm (i.e.
deposition) event (Hutchins, Ditullio, Hudson,
Wells, unpubl. data). A mechanism of direct con-
tact between the iron source (dust particle) and the
buoyant Trichodesmium colonies would avoid the
problems associated with dilute iron chemistry and
loss of particulate iron from the photic zone. Based
on dust flux measurements, dissolution rates and
Trichodesmium productivity’s, Rueter and his colleagues (Rueter et al., 1992) estimated that half of
the total iron used biologically in the North Tropical Atlantic could be introduced via this route.
Characteristics of the environment may influence the
strategies employed
It is likely that the above strategies by which
phytoplankton may acquire iron shift in importance among different oceanographic
environments, not only because of changes in the
phytoplankton assemblage but also because of different external inputs and chemical speciation of
iron. Ultimately, iron uptake strategies will have
a pivotal role in phytoplankton/iron
interactions
in seawater, but this level of understanding is presently far from our grasp. A blend of laboratory and
field experimentation
focused on isolating and
quantifying the different pathways for iron uptake
by phytoplankton should provide the first steps
towards this goal.
(4) How do microbes adapt biochemically
physiologically
to Fe availability
and
in the ocean?
Understanding how cells adapt to low iron conditions in seawater depends on identifying the biochemical roles for iron and which cellular processes
are most strongly dependent on iron. This information then can be used to develop specific tools
for evaluating the iron-related physiological state
of organisms in different oceanographic regimes
and over various times scales (hours to months).
Cellular processes most strongly regulated by iron
Very few studies of the biochemical roles of iron
have examined marine phytoplankton (e.g. Glover,
1977; Sandmann, 1985; Rueter and Ades, 1987;
Doucette and Harrison, 1991; Greene et al., 1991;
Geider et al., 1993) and even fewer have involved
marine
heterotrophic
bacteria
(Haygood,
abstract). Nonetheless, several candidate meta-
170
M.L. Wells et al./Marine Chemistry 48 (1995) 157-182
bolic processes are expected to be particularly
sensitive to the iron supply to phytoplankton,
including:
. nitrogen assimilation,
(4
. N2 fixation,
- photosynthetic and respiratory electron transport,
- porphyrin biosynthesis, and
. removal of reactive oxygen species.
(4
Nutrient limitation affects microbial growth
rates by impairing essential biochemical functions. Major nutrients, such as C, N, and P, have
essential roles as structural components (carbohydrates and proteins), energy currency (ATP),
and information
storage (nucleic acids). Like
other micronutrients, iron plays a largely catalytic
role as a cofactor of enzymes and redox proteins
including those required for the assimilation of the
major nutrients. Iron-containing
proteins are
essential for photosynthetic and respiratory electron transport, and are directly involved in nitrate
and nitrite reduction, sulfate reduction, N2 fixation, chlorophyll synthesis, and a number of
other biosynthetic
and degradative reactions,
including those involved in detoxification of O2
radicals (Geider and La Roche, 1994, abstract).
Because of the multiplicity of functions of iron,
many cellular processes are likely to be affected
simultaneously by iron-limitation. Specific metabolic pathways, however, are known to require
large amounts of iron, including N2 fixation and
NO, reduction (Raven, 1988). Laboratory and
field evidence suggests that these cellular processes
may be restricted in iron-poor environments
(Rueter, 1988; Price et al., 1991, 1994; Timmermans et al., 1994).
While obtaining a better understanding of the
pathways for metabolic use of iron is a central
goal for marine algal physiologists, there are
several areas that are particularly lacking in
information:
(a>How flexible is “iron use efficiency” (i.e.
C:Fe/time) in the heterotrophic
and autotrophic biota and how does it respond to iron
availability?
(b) What are the minimum iron requirements for
growth of organisms, to what extent can iron
catalysts be replaced by other molecules of
similar function, and how are iron requirements regulated?
How does the biochemistry of oceanic plankton with low Fe:C ratios differ from that of
other organisms?
What are the kinetics of physiological and
biochemical responses to changes in iron
availability?
“Use efficiency” is a term employed by plant
physiologists to describe the rate of photosynthesis (or biomass accumulation) per unit of limiting resource (Raven, 1988). In autotrophs, the rate
of carbon dioxide fixation per unit cellular iron, or
the iron use efficiency (IUE), is the product of the
carbon to iron ratio (C:Fe, with units of mol
organic carbon per mol cellular iron) and the specific growth rate (p, with units of dd’): IUE = (C:Fe)
p. It thus incorporates stoichiometric information
on elemental composition with kinetic information
on growth rates into a single term. Like the assimilation number, which is the rate of CO2 fixation per
unit chlorophyll per unit time, the IUE is affected
by intra- and interspecific factors.
Raven (1988, 1990) calculated theoretical, maximum iron-use efficiencies for phytoplankton
by
considering the iron content and specific reaction
rate of iron-containing catalysts in the photosynthetic and respiratory pathways and in N assimilatory pathways. Though the calculations derived
from data on Fe-sufficient organisms which were
not selected for genetic acclimation to low iron
conditions, they demonstrate the high cost of N2
fixation and the moderate cost of NO; assimilation
relative to growth on NH:. Nitrate assimilation
and reduction increases the iron requirement for
growth (relative to NH,$ assimilation) by about
60% and N2 fixation increases this requirement
by lOO-fold. Light also is expected to alter the
iron requirements of phototrophs, increasing it by
as much as 50 times at low photon flux densities of
1 uE rnp2 s-l. Experimental tests using Thalassiosira weissflogii support the predicted increase of Fe
quota under low light (Strzepek and Price, in
prep.).
Photoautotrophs may decrease their Fe requirements and still grow quickly (i.e. increase their
M.L. Wells et aLlMarine Chemistry 48 (1995) 157-182
IUE) by replacing Fe catalysts with those which
contain no metal and by altering metabolic pathways. For example, if ATP for biosynthesis is
generated directly by photophosphorylation
(e.g.
as opposed to CO2 fixation into carbohydrate followed by ATP synthesis during mitochondrial oxidation of carbohydrate) IUE could be increased by
about 25% (Raven, 1988). The maxima could be
further increased if flavodoxin and plastocyanin
substitute for ferredoxin and cytochrome c in the
photosynthetic
electron transfer chain. Raven
(1988) argued for a modest (- 10%) decrease in
Fe requirement with this substitution, however,
Anderson and Erder (this workshop) estimate a
much greater savings (> 80%) using lower Fe:C
ratios. Even given this intraspecific and phenotypic
plasticity,
the discrepancies
between
observed and theoretical iron-use efficiencies of
the phototrophs that have been studied cannot be
resolved (Sunda et al., 1991). Future research must
address the fundamental
issues of iron biochemistry and physiology in a wider range of
oceanic and coastal phytoplankton isolates.
Although the information on C:Fe and IUE of
relevant oceanic phytoplankton clones is limited
(Geider and La Roche, 1994), there is currently
no equivalent information for other members of
the microbial loop. It is anticipated that for a
given growth rate, the iron requirements of heterotrophs will be lower (i.e. C:Fe higher) than those of
autotrophs because they lack iron-rich photosynthetic membranes (Raven, 1988, 1990). However,
bacteria and ciliates have the potential to achieve
specific growth rates that exceed those of the
phytoplankton.
High cellular iron contents (i.e.
low C:Fe) may thus be required to sustain these
high growth rates, even though heterotrophs have
a higher intrinsic IUE.
The metabolic iron requirement of different
phytoplankton
species varies widely. The calculated, minimum iron requirements
for algal
growth (Raven, 1988, 1990) are greater than
measured values by many times, but the organisms
from which these values were calculated were not
iron stressed. Carbon to iron ratios vary from
< 2000 C:Fe in the freshwater Nz-fixing cyanobacterium Anabaena sp. (Hutchins et al., 1991) to
N 500,000 C:Fe in the oceanic diatom Thalassiosira
oceanica (Sunda
171
et al., 1991). With these iron
quotas, the organisms achieve specific growth
rates of N 0.03 d-’ and N 1.O d-' , respectively,
demonstrating the great variability in IUE. Excluding the N2 fixers, which have a very high ironrequirement (Raven, 1988), the range of C:Fe
ratios is still 50-fold. The red tide dinoflagellate,
GJlmnodinium sarzguineum, has a C:Fe ratio of
< 10,000 (Doucette and Harrison, 1991) and in
some coastal cyanobacterial
strains (Brand,
1991a), and in diatoms from coastal upwelling
regimes (Bruland et al., 1991) the C:Fe ratio is
N 20,000. The high iron requirements of coastal
isolates, and their lower IUE compared to oceanic
species, suggests the potential for iron-limitation in
coastal seawaters even considering the substantially higher quantities of total iron in these
waters.
While cellular iron requirements
provide
guidance towards identifying species which have
economized their iron use, iron use efficiencies
have greater ecological relevance because they
quantify potential growth rates per unit iron
quota. But physical constraints which regulate
iron transport into the cell also must be considered. Decreasing cell size is accompanied by
larger surface area to volume ratios and better
diffusional characteristics, so that small cells having low IUE can compete with larger cells displaying higher IUE. For example, three different
oceanic species in culture ranging in size from 1.5
to 6 pm all had similar iron-limited growth rates at
the lowest Fe’ concentration despite having very
different cellular iron requirements (Sunda and
Hunstman, in prep.). This result was achieved by
an inverse relationship between iron transport
kinetics (per unit carbon) and iron use efficiency.
Larger cells, such as T. oceanica, must then evolve
higher use efficiency to overcome the unfavorable
transport kinetics. Thus, cellular iron quotas and
use efficiencies alone are not enough to anticipate
the relative success of different members of the
algal assemblage.
Explanations for the tremendous variation in
C:Fe ratios of phytoplankton are presently largely
speculative as we yet have little information on the
biochemical differences among algae. This speculation forms the basis for seeking indicators of iron
172
M.L.
Wells et aLlMarine
stress in phytoplankton which is discussed below.
Similarly, we have little information on the kinetics
of physiological and biochemical responses to
changes in iron availability. For example, we cannot
define clearly between physiological and biochemical
responses that represent short term acclimation to
iron limitation and more long range genetic “acclimation” patterns to a low iron environment. These
issues will be important for predicting population
responses to fluctuations in iron supply originating
from changes in external iron inputs or short term
variations in iron speciation.
Biochemical, molecular and physiological indicators
of Fe status of plankton communities
The metabolic consequences of iron deprivation
to marine algae and heterotrophic bacteria are
poorly understood. In higher plants, the activity
of iron-containing enzymes has been examined
under conditions of iron deficiency. Catalase
(Leidi et al., 1986) and lipoxygenase (Boyer and
Ploeg, 1986) activities, for example, decrease
under low iron and provide a measure of iron
stress in these organisms. In marine phytoplankton, the ratio of cytochrome f: chlorophyll a
declines in iron-limited cultures of Phaeodactylum
tricornutum and Zsochrysis galbana (Glover, 1977).
Variations in ferredoxin content also have been
used to infer iron-limitation for zooxanthellae on
Davies Reef, Australia (Entsch et al., 1983) and
Trichodesmium
colonies from the Caribbean
(Rueter et al., 1992). In the approaches outlined
above, a decline in enzyme activity or change in
biochemical content is observed in response to
iron-limitation. Iron limitation also appears to alter
the cellular abundance of photosystem I and II,
increase the number of non-functional reaction centers, impair energy transfer efficiency in photosystem
II, and reduce intersystem electron flow (Greene et
al., 1992; Geider et al., 1993; Geider and La Roche,
1994; Green et al., abstract). However, all of the
above indicators may be susceptible to interferences
by factors other than iron-limitation, such as toxicity
or limitation by other elements.
Promising candidate indicators of iron stress and
iron limitation in marine algae include:
. cytochrome
c/plastocyanin
ratios,
Chemistry
48 (1995) 157-182
. ferredoxin/flavodoxin ratios,
. fluorescence characteristics,
- iron transport rates and proteins, and
- immuno-probes, mRNA.
Experimental tests to establish the importance of
iron limitation in the sea have relied largely on
enrichment assays in bottles (Martin et al., 1989,
1991; de Baar et al., 1990; Buma et al., 1991; Coale,
1991; Helbling et al., 1991; Takeda et al., submitted) and most recently on enrichment of a
60 km* patch of surface ocean (Martin et al., 1994).
In these experiments, iron concentrations are artificially increased and the response of the plankton
community is assayed for a variety of physiological
indicators, most of which measure the bulk community response to the iron additions, rather than
the response of individual species or classes of
of
organisms. More recently, size-fractionation
the induced response as well as pigment analyses
have shown that iron additions differentially affect
separate components of the planktonic assemblage
(Buma et al., 1991; DiTullio et al., 1993; Price et al.,
1994; Barber et al., abstract). Improved methods
which instantaneously detect and quantify the
nature and extent of iron limitation of the separate
components of the diverse planktonic assemblage
are needed (i.e. an autecological rather than a
systems approach).
A number of biochemicals may potentially serve
as diagnostic indicators of iron-limitation. One
candidate pathway worthy of investigation is the
cytochrome c/plastocyanin system, whereby iron
limitation results in the substitution of coppercontaining plastocyanin for cytochrome c, which
contains iron (Sandmann and Boger, 1980).
Another promising indicator for heterotrophic
bacteria as well as eukaryotic and prokaryotic
phytoplankton is based on the substitution of the
non-iron containing protein flavodoxin for the
catalytic protein ferredoxin (the terminal electron
donor to NADP in the photosynthetic electron
transport chain) under conditions of iron limitation (e.g. Zumft and Spiller, 1972; Entsch et al.,
1983; Laudenbach et al., 1988; La Roche et al.,
1993).
The power and utility of the “diagnostic indicator” approach is already apparent in ongoing
M.L.
Wells et aLlMarine
studies of ferredoxin and flavodoxin by several
research groups (Geider and La Roche, 1994;
Anderson and Erdner, abstract; La Roche and
Geider, abstract). Although early studies of this
indicator suffered from the need to collect large
quantities of algae, tools now are under development to detect these two target proteins in microgram quantities using HPLC, at nanogram levels
using Western blots of protein extracts, and potentially at picogram levels in individual cells using
immunofluorescence.
These methods potentially
can separate the pelagic community response at
several different ecological levels. Whereas HPLC
can only provide data for the bulk community or
for size fractions of that community, antibody
detection on Western blots can resolve class- or
species-specific responses. Immunocytochemical
techniques have the potential to reveal the physiological status of individual cells; flow cytometric
analyses would then help establish the overall
health of that species population. Attention also
is being directed to molecular characterization of
the two proteins, which can lead to probes for their
genes, or to the mRNA transcripts of those genes,
which could offer similar levels of resolution and
specificity.
Direct application of HPLC and antibody-based
techniques to natural waters has been constrained
by the large amount of water that must be filtered
to provide the biomass for HPLC measurements,
and by the lack of broad cross-reactivity in the
antibodies that have thus far been obtained.
Nevertheless, the concept is sound and soon
should be applicable to natural communities.
Results from laboratory studies already demonstrate the information that can be uniquely provided by the use of diagnostic indicators. For
example, one surprising observation from studies
of laboratory cultures is that expression of the
flavodoxin gene occurs in a wide range of marine
phytoplankton
species, including isolates from
coastal regions where iron has been presumed to
be in high abundance (Anderson and Erdner,
abstract; La Roche and Geider, abstract). The
implication is that iron limitation may be a problem common to many oceanic regimes, not just
those where ambient iron concentrations are very
low (see below).
Chemistry
48 (1995)
157-182
113
One new approach yielding exciting insight into
the effects of iron on natural phytoplankton
populations is fast repetition rate (FRR) fluorometry (Kolber and Falkowski, 1992). The FRR
fluorometer measures the change in the quantum
yield of in vivo chlorophyll fluorescence resulting
from exciting photosystem II (PSII) with a train of
subsaturating flashes. The cumulative stimulation
of PSI1 by these flashes induces a saturation profile,
the amplitude of which is quantitatively proportional to the quantum efficiency of PSII. The
rate of saturation is quantitatively proportional
to the functional photon absorption cross section
of the PSI1 reaction center. In other words, FRR
fluorometry provides an instantaneous measurement of the community-scale efficiency of algal
light harvesting systems which infers a metabolic
status of the phytoplankton
assemblage. Low
photosynthetic efficiency can result from a poor
supply of nutrients, toxic metal (e.g. Cu) effects,
and photo damage of the light harvesting system.
For example, iron limitation leads to a marked
decrease in quantum efficiency and an increase in
absorption cross section in laboratory cultures
(Greene et al., 1992). In the HNLC waters of the
equatorial Pacific, there was a high degree of
inactivation of PSI1 reaction centers and low
photosynthetic energy conversion efficiency in all
phytoplankton
size classes, but after mesoscale
iron enrichment, in-situ photosynthetic efficiency
increased by 70% and the absorption cross
section decreased by a factor of two, consistent
with an alleviation of iron stress (Kolber et al.,
1994). These changes were observed in all size
classes including picoplankton,
implying that
small size may offer no refuge from iron
limitation
in this low iron
environment.
Similar effects of iron enrichment to these
waters also have been found in on-deck
culture experiments (Greene et al., 1994). This
rapid, non-intrusive methodology is certain to
be an important tool for probing the physiological
state
of
natural
phytoplankton
populations.
Its further refinement in conjunction with studies of iron speciation may offer
the best approach in the future for linking algal
physiology and the ambient iron chemistry in
seawater.
174
M.L. Wells et aLlMarine Chemistry 48 (199.5) 1.57-182
The applicability of laboratory studies to natural
plankton communities
Two central issues must be considered when
extrapolating laboratory test results to natural
phytoplankton communities.
(1) How general are the responses measured with
individual organisms and what is the extent of
intraspecific and interspecific variability?
(2) How representative of the indigenous microbiota are the organisms that we presently
study in culture?
Field studies (Martin and Gordon, 1988; de Baar
et al., 1990; Martin et al., 1990b, 1991; Buma et al.,
1991; Helbling et al., 1991; Takeda et al., submitted) have documented how iron can influence
total phytoplankton biomass and productivity, yet
we know that the phytoplankton community comprises a diversity of species with different adaptations and iron requirements. Although we still
cannot culture many phytoplankton, particularly
oceanic species, we do have in culture a number
of clones, both neritic and oceanic, that are dominant in their communities and appear to be reasonably representative.
By contrast, analysis of
ribosomal RNA isolated from bulk water samples
has revealed a previously unrecognized diversity of
bacteria and Arches, of which no representatives
are in culture (Giovannoni et al., 1990; Fuhrman et
al., 1992). Because many of these microbes had
diverged from the primary lineage prior to the development of an oxidizing atmosphere, when iron
became much less available, novel adaptations to
deal with subsequent low iron conditions may have
arisen in the different groups. Thus, extrapolating
from one group to another may be unwise.
Broad differences among phytoplankton in their
adaptations to iron availability have been identified. Laboratory studies indicate that cyanobacteria have a higher cellular iron requirement than
most eukaryotic phytoplankton
(Brand, 1991a)
which is due in part to a higher PSI/PSI1 ratio
(Raven, 1990). Yet to be examined are the iron
growth requirements of the widespread and abundant prochlorophytes. Related to this question of
iron requirements of representative groups of phytoplankton is the question of whether or not we
have even identified all the major groups of
phytoplankton in the ocean. The fact that the two
most numerically abundant groups of phytoplankton, the cyanobacteria and prochlorophytes, have
only recently been discovered does not give us
much confidence. The laboratory observation of
higher iron requirements in cyanobacteria than in
most eukaryotic phytoplankton agrees well with
the global distribution of these two groups with
respect to the relative inputs of iron and major
nutrients to the photic zone (Brand, 1991a). However, this observation has not successfully predicted the outcome of bottle and mesoscale
experimental enrichments. In bottle enrichments,
cyanobacteria populations often decline with iron
additions (DiTullio, abstract) while all phylogenetic groups, including both cyanobacteria and
eukaryotes, increased in the mesoscale iron enrichment experiment in the equatorial Pacific (Barber
et al., 1994). Current efforts to explain the discrepancies between single species laboratory
studies, bottle enrichments in the field, and mesostale enrichments have focused on the complex
interactions between phytoplankton, their grazers
and the microbial loop.
In addition to the importance of studying representative species of the phytoplankton community,
it is important to study representative genotypes.
Genetic variability within populations and genetic
differentiation between populations are well known
and probably occur in all phytoplankton species
(Brand, 1991b). It is therefore important to use
local isolates in any site specific laboratory
studies. An excellent example of this with respect
to trace metals is the study of Jensen et al. (1974) in
which a clone of Skeletonema costatum from a
metal polluted fjord was much more tolerant of
zinc toxicity than a clone of the same species
from a relatively pristine fjord.
While genetic variation can be a complicating
factor, it can also be used to our advantage as an
indicator of past selective pressure. Brand et al.
(1983) used the differences in iron requirements
between neritic and oceanic phytoplankton species
to argue that iron limitation of growth rates must
have been the selective force leading to such
differences.
It is important that iron manipulation experiments with natural population or laboratory
M.L. Wells et al./Marine Chemistry 48 (1995) 157-182
cultures be conducted as near to ambient concentrations as possible. Experiments using metal concentrations lo-100 fold above natural levels may
affect the speciation and reactivity of not only iron
but also that of other bioactive metals, particularly
if iron solubility is exceeded. For oceanic clones,
where little work has been done, the new evidence
of strong iron-binding ligands may complicate culturing studies which employ nutrient-enriched seawater media and synthetic ligands to buffer metal
activity. Development of standardized protocols
for media preparation and growing open ocean
clones should help reduce these uncertainties.
(5) What are the ecological and biogeochemical
implications of changing iron availability in the
oceans?
Of all of the aspects discussed during the workshop, the possible implications of changing iron
availability on the ecology and biogeochemistry
of ocean systems were perhaps the most speculative. Iron will become an important factor only
in those places or times when the supply rates
approach or are below the level of iron demand
by the plankton community. Temporal or spatial
variations in iron supply could influence:
- phytoplankton
. size structure
biomass and production rates,
of the phytoplankton
assem-
blage,
- species composition within size fractions, and
- trophic dynamics and thus export production.
The present focus of iron research in the sea
concentrates on HNLC regions where relatively
low and constant phytoplankton biomass coexists
with high concentrations of the major nutrients.
These HNLC regions, including the subarctic Pacific, the equatorial Pacific and the Southern Ocean,
are characterized by a high input of major nutrients
via upwelling or vertical mixing but a low external
supply of iron. Characteristics of the algal assemblages occurring in these regions are consistent
with iron (and other trace metals) being an important factor. By virtue of their small size, picoplankton (< 2.0 pm) are extremely well adapted to
maximize iron uptake. In fact, Hudson and
115
Morel (1993) argued that transport limitation at
low concentrations of iron largely explain why
HNLC ecosystems are dominated by picoplankton. They apparently flourish because favorable
surface area to volume ratios and diffusion characteristics optimize iron acquisition per unit of biomass, though elevated iron transport rates (Price et
al., 1994) and FRR fluorometry (Greene et al.,
1994; Kolber et al., 1994) suggest that picoplankton in the equatorial Pacific still are experiencing
iron stress. Larger cells do not compete effectively
for the scarce iron resource and are curtailed to low
growth rates.
It also is expected that iron can strongly influence the resultant food web structure of the ecosystem. Picoplankton
are fed upon by small
herbivores which can grow faster than their prey
(Banse, 1992) and thus can quickly respond to any
increase in picoplankton growth rates. This quick
response is argued to maintain biomass at a low
and nearly constant level in HNLC regions
(Banse, 1991; Frost, 1991; Miller et al., 1991;
Banse and English, 1994), though the effects of
light and macronutrients also may play an important role in the Southern Ocean (Dugdale and
Wilkerson, 1990, 1991; Mitchell et al., 1991;
Nelson and Smith, 1991). In other words, phytoplankton populations might be grazer-controlled in
an iron-limited ecosystem (the so-called ecumenical
hypothesis). However, picoplankton biomass levels
still can increase when perturbations in nutrient
inputs are sufficiently large, such as in the mesoscale iron enrichment experiment (Martin et al.,
1994), suggesting that iron may be more important
than grazing in controlling algal biomass in these
waters.
In HNLC systems, temporary increased inputs
of iron (by a series of major storms, volcanic eruptions, etc.) may cause a shift towards an ultraplankton (- 2-5 pm) dominated system (Martin
and Fitzwater, 1988; Martin et al., 1990a,b; de
Baar et al., submitted) or simply increase the production of all size classes of phytoplankton (Kolber
et al., 1994; Martin et al., 1994). The net effect in
both cases should be to increase export production
until the iron resource is returned to pre disturbance levels and the ecosystem moves back to one
sustained primarily by regenerated iron (unless
176
M.L. Wells et al./Marine
another resource first becomes limiting). Such
periodic inputs of iron would serve to maintain
an assemblage of phytoplankton
species whose
iron requirements range from extremely low to
moderate, as appears to be the case in remote
HNLC regions.
In subtropical waters, increases in iron input
could have a similar effect but for entirely different
reasons. Because phytoplankton in these systems
appear to be N-limited, most species should
experience no direct effect from an increased
supply of iron. However, Trichodesmium sp. are
not limited by low concentrations of NO; but are
likely stressed or limited by iron given their
extremely high iron requirements for N2 fixation
(see above). Increased iron inputs thus might
increase
their production
significantly
and
indirectly stimulate overall productivity
and
export production.
Even though iron concentrations in coastal and
shelf regions are substantially higher than in
oceanic surface waters, changing iron availability
could limit certain populations and lead to a shift
in the dominant species within the phytoplankton
assemblage. For example, Doucette and Harrison
(1990) obtained results demonstrating the importance of iron for dinoflagellates and Wells et al.
(1991b) have argued that low iron conditions may
limit the distribution of dinoflagellates in the Gulf
of Maine. The observation that flavodoxin is
expressed in coastal diatom species (Anderson
and Erdner, abstract: La Roche and Geider,
abstract) strongly suggests, from what we know
about the substitution of flavodoxin for ferredoxin, that these cells are biochemically prepared
to deal with iron limitation. It seems unlikely that
coastal phytoplankton would have this adaptive
ability without the need to express it occasionally.
Unlike the stable HNLC regions, iron depletion
in coastal and shelf waters may result from organism growth and the seasonal accumulation of high
biomass levels (e.g. the coastal spring bloom). This
represents a different type of iron limitation than
seen in open ocean systems. Such bloom related
depletion has two important implications, first
that cells which can store excess levels of iron
prior to the bloom (e.g. diatoms) may have a
competitive advantage, and second, that use of
Chemistry 48 (1995) 157-182
iron sparing mechanisms (e.g. flavodoxin for ferredoxin) may be needed to help continue growth
after iron depletion. This pattern of iron depletion
following growth presumably also is prevalent in
other environments where the chlorophyll and
standing stocks vary, such as the edges of river
plumes and along ice margins (de Baar et al., submitted). Similar considerations also should apply
in non-HNLC open ocean environments which
characteristically experience spring time blooms,
such as in the North Atlantic.
3. Summary
The perceptions of ocean chemists and algal
physiologists have shifted enormously over the
past two decades, from attempts to correlate
metal uptake by phytoplankton with bulk measurements of total “dissolved” metals, to recognizing
that metal speciation is the critical factor. This
evolution in thinking began with well-defined,
metal buffered laboratory culture experiments
and the principles learned there have appeared to
work well for predicting the biological effects of
various metals (e.g. Cu, Zn, and Mn) in natural
systems; until iron. It became apparent during the
workshop discussions that two main factors contribute to the vexing difficulty in characterizing the
relationship between iron chemistry and phytoplankton dynamics.
First, the marine chemistry of iron is more complex and less explored than many other bioactive
metals. The central issues that elicited surprise and
chagrin at the workshop were that: (1) there is no
clear consensus about the inorganic speciation of
Fe(II1) in seawater and as a result we still do not
know the inorganic solubility of iron in seawater
with respect to any of the solid iron phases present,
(2) inorganic iron species may in fact be a minor
component of soluble iron chemistry with organic
complexation instead being the dominant form,
and (3) as yet we cannot explain the apparent persistence or speciation of Fe(I1) in surface waters.
The second complicating factor is the diversity of
iron uptake strategies likely employed by phytoplankton. Up until now, we have modeled iron
acquisition by phytoplankton in much the same
M.L. Wells et al./Marine Chemistry 48 (199.5) 157-182
way as for other metals, namely the reaction of
inorganic metal species with cell surface ligand
sites followed by transport into the cell. This
“classical” uptake mechanism has been verified
for iron and well characterized in laboratory culture and modeling studies. We now realize that
while iron uptake typically is correlated with free
hydrated Fe 3+ in constant pH seawater containing
different synthetic chelators, it is the more abundant, kinetically labile hydrolysis species comprising Fe(II1)’ and Fe(I1)’ that actually control uptake
rates. However, very recent findings suggesting that
the bulk of soluble iron may be organically bound
in natural systems means that labile inorganic
species might fall to extremely low levels, perhaps
requiring photochemical or other redox mechanisms to provide a constant supply of metal.
In addition to direct uptake of Fe(III)/, we now
have glimpsed several other tricks marine algae
might use to acquire iron when this resource
becomes scarce. Cells might employ the release
and uptake of siderophores, become siderophore
“pirates” by utilizing the siderophore-Fe
complexes of other species, or reductively dissociate
“generic” organic-iron complexes at the cell surface. There presently is no direct evidence as to
the significance of these alternate iron uptake
strategies in marine waters.
Though a central goal of the workshop was to
better define iron availability to help develop
analytical tools for quantifying this bioavailable
fraction in seawater, this task is not possible
given the foundation of our present knowledge.
Quantification of iron “availability” in natural
waters awaits better characterization of the marine
chemistry of iron in conjunction with exploration
of the uptake strategies used by different members
of the algal community.
An issue raised repeatedly during the discussions
was that while interest in the role of iron has
focused on HNLC regions, there is increasing evidence that iron also may be important in other
oceanic and even neritic domains. At least some
neritic phytoplankton species have the ability to
switch from iron-rich to non-iron containing electron transfer proteins under iron deplete conditions; an adaptive strategy which implies that
these nearshore organisms become limited or
111
stressed by iron availability at certain times. By
influencing
algal species composition,
iron
chemistry and supply may interact with other biological factors in regulating the food web structure
of many marine ecosystems.
What emerged then from this workshop was a
shift in the paradigm of iron biogeochemistry in
seawater, from one in which iron/phytoplankton
interactions behave similarly as for other bioactive
metals to one in which iron stands as truly unique.
It is certain that unraveling the rich complexity of
interaction between iron chemistry and the biota
will need the frank cross-disciplinary discussion
and collaboration that characterized this workshop.
Acknowledgements
We wish to thank the staff at the Bermuda Biological Station for Research for their gracious and
friendly help before, during and after the workshop. We thank the participants
for their
unbridled
enthusiasm
and openness
which
repeatedly led to exciting speculation and new conceptual fantasies; an interaction unusual in the
present difficult funding situation. We express our
appreciation to Eden Rue for her additional
assistance before and during the workshop. We
thank the working group leaders - Kenneth
Coale, Whitney King, George Luther, Frank
Millero, and Charlie Trick - who helped guide the
working group discussions towards formulating a
cohesive list of issues. We also thank Bill Sunda,
Dorothy Swift, Richard Greene and Hein de Baar
for their detailed comments and suggestions on
earlier versions of this manuscript as well as other
participants who provided their views. We give
particular thanks to Neil Andersen for spearheading the search for funding to make this workshop
possible. This workshop was sponsored by NSF in
cooperation with ONR, NASA, NOAA, and DOE.
Appendix 1. List of abstract titles
Alison Butler - Mechanisms of iron acquisition by marine microorganisms: bacterial siderophores
Donald M. Anderson and Deana L. Erdner - Molecular probes
for iron nutritional
status in marine phytoplankton
178
M.L. Wells et al.lMarine Chemistry 48 (1995) 157-182
Richard Barber and the Iron Ex group - The in situ phytoplankton
response to natural and experimental iron enrichment
Larry E. Brand - Simultaneous nutrient limitation by iron, zinc
and manganese in marine phytoplankton
Kenneth Coale and the Iron Ex group - Mesoscale iron
enrichment and Galapagos plume studies in the equatorial
Pacific
Alvin L. Crumbliss - Chemical models related to siderophore
mediated iron bioavailability
Hein J.W. de Baar, Maria A. van Leeuwe, Renate Scharek,
Jeroen T.M. de Jong, Klaas R. Timmermans, Bettina M.
Liischer and Rob F. Nolting - Iron and plankton in the
Antarctic Circumpolar Current
Giacome R. DiTullio - Importance of Fe-availability on sizefractionated 14C :59Fe uptake ratios and phytoplankton
ecology in the tropical Pacific ocean
Martha Gledhill and Constant M.G. van den Berg - The
speciation of iron in the North Sea measured by cathodic
stripping voltammetry
Richard M. Greene, Zbigniew Kolber and Paul Falkowski Photosynthetic
energy conversion efficiency and iron
limitation in the equatorial Pacific
David A. Hutchins - Bioavailability of biological iron in high
and low iron environments
Margo G. Haygood - Iron physiology of marine bacteria
Robert Hudson and Kenneth W. Bruland - Iron transport
mechanisms in marine microorganisms: chemical and
physical constraints
Kenneth S. Johnson and the MLML group - Iron photochemistry and analytical chemistry
Dana R. Kester - Marine redox cycle of iron
Julie La Roche and Richard J. Geider - Evaluating the use of
flavodoxin as a molecular marker of phytoplankton iron
status
D. Whitney King, Robb A. Aldrich and Heather A. Lounsbury Investigation of the redox dynamics of iron in aquatic
systems
Katsuhiko Matsunaga, Kenshi Kuma, Yoshihiro Suzuki and I.
Kudo - Bioavailable iron in submicron colloids
Chris I. Measures, J. Juan and J.A. Resing - Determination of
iron in seawater by flow injection analysis using in-line
preconcentration and spectophotometric detection
Francois M.M. Morel - Cycling and availability of iron in the
equatorial Pacific
Rodney T. Powell and William M. Landing - Colloidal iron:
oceanic and estuarine size distributions and reactivity
Neil M. Price and M.T. Maldonado - Iron acquisition by marine
diatoms consumming different nitrogen substrates
Eden L. Rue and Kenneth W. Bruland - Complexation of
iron(II1) by natural organic ligands in the central North
Pacific as determined
by competitive equilibration/
adsorptive cathodic stripping votammetry
John Rueter - Iron nutrition for two strains of oceanic
cyanobacteria: the role of aeolian dust
Barbara Sulzberger - Kinetics of photoreductive dissolution of
colloidal iron(II1): its dependence on the type of colloidal
iron(II1)
William G. Sunda and Susan A. Huntsman - Iron uptake
and growth limitation in oceanic and coastal phytoplankton
Shigenobu Takeda - Iron enrichment experiments in the
equatorial Pacific: availability of added iron to phytoplankton
Steven W. Taylor, George W. Luther III, J. Herbert Waite and
Brent Lewis - Polarographic and spectrophotometric
investigation of iron(II1) complexation to 3,4-dihydroxyphenylalanine- containing peptides and proteins from
Mytilus edulis, and I-nitroso-2-naphthol
Charles G. Trick - Physiological changes in the marine
cyanobacterium Synechococcus sp. PCC7002 exposed to
low ferric ion levels
Constant van den Berg - Determination of organic complexation of iron in the western Mediterranean
Bettina M. Voelker and D.L. Sedlak - Iron reduction by photoproduced superoxide in seawater
T. David Waite, R. Szymczak and Q. Espey - Recent observations of photochemically mediated iron transformations in
seawater
Jingfeng Wu and George W. Luther III - Size- fractionated iron
concentrations in the water column of the Northwest
Atlantic ocean
Guoshun Zhuang, Zhen Yi and Gordon T. Wallace - An
improved HPLC method for the determination of Fe(H) in
aerosols, rainwater and seawater
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