FEMS Microbiology Ecology 29 (1999) 1^11
MiniReview
Marine bacteria and biogeochemical cycling of iron in the oceans
Philippe D. Tortell a; *, Maria T. Maldonado b , Julie Granger b , Neil M. Price
a
b
Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544, USA
b
Department of Biology, McGill University, Montreal, Que. H3A 1B1, Canada
Received 18 June 1998; received in revised form 2 December 1998 ; accepted 8 December 1998
Abstract
Prokaryotic microbes play a critical role in oceanic Fe cycling. They contain most of the biogenic Fe in offshore waters and
are responsible for a large portion of the Fe uptake by the plankton community. In the subarctic North Pacific, surface
populations of heterotrophic species assimilate more than 50% of the dissolved Fe and thus compete directly with
phytoplankton for this limiting resource. In oligotrophic tropical and subtropical waters, photosynthetic bacteria become more
important in Fe cycling as the number of unicellular cyanobacteria increases and the nitrogen-fixing Trichodesmium, which
contains most of the biogenic Fe in the mixed layer, becomes abundant. Like their terrestrial counterparts, heterotrophic and
phototrophic marine bacteria produce Fe-binding siderophores that are involved in Fe acquisition. Evidence exists that
bacteria may modify Fe chemistry in the sea through the production of these ligands and thereby play a significant role in
regulating production of eukaryotic phytoplankton. z 1999 Federation of European Microbiological Societies. Published
by Elsevier Science B.V. All rights reserved.
Keywords : Iron chemistry; Marine bacterium ; Iron cycling
1. Introduction
Over the past decade and a half, it has become
apparent that trace metals can a¡ect oceanic primary
productivity as limiting nutrients and toxic inhibitors
[1]. As a result, the biogeochemical cycling of trace
metals in the oceans has become a subject of great
interest and research. Of all trace metals, Fe has thus
far received most attention. Shipboard bioassay experiments [2^4] as well as two mesoscale in situ Fe
fertilizations [5,6] have demonstrated that low Fe
availability constrains phytoplankton growth in several large open ocean regions where primary production is low despite high concentrations of major nu* Corresponding author.
trients (nitrate, phosphate, silicate). Iron limitation
in these regions decreases the e¤ciency of the `biological carbon pump' through which CO2 is consumed in surface waters and transported as sinking
particulate organic carbon to the deep sea. Given the
importance of this process in controlling atmospheric
CO2 [7], Fe biogeochemistry in the oceans has important implications for global carbon cycling and
climate studies.
At present, our understanding of Fe biogeochemistry is incomplete. Many of the processes that are
thought to control dissolved and particulate Fe concentrations in the oceans are known from laboratory
studies that mimic natural conditions with varying
degrees of success. One fact that is clear, however,
is that biological processes are of great importance.
0168-6496 / 99 / $20.00 ß 1999 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 6 4 9 6 ( 9 8 ) 0 0 1 1 3 - 5
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P.D. Tortell et al. / FEMS Microbiology Ecology 29 (1999) 1^11
Most investigations of Fe-biota interactions to date
have focused on eukaryotic phytoplankton, which
account for the bulk of CO2 export. Relatively little
attention has been given to the role of phototrophic
and heterotrophic bacteria in Fe cycling. This is in
marked contrast to studies of ocean carbon and nitrogen dynamics where these organisms have been
shown to play a central role [8,9].
In this review, we discuss the biogeochemistry of
Fe in the oceans with speci¢c emphasis on the role of
prokaryotes. In particular, we highlight several
mechanisms through which photosynthetic cyanobacteria and heterotrophic bacteria may a¡ect Fe
bioavailability and cycling. Despite the recently discovered abundance of Archaea in the oceans [10,11]
we shall not explicitly consider these organisms for
lack of relevant physiological data. Given the tremendous advances in aquatic microbiology and rapid development of new molecular techniques, it may
soon be possible to examine bacterial-Fe interactions
across the broad taxonomic and biochemical diversity of marine prokaryotes. Our discussion points to
several important questions that will need to be addressed by such future research.
2. Biogeochemistry of dissolved iron in the sea
Although Fe is the fourth most abundant element
in the earth's crust, its concentration in most ocean
waters is vanishingly low. Until recently, ubiquitous
contamination of samples during collection and
analysis prevented accurate determination of true
dissolved Fe levels. The advent of ultra-trace metal
clean techniques [12], however, facilitated accurate
and systematic measurements of Fe in the world's
oceans [13] and provided new insight into the role
of marine biota in Fe cycling. Below, we brie£y summarize the most salient features of dissolved Fe distributions and chemistry, and discuss recent evidence
for bacterial control of Fe speciation and solubility.
Like nearly all trace metals, dissolved Fe concentrations show strong horizontal and vertical gradients, decreasing by as much as several hundredfold from coastal to o¡shore waters and increasing
signi¢cantly with depth in the upper 500 meters [13].
Coastal waters receive large inputs of Fe from rivers
and anoxic sediments [13] whereas o¡shore regions
Fig. 1. Vertical distribution of dissolved iron and of two classes
of iron-binding ligands, L1 and L2, in the central North Paci¢c.
The data are replotted from [22].
rely mainly on atmospheric dust deposition and/or
upwelling of deep waters as sources of Fe [14,15].
We shall focus our discussion on oceanic (o¡shore)
regions that account for s 90% of marine primary
productivity and are characterized by persistently
low Fe availability. Such low Fe levels demonstrably
limit primary productivity in high-nutrient oceanic
regions [6] and may impose a signi¢cant stress on
phytoplankton in oligotrophic subtropical gyres. Recent work has demonstrated that coastal upwelling
regimes can also be Fe-de¢cient [16].
Dissolved Fe concentrations in o¡shore waters of
the Paci¢c, Atlantic, and Southern Oceans average
0.07 þ 0.04 nmol kg31 at the surface ( 6 200 m) and
0.76 þ 0.25 nmol kg31 at depth ( s 500 m) [13]. Such
surface depletion is typical of `bioactive' micro- and
major nutrients and results from biological uptake in
the upper water column followed by regeneration
during oxidation of organic detrital matter as it sinks
to the sea £oor. Heterotrophic bacteria in surface
and subsurface layers of the oceans mediate this latter process [9].
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P.D. Tortell et al. / FEMS Microbiology Ecology 29 (1999) 1^11
Despite its nutrient-type pro¢le, the oceanic distribution of Fe di¡ers signi¢cantly from that of other
essential trace metals such as Zn [17]. Most of these
metals are highly soluble in seawater, whereas Fe is
present to a large extent in particulate phases (silicates, aluminosilicates and oxyhydroxides) (see [18]).
The `dissolved' Fe fraction (operationally de¢ned as
6 0.4 Wm) consists largely of colloidal hydrolysis species such as Fe(OH)3 which are rapidly scavenged
out of the water column by coagulation and adsorption onto sinking particulate material. As a result,
the dissolved Fe pool in deep waters turns over rapidly with an ocean residence time on the order of 100
years [19].
Unlike other particle reactive metals (e.g. Al, Mn,
Pb) whose dissolved concentrations decrease with
depth due to removal by particulate adsorption,
deep water ( s 500 m) dissolved Fe concentrations
appear to be relatively invariant (Fig. 1). This anomalous behavior is at least partially attributable to the
presence of high a¤nity organic ligands which speci¢cally complex Fe(III) and increase its apparent
solubility by enhancing colloid dissolution and reducing scavenging rates [13]. Indeed, UV oxidation
of the organic ligands in open ocean water samples,
has been shown to decrease the relative proportion
of truly dissolved ( 6 0.025 Wm), i.e. non-colloidal,
Fe [20]. Several groups have examined organic Fe
complexation using highly sensitive electrochemical
techniques [21^23]. Their results suggest that greater
than 99.9% of dissolved Fe is bound by ligands
which appear to fall into two classes. The stronger
ligand class (L1) is present in surface waters at concentrations of 0.4^1.0 nM and has an inorganic Fe
conditional stability constant1 of approximately
1013 M31 while the L2 class is found throughout
the water column at approximately 1.5 nM with an
inorganic Fe conditional stability constant of about
3U1011 M31 (Fig. 1). As a result of this organic
complexation, inorganic Fe concentrations are less
than 0.1 pM (i.e. 0.0001 nM) in surface waters and
1
The conditional stability constant (KP) is de¢ned as: [FeL]/
([FeP]W[LP]); where [FeL] is the concentration of organically complexed Fe, and [FeP] and [LP] are the concentrations of unbound Fe
and ligand respectively. KP expresses the relative thermodynamic
stability of organic Fe complexes at speci¢ed values of pH and
ionic strength (8.1 and 0.7 M in seawater).
3
show more pronounced vertical gradients than total
dissolved Fe [22]. As discussed below, this has important implications for biological Fe acquisition.
The sources and chemical structures of the Fe
complexing agents are presently unknown although
it is clear that these organic compounds must be of
biogenic origin. A prominent hypothesis is that they
are bacterial siderophores ^ highly speci¢c Fe-binding compounds utilized for Fe acquisition [24]. Several lines of indirect evidence are consistent with this
hypothesis. Laboratory cultures of oceanic heterotrophic and phototrophic bacteria produce siderophores when grown under low Fe levels typical of
oceanic environments [25,26]. In the ¢eld, evidence
of in situ production of hydroxamate containing
siderophores has been obtained from coastal cyanobacterial mats [27]. Furthermore, the conditional
stability constant of the strong organic ligand class
(L1) in oceanic waters is similar to that of the well
studied siderophore desferrioxamine B (1016:5 M31 )
[22]. Although the siderophores of a number of cultured marine strains have been isolated and studied
(e.g. [28,29]), the low concentration of Fe-binding
ligands in seawater precludes the precise structural
characterization necessary to trace their biotic origins. Identifying the biological sources and chemical
structures of organic Fe ligands in seawater will be
one of the great challenges facing oceanographers
over the next decade.
3. Biological Fe acquisition
If, indeed, the organic Fe chelators in seawater are
predominantly of bacterial origin, prokaryotes may
be largely controlling the availability of dissolved Fe
to eukaryotic phytoplankton through complexation
and assimilation. Laboratory studies indicate that
certain strains of heterotrophic marine bacteria utilize Fe bound to siderophores, including those that
do not produce them [30]. At least some marine bacteria may thus rely on Fe-siderophore complexes as
the sole Fe source in situ. Measurements of the
chemical speciation of Fe during the second Equatorial Paci¢c Fe enrichment experiment [6] showed a
rapid ( 6 1 day), four-fold increase in L1 concentration that resulted in the complexation of nearly all
the added Fe [31]. As the complexation preceded the
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P.D. Tortell et al. / FEMS Microbiology Ecology 29 (1999) 1^11
increase in phytoplankton biomass it may have in£uenced the community response to Fe enrichment
by a¡ecting Fe bioavailability.
The chemical speciation of Fe has been shown to
determine the extent to which it can be taken up by
phytoplankton [32]. While early work suggested that
phytoplankton access only inorganic forms of Fe, we
have recently learned that diatoms are able to utilize
Fe bound to a number of organic chelators including
siderophores via a cell surface reductase mechanism
[33]. Evidence of Fe-siderophore use by natural assemblages of plankton in the subarctic Paci¢c has
also been obtained [34]. Both autotrophic and heterotrophic plankton are apparently able to take up Fe
bound to desferrioxamine B and E, and large phytoplankton reduce the Fe in the chelates extracellularly. The ability of indigenous plankton to utilize
Fe complexed to siderophores provides indirect evidence that these bacterial compounds may be important in Fe cycling in situ. Very recent work [35] indicates that marine photosynthetic £agellates may
acquire Fe by ingesting bacteria which, as discussed
below, constitute a large pool of particulate Fe in
open ocean waters.
4. Biogenic iron in oceanic surface waters
The majority of Fe in seawater resides in particulate phases whose biogeochemical cycling is as
poorly understood as that of the dissolved species.
Total particulate Fe ( s 0.4 Wm) shows high spatial
variability and is only weakly correlated with dissolved Fe levels [13]. Understanding the dynamics
of the particulate Fe reservoir requires information
on its partitioning between lithogenic (aluminosilicate), detrital (non-living organic), and biogenic (living) pools. Recent data indicate that bacteria comprise a large part of the biogenic Fe in open ocean
waters and play a critical role in Fe cycling.
Quantifying the size of biogenic Fe pools requires
estimates of biomass (mol C) and Fe requirements
(commonly referred to as Fe quota, expressed as mol
Fe cell31 or Wmol Fe mol C31 ) of all producers and
consumers in the ecosystem. While biomass data are
available from several long-term oceanographic stations, relatively few measurements of Fe:C ratios
have been reported for representative open ocean
plankton groups. Nonetheless, it appears that marine
prokaryotes (cyanobacteria and heterotrophic bacteria) have a signi¢cantly higher Fe content than eukaryotic phytoplankton. Under Fe-de¢cient culture
conditions, oceanic diatoms (eukaryotic algae) have
an average Fe quota of 3.0 þ 1.5 Wmol Fe mol C31
[36] while open ocean heterotrophic and photosynthetic bacteria have Fe:C ratios of approximately
7.5 þ 1.7 (¢ve isolates) and 19 (one isolate), respectively [37,38]. Some of these laboratory data have
been substantiated by ¢eld studies at Station Papa
in the subarctic North Paci¢c Ocean, an o¡shore,
Fe-limited region [2]. At this station, Fe:C ratios of
eukaryotic phytoplankton and heterotrophic bacteria
are 3.7 þ 2.3 and 6.1 þ 2.5 Wmol Fe mol C31 , respectively [34,37], values similar to those measured in Felimited cultures. To our knowledge, no ¢eld measurements of Fe:C ratios of coccoid cyanobacteria
have yet been reported.
The high Fe quotas of marine bacteria, although
possibly surprising to oceanographers, are not unexpected based on the biochemistry and physiology of
these organisms. Both phototrophic and heterotrophic prokaryotes require large quantities of Fe as a
redox catalyst in their respective photosynthetic and
respiratory electron transport chains. Indeed, simple
computation of the Fe content of a heterotrophic
bacterium, derived from the concentration of Fe-requiring catalysts, indicates that the majority of the
cellular Fe is compartmentalized in the respiratory
electron chain (Table 1). This conclusion is supported by empirical data from Corynebacterium diphtheriae [39]. More data are needed to determine
whether the Fe quotas of photosynthetic marine bacteria are truly higher than those of their heterotrophic counterparts.
High bacterial Fe:C ratios change our conceptual
models of biological Fe cycling in the open ocean.
Tortell et al. [37] constructed a preliminary biogenic
Fe budget for Station Papa by summing the Fe content (biomass multiplied by Fe quota) of eukaryotic
phytoplankton, cyanobacteria and heterotrophic
bacteria. Their results suggested that the prokaryotes
constitute a striking 80% of the biogenic Fe in this
system with heterotrophic bacteria alone accounting
for half of this Fe pool. We have recalculated the
Station Papa biogenic Fe budget (Table 2) using 3year average plankton Fe:C ratios recently deter-
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P.D. Tortell et al. / FEMS Microbiology Ecology 29 (1999) 1^11
5
Table 1
Theoretical Fe requirement of Escherichia coli growing aerobically in Fe-replete medium containing yeast extract as a C substrate
Enzyme complex
[Complex]a
(Wmol g protein31 )
Fe contentb
(atom/complex)
NADH-Q reductase
0.206
Succinate-Q dehydrogenase
0.335
Cytochrome b1
Cytochrome oxidase
Aconitase
Superoxide dismutase
Catalase
Total
Total measurede
0.335
0.124
0.018
0.530
0.011
^
2U2Fe-2S
6U4Fe-4S
1U2Fe-2S
1U3Fe-4S
1U4Fe-4S
1
2
1U4Fe-4S
1
1
^
[Fe]c (mol
Fe/cellU10320 )
Fe quotad (Wmol
Fe mol C31 )
89.4
46.7
5.19
3.84
1.15
8.22
0.17
155
216
43
61
Note that 94% of the cellular Fe is found in the respiratory chain, associated with NADH-Q reductase, succinate-Q reductase, cytochrome b1 ,
and cytochrome oxidase complexes.
a
Concentrations Fe containing catalysts were obtained from published values and from measurements of catalytic activity and turnover.
NADH-Q reductase was determined from the di¡erence between total £avin (0.541 nmol mg protein31 [63]) minus £avin associated with
succinate dehydrogenase, where one £avin equivalent exists per NADH-Q or succinate dehydrogenase equivalent. Succinate dehydrogenase
was assumed to be equimolar with cytochrome b1 . Cytochrome b1 and cytochrome oxidase were obtained from [63]. Aconitase concentration
was determined from enzyme activity of E. coli (63.5 nmol cis-aconitate min31 mg protein31 [64]), turnover of puri¢ed enzyme in yeast (50
Wmol cis-aconitate min31 ), and molecular weight (68 500 g mol31 [65]). Fe-SOD concentration was determined from enzyme activity for E.
coli (5.6 U mg protein31 [66]), turnover of puri¢ed bovine Cu/Zn-SOD (3300 U mg protein31 [67]), and molecular mass of Fe-SOD (40 000 g
mol31 ) and Cu/Zn-SOD (32 000 g mol31 ) [68]. Concentration of catalase was obtained from Corynebacterium diphtheridae [39].
b
Includes heme and non-heme (Fe-S clusters) Fe.
c
Iron content normalized per cell assuming 1.55U10313 g protein cell31 .
d
Iron content normalized per cellular C assuming 0.17 pg C Wm33 [69] and a cell volume of 2.5 Wm3 [70].
e
[70].
mined for this oceanic region [34]. The result is very
similar to that obtained previously [37]. The total
biogenic Fe that we calculated (V17 pM) falls well
below the 560 pM particulate Fe previously reported
for the subarctic Paci¢c Ocean [40] suggesting that
lithogenic and/or detrital Fe pools are large and/or
the biogenic Fe pool is signi¢cantly underestimated.
Fig. 2 illustrates our current understanding of Fe
cycling among biogenic pools in these waters.
Recently, Price and Morel [18] expanded the Station Papa biological Fe budget to include metazoan
and protozoan grazers using Fe:C ratios [41] and
biomass estimates [42,43]. Addition of the Fe contained in these organisms increases the calculated
Table 2
Iron content and steady-state Fe assimilation rates of autotrophic and heterotrophic plankton at Station Papa in the subarctic Paci¢c
Ocean (after [37])
Plankton group
Biomass
(Wmol C l31 )
Fe quotaa
(Wmol Fe mol C31 )
Biogenic Fe
(pmol l31 )
Turnover rate
(day31 )
Fe uptake rate
(fmol Fe l31 h31 )
Eukaryotic phytoplankton
Cyanobacteria
Heterotrophic bacteria
Total
1.41 þ 0.49
0.24 þ 0.16
1.18 þ 0.37
2.83
3.7 þ 2.3
19
6.05 þ 2.5
5.22
4.56
7.13
16.9
0.25
0.2
0.06
54
38
18
110
a
Eukaryotic phytoplankton and heterotrophic bacteria Fe quotas measured at Station Papa in the subarctic Paci¢c Ocean during three
consecutive years (average þ S.D. [34]). Cyanobacteria Fe quotas are laboratory measurements [38].
FEMSEC 997 21-4-99
Fig. 2. Biological Fe cycle in the upper and deep ocean in the open subarctic Paci¢c Ocean. Circles represent the relative biogenic iron pools in the upper ocean derived from
¢eld measurements of Fe:C ratios of plankton and annual averages of their C standing stocks, as reported in Table 1. Arrows indicate the £ow among Fe pools, including
iron uptake by heterotrophic bacteria, phototrophic cyanobacteria and eukaryotic phytoplankton. Iron inputs to the surface ocean are restricted to aeolian deposition [14] and
deep water upwelling [15]. Loss from the surface is mediated by sinking particles. Note that protozoa and metazoa, which contribute to primary production and graze on
smaller plankton, are not included in this model. They likely represent a signi¢cant biogenic iron pool and contribute to the sinking particle £ux and surface remineralization
of Fe [18].
6
P.D. Tortell et al. / FEMS Microbiology Ecology 29 (1999) 1^11
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P.D. Tortell et al. / FEMS Microbiology Ecology 29 (1999) 1^11
7
Table 3
Biogenic Fe budget for surface waters of the northern Sargasso Sea ( 6 200 m)
Plankton group
Biomassa (Wmol C l31 )
Fe quotab (Wmol Fe mol C31 )
Biogenic Fe (pmol l31 )
Eukaryotic phytoplankton
Cyanobacteria
Prochlorococcus
Heterotrophic bacteria
Heterotrophic protozoa
Total
Trichodesmium
Total
0.47 þ 0.09
0.16 þ 0.48
0.39
0.57 þ 0.12
0.31 þ 0.01
1.90
0.12 þ 0.1 colonies l31
3.0
7.5^19
7.5^19
7.5
12
1.41
1.2^3.04
2.92^7.41
4.27
3.72
13.52^19.85
27.6
41.12^47.45
0.23 nmol colony31
a
Data are averages of spring and fall data as reported [46,47]. Biomass of Trichodesmium is reported as abundance in colonies l31 [51].
Data for eukaryotic phytoplankton and heterotrophic bacteria are from laboratory measurements as in Tortell et al. [37]. Iron quotas for
protozoa are taken from Chase and Price [41]. For the photosynthetic prokaryotes, we have assumed upper and lower limits for Fe:C ratios
corresponding to laboratory measurements of cyanobacteria and heterotrophic bacteria. Iron content of Trichodesmium is reported per colony
[53].
b
biogenic Fe to V38 pM with bacteria accounting for
40% of the total.
The biogenic Fe budget of the subarctic North
Paci¢c may not be representative of tropical and
subtropical oceanic ecosystems where photosynthetic
prokaryotes dominate phytoplankton biomass [44]
and primary productivity [45]. In these waters, bacteria may be even more important in Fe cycling. To
test this idea, we have constructed a preliminary biological Fe budget for the northern Sargasso Sea (Table 3) using published biomass data [46,47]. and laboratory Fe quota measurements (which include a
number of Sargasso Sea isolates [37]). A salient feature of this open ocean region is the abundance of
Prochlorococcus [48,49] that are not found at Station
Papa. Because no Fe quotas have been reported for
these organisms and the cyanobacterial estimates are
based on a single Paci¢c Ocean strain we have chosen upper and lower limits of 19 and 7.5 Wmol Fe mol
C31 for photosynthetic prokaryotes. (These values
represent Fe:C ratios of cyanobacteria and heterotrophic bacteria measured in the laboratory.) If we
include in the budget only unicellular organisms that
assimilate Fe directly from solution, prokaryotes account for 86^91% of the biogenic Fe depending on
our choice of Fe:C ratios (Table 3). Considering
protozoan grazers in the model reduces the contribution of prokaryotes to 62^74% and brings the total
biological Fe to ca. 20 pM. This value is similar to
the biogenic Fe calculated for the subarctic Paci¢c
yet substantially less than the 750 pM particulate Fe
measured in the northwest Atlantic [50]. Despite the
higher abundance of photosynthetic prokaryotes in
the Sargasso Sea, heterotrophic bacteria appear to
contribute substantially to the biological Fe pool
constituting between ca. 30^50% of the Fe stocks
in producers (Table 3).
The above calculation does not include photosynthetic Trichodesmium colonies that are abundant in
tropical waters [51,52] and have an elevated Fe demand associated with N2 ¢xation [53]. Such high Fe
requirements may possibly limit the abundance of
these organisms and the extent of N2 ¢xation in
the sea [53]. As shown in Table 3, it appears that
Trichodesmium could potentially constitute the majority of biological Fe in subtropical gyres containing
as much Fe as all other organisms combined. The
means by which they acquire this Fe is uncertain
although some novel mechanisms such as utilization
of colloidal Fe have been proposed [53].
In addition to contributing substantially to standing stocks of biogenic Fe, prokaryotes appear to
assimilate a large fraction of the dissolved Fe.
Steady-state Fe uptake rates calculated from the total Fe in each biological pool and published estimates of mean turnover times suggest that heterotrophic bacteria and cyanobacteria account for
approximately 20 and 30% of total community uptake respectively at Station Papa [37] (Table 2, Fig.
2). These calculations are supported by direct measurements of size-fractionated 55 Fe uptake rates
which show that 55 þ 20% of Fe assimilation at this
station is due to prokaryotes (0.2^1.0 Wm) [34]. Normalized to carbon biomass, bacterial Fe uptake rates
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8
P.D. Tortell et al. / FEMS Microbiology Ecology 29 (1999) 1^11
are signi¢cantly faster, by 1.6 times, than those of
eukaryotic phytoplankton, presumably re£ecting
their high Fe requirements for growth [37].
Spatial and temporal variability in biomass,
growth rates, and Fe quotas of various plankton
groups will undoubtedly in£uence their contributions
to biogenic Fe pools and assimilation rates. Maldonado and Price [34] found that the relative contribution of bacteria and phytoplankton to community Fe
uptake in the subarctic Paci¢c was signi¢cantly
(P 6 0.001) correlated with their biomass ratios, but
independent of their production rates. In a multiyear survey of ¢ve stations, they showed that bacteria accounted for an average of 58 þ 23% of community Fe uptake in this ocean region. Over larger
spatial scales in bodies of water di¡ering greatly in
trophic status, Fe assimilation by bacteria appears to
vary widely and correlate poorly to bacterial production or biomass [54]. Much more data will be needed
to quantify and understand the patterns of biological
Fe uptake.
5. Iron limitation of bacteria in the sea
While a number of studies have examined the effects of Fe de¢ciency on phytoplankton growth in
coastal and oceanic waters, very little is known
about the potential for Fe limitation of bacterial
production. The high Fe demand of prokaryotes
and the observation that Fe:C ratios of Fe-limited
cultures are very similar to those in the ¢eld suggest
that bacteria may su¡er Fe de¢ciency in situ [37].
Indeed, Behrenfeld et al. [55]. have shown that the
indigenous populations of photoautotrophs composed primarily of Synechococcus and Prochlorococcus are Fe-stressed in the equatorial Paci¢c Ocean,
even though the biomass of these organisms does not
appear to increase greatly in response to Fe enrichments [6,56]. E¤cient grazing by rapidly growing
protozoans seems to be at least partly responsible
for this apparent paradox [57]. Despite high grazing
pressure, the abundance of heterotrophic bacteria
has been shown to increase in Fe-amended Equatorial Paci¢c samples [57]. This e¡ect, however, was
attributed to elevated dissolved organic matter
(DOM) levels associated with the stimulation of
large phytoplankton growth rather than Fe limita-
tion per se. By comparison, Fe enrichment of pre¢ltered Southern Ocean water (free of phytoplankton
and grazers) signi¢cantly stimulated the growth of
heterotrophic bacterial populations suggesting that
they are truly Fe-de¢cient [58]. It is important to
note that photosynthetic and heterotrophic prokaryotes have unique requirements for light and DOM
respectively whose availability can partially determine the extent to which they may be Fe-limited
(e.g. [59]). Future ¢eld studies will need to consider
these interactions.
Tortell et al. [37]. examined the physiological effects of Fe limitation on open ocean heterotrophic
bacteria and the relationship between Fe and C metabolism in these organisms. Under Fe-de¢cient culture conditions, respiratory electron transport activity was reduced resulting in a signi¢cant decrease in
carbon growth e¤ciencies and an e¡ective co-limitation by Fe and C. Recent ¢eld studies examining the
responses of heterotrophic bacteria to Fe and C additions have provided some preliminary support of
this co-limitation hypothesis ([54]). It appears, therefore, that Fe availability may a¡ect the pathways of
carbon metabolism in heterotrophic bacteria and in£uence the relative importance of the microbial loop
as a `carbon sink' or `carbon link' (sensu Azam et al.
[8])
6. Conclusions and future prospects
It is now becoming apparent that marine bacteria
play a critical role in the biogeochemical cycling of
Fe in the oceans. By virtue of their high, Fe-rich
biomass in o¡shore waters these organisms take up
and sequester large quantities of Fe thereby competing directly with phytoplankton for this potentially
limiting resource. This competition may be mediated
in part through the production of siderophores by
which bacteria may be largely controlling Fe speciation and solubility in seawater.
Our current understanding of Fe-bacterial interactions is based on a very small amount of data. We
need much more information on the Fe quotas of a
variety of bacteria (and phytoplankton) in the laboratory and in the ¢eld as well as characterization of
the Fe-binding ligands. This should provide insight
into how bacteria satisfy their nutritional Fe require-
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P.D. Tortell et al. / FEMS Microbiology Ecology 29 (1999) 1^11
ments and elucidate the competitive interactions
among di¡erent plankton groups. Future studies
need to examine the extent to which bacteria are
Fe-limited in situ. The use of physiological rate
measurements [37] and molecular probes [60] will
be critical in addressing this question.
While we have focused our attention on Fe in this
review, we believe that bacteria are likely to be important in the oceanic cycling of other trace metals
as well. Most bioactive metals thus far examined are
largely complexed by strong organic ligands of unknown origin [1]. Cyanobacteria have already been
implicated as producers of Cu-speci¢c ligands [61].
Recent work suggests that limitation by trace metals
other than Fe may possibly constrain phytoplankton
growth in the sea [62]. The extent to which this may
be true of bacteria remains to be discovered.
Acknowledgments
Franc°ois Morel, Phoebe Lam, Klaus Keller and
Anne Krapiel provided helpful comments. Funding
for this work was provided by grants from the Natural Sciences and Engineering Research Council of
Canada.
References
[1] Bruland, K.W., Donat, J.R. and Hutchins, D.A. (1991) Interactive in£uences of bioactive trace-metals on biological production in oceanic waters. Limnol. Oceanogr. 36, 1555^1577.
[2] Martin, J.H. and Fitzwater, S.E. (1988) Iron de¢ciency limits
phytoplankton growth in the north-east Paci¢c subarctic. Nature 331, 341^343.
[3] deBaar, H.J.W., Buma, A.G.J., Nolting, R.F., Cadee, G.C.,
Jacques, G. and Treguer, P.J. (1990) On iron limitation of the
Southern Ocean: experimental observations in the Weddell
and Scotia Sea. Mar. Ecol. Prog. Ser. 65, 105^122.
[4] Price, N.M., Andersen, L.F. and Morel, F.M.M. (1991) Iron
and nitrogen nutrition of equatorial Paci¢c plankton. Deep
Sea Res. I 38, 1361^1378.
[5] Martin, J.H. et al. (1994) Testing the iron hypothesis in ecosystems of the Equatorial Paci¢c Ocean. Nature 371, 123^129.
[6] Coale, K.H. et al. (1996) A massive phytoplankton bloom
induced by an ecosystem-scale iron fertilization experiment
in the equatorial Paci¢c Ocean. Nature 383, 495^501.
[7] Sarmiento, J.L. and LeQueère, C. (1996) Oceanic carbon dioxide uptake in a model of century-scale global warming. Science 274, 1346^1350.
9
[8] Azam, F., Fenchel, T., Field, J.G., Gray, J.S., Meyer-Reil,
L.A. and Thingstad, T.F. (1983) The ecological role of
water-column microbes in the sea. Mar. Ecol. Prog. Ser. 10,
257^263.
[9] Cho, B.C. and Azam, F. (1988) Major role of bacteria in
biogeochemical £uxes in the ocean's interior. Nature 332,
441^443.
[10] Delong, E.F. (1992) Archaea in coastal marine environments.
Proc. Natl. Acad. Sci. USA 89, 5685^5689.
[11] Fuhrman, J.A., McCallum, K. and Davis, A.A. (1992) Novel
major archaeabacterial group from marine plankton. Nature
356, 148^149.
[12] Bruland, K.W., Franks, R.P., Knauer, G.A. and Martin, J.H.
(1979) Sampling and analytical methods for the determination
of copper, cadmium, zinc, and nickel at the nanogram per liter
level in seawater. Anal. Chim. Acta 105, 233^245.
[13] Johnson, K.S., Gordon, R.M. and Coale, K.H. (1997) What
controls dissolved iron concentrations in the world ocean?
Mar. Chem. 57, 137^161.
[14] Duce, R.A. and Tindale, N.W. (1991) Atmospheric transport
of iron and its deposition in the ocean. Limnol. Oceanogr. 36,
1715^1726.
[15] Coale, K.H., Fitzwater, S.E., Gordon, R.M., Johnson, K.S.
and Barber, R.T. (1997) Control of community growth and
export production by upwelled iron in the equatorial Paci¢c
Ocean. Nature 379, 621^624.
[16] Hutchins, D.A. and Bruland, K.W. (1998) Iron-limited diatom growth and Si:N uptake ratios in a coastal upwelling
regime. Nature 393, 561^564.
[17] Bruland, K.W. (1980) Oceanographic distributions of cadmium, zinc, nickel, and copper in the North Paci¢c. Earth Planet. Sci. Lett. 47, 176^198.
[18] Price, N.M. and Morel, F.M.M. (1998) Biological cycling of
iron in the oceans. In: Metal Ions in Biological Systems (Sigel,
A. and Sigel, H., Eds.), Vol. 35, pp. 1^36. Marcel Dekker,
New York.
[19] Donat, J.R. and Bruland, K.H. (1994) Trace elements in the
oceans. In: Trace Metals in Natural Waters (Salbu, B. and
Steinnes, E., Eds.), pp. 247^281. CRC Press, Boca Raton,
FL.
[20] Kuma, K., Katsumoto, A., Kawakami, H., Takatori, F. and
Matsunaga, K. (1998) Spatial variability of Fe(III) hydroxide
solubility in the water column of the northern North Paci¢c.
Deep Sea Res. 45, 91^113.
[21] Gledhill, M. and van den Berg, C.M.G. (1994) Determination
of complexation of iron(III) with natural organic complexing
ligands in seawater using cathodic stripping voltammetry.
Mar. Chem. 47, 41^54.
[22] Rue, E.L. and Bruland, K.W. (1995) Complexation of Fe(III)
by natural organic ligands in the central North Paci¢c as
determined by a new competitive ligand equilibration/absorptive cathodic stripping voltammetric method. Mar. Chem. 50,
117^138.
[23] Wu, J. and Luther, G.W. III (1995) Evidence for the existence
of Fe(III) organic complexation in the surface water of the
Northwest Atlantic ocean by a competitive ligand equilibrium
method and a kinetic approach. Mar. Chem. 50, 159^177.
FEMSEC 997 21-4-99
10
P.D. Tortell et al. / FEMS Microbiology Ecology 29 (1999) 1^11
[24] Winkelman, G. (Ed.) (1991) Handbook of Microbial Iron
Chelates. CRC Press, Boca Raton, FL.
[25] Trick, C.G. (1989) Hydroxamate-siderophore production and
utilization by marine eubacteria. Curr. Microbiol. 18, 375^
378.
[26] Wilhelm, S.W. and Trick, C.G. (1994) Iron-limited growth of
cyanobacteria : Multiple siderophore production is a common
response. Limnol. Oceanogr. 39, 1979^1984.
[27] Estep, M., Armstrong, J.E. and van Baalen, C. (1975) Evidence for the occurrence of speci¢c iron(III)-binding compounds in near-shore marine ecosystems. Curr. Appl. Microbiol. 30, 186^188.
[28] Haygood, M.G., Holt, P.D. and Butler, A. (1993) Aerobactin
production by planktonic marine Vibrio sp. Limnol. Oceanogr. 38, 1091^1097.
[29] Reid, R.T., Live, D.H., Faulkner, D.J. and Butler, A. (1993)
A siderophore from a marine bacterium with an exceptional
ferric ion a¤nity constant. Nature 366, 455^458.
[30] Granger, J. and Price, N.M. (1999) The importance of siderophores in iron nutrition of heterotrophic marine bacteria.
Limnol. Oceanogr. (in press).
[31] Rue, E.L. and Bruland, K.W. (1997) The role of organic complexation on ambient iron chemistry in the equatorial Paci¢c
Ocean and the response of a mesoscale iron fertilization experiment. Limnol. Oceanogr. 42, 901^910.
[32] Hudson, R.J.M. and Morel, F.M.M. (1990) Iron transport in
marine phytoplankton: kinetics of cellular and medium coordination reactions. Limnol. Oceanogr. 35, 1002^1020.
[33] Maldonado, M.T. and Price, N.M. (1999) Nitrate regulation
of Fe reduction and transport in Fe-limited Thalassiosira oceanica. Limnol. Oceanogr. (submitted).
[34] Maldonado, M.T. and Price, N.M. (1999) Utilization of iron
bound to strong organic ligands by phytoplankton communities in the subarctic Paci¢c Ocean. Deep Sea Res. (in press).
[35] Maranger, R., Bird, D.F. and Price, N.M. (1998) Iron acquisition by photosynthetic marine phytoplankton from ingested
bacteria. Nature 396, 248^251.
[36] Maldonado, M.T. and Price, N.M. (1996) In£uence of N substrate on Fe requirements of marine centric diatoms. Mar.
Ecol. Prog. Ser. 141, 161^172.
[37] Tortell, P.D., Maldonado, M.T. and Price, N.M. (1996) The
role of heterotrophic bacteria in iron-limited ocean ecosystems. Nature 383, 330^332.
[38] Brand, L. (1991) Minimum iron requirements of marine phytoplankton and their implications for the biogeochemical control of new production. Limnol. Oceanogr. 36, 1756^1771.
[39] Righelato, R.C. (1969) The distribution of iron in iron-de¢cient toxin-synthesizing and in excess-iron non-toxin-synthesizing Corynebacterium diphtheriae. J. Gen. Microbiol. 58,
411^419.
[40] Martin, J.H., Gordon, R.M., Fitzwater, S.E. and Broenkow,
W.W. (1989) VERTEX: phytoplankton/iron studies in the
Gulf of Alaska. Deep Sea Res. 36, 649^680.
[41] Chase, Z. and Price, N.M. (1998) Metabolic consequences of
iron de¢ciency in heterotrophic marine protozoa. Limnol.
Oceanogr. 42, 1673^1684.
[42] Parsons, T.R. and Lalli, C.M. (1988) Comparative oceanic
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
[52]
[53]
[54]
[55]
[56]
[57]
ecology of the plankton communities of the subarctic Atlantic
and Paci¢c oceans. Oceanogr. Mar. Biol. 26, 317^359.
Strom, S.L., Postel, J.R. and Booth, B.C. (1993) Abundance,
variability, and potential grazing impact of planktonic ciliates
in the open sub-arctic Paci¢c Ocean. Prog. Oceanogr. 32, 185^
203.
Li, W.K.L., Subba Rao, D.V., Harrison, W.G., Smith, J.C.,
Cullen, J.J., Irwin, B. and Platt, T. (1983) Autotrophic picoplankton in the tropical ocean. Science 219, 292^295.
Platt, T., Subba Rao, D.V. and Irwin, B. (1983) Photosynthesis of picoplankton in the oligotrophic ocean. Nature 301,
702^704.
Li, W.K.W., Dickie, P.M., Irwin, B.D. and Wood, A.M.
(1992) Biomass of bacteria, cyanobacteria, prochlorophytes
and photosynthetic eucaryotes in the Sargasso Sea. Deep
Sea Res. 39, 501^519.
Caron, D.A. et al. (1995) The contribution of microorganisms
to particulate carbon and nitrogen in surface waters of the
Sargasso Sea near Bermuda. Deep Sea Res. 42, 943^947.
Chisholm, S.W., Olson, R.J., Zettler, E.R., Goericke, R.,
Waterbury, J.B. and Welschmeyer, N.A. (1988) A novel
free-living prochlorophyte abundant in the oceanic euphotic
zone. Nature 334, 340^343.
Goericke, R. and Welschmeyer, N.A. (1993) The marine prochlorophyte Prochlorococcus contributes signi¢cantly to phytoplankton biomass and primary production in the Sargasso
Sea. Deep Sea. Res. 40, 2283^2294.
Wu, J. and Luther, G.W. III (1994) Size-fractionated iron
concentrations in the water column of the western North Atlantic Ocean. Limnol. Oceanogr. 39, 1119^1129.
Carpenter, E.J. and Romans, K. (1991) Major role of the
cyanobacterium Trichodesmium in nutrient cycling in the
North Atlantic Ocean. Science 254, 1356^1358.
Capone, D.G., Zehr, J.P., Paerl, H.W., Bergman, B. and Carpenter, E.J. (1997) Trichodesmium, a globally signi¢cant marine cyanobacterium. Science 276, 1221^1229.
Rueter, J.G., Hutchins, D.A., Smith, R.W. and Unsworth,
N.L. (1992) Iron nutrition of Trichodesmium. In: Marine Pelagic Cyanobacteria : Trichodesmium and other Diazotrophs
(Carpenter, E.J., Capone, D.G. and Rueter, J.G., Eds.), pp.
289^306. Kluwer Academic, Dordrecht.
Kirchman, D.L. and Hutchins, D.A. (1998) Uptake of iron in
bacteria and phytoplankton in the Ross Sea, California upwelling and coastal Delaware waters. Ocean Sciences Meeting,
San Diego, CA. EOS Trans. Am. Geophys. Union 79, Suppl.
Behrenfeld, M.S., Bale, A.S., Kolber, Z.S., Aiken, J. and Falkowski, P.G. (1996) Con¢rmation of iron limitation of phytoplankton photosynthesis in the equatorial Paci¢c Ocean.
Nature 383, 508^511.
Zettler, E.R., Olson, R.J., Binder, B.J., Chisholm, S.W., Fitzwater, S.E. and Gordon, R.M. (1996) Iron-enrichment bottle
experiments in the equatorial Paci¢c: Responses of individual
phytoplankton cells. Deep Sea Res. II 43, 1017^1029.
Price, N.M., Ahner, B.A. and Morel, F.M.M. (1994) The
Equatorial Paci¢c Ocean: Grazer controlled phytoplankton
populations in an iron-limited ecosystem. Limnol. Oceanogr.
39, 520^534.
FEMSEC 997 21-4-99
P.D. Tortell et al. / FEMS Microbiology Ecology 29 (1999) 1^11
[58] Pakulski, J.D., Co¤n, R.N., Kelley, C.A., Holder, S.L.,
Downer, R., Aas, P., Lyons, M.M. and Je¡rey, W.H. (1996)
Iron stimulation of Antarctic bacteria. Nature 383, 133^
134.
[59] Sunda, W.G. and Huntsman, S.A. (1997) Interrelated in£uence of iron, light and cell size on marine phytoplankton
growth. Nature 390, 389^392.
[60] LaRoche, J., Boyd, P.W., McKay, R.M.L. and Geider, R.J.
(1996) Flavodoxin as an in situ marker for iron stress in phytoplankton. Nature 382, 802^805.
[61] Mo¡et, J.W., Zika, R.G. and Brand, L.E. (1990) Distribution
and potential sources and sinks of copper chelators in the
Sargasso Sea. Deep Sea Res. 37, 27^36.
[62] Morel, F.M.M., Reinfelder, J.R., Roberts, S.B., Chamberlain,
C.P., Lee, J.G. and Yee, D. (1994) Zinc and carbon co-limitation of marine phytoplankton. Nature 369, 740^742.
[63] Kim, I.C. and Bragg, P.D. (1971) Properties of nonheme iron
in a cell envelope fraction from Escherichia coli. J. Bacteriol.
107, 644^670.
[64] Yamamoto, I. and Ishimoto, M. (1975) E¡ect of nitrate re-
[65]
[66]
[67]
[68]
[69]
[70]
11
duction on the enzyme levels in carbon metabolism in Escherichia coli. J. Biochem. 78, 307^315.
Suzuki, T., Yamazaki, O., Nara, K., Akiyama, S., Nakao, Y.
and Fukada, H. (1975) The aconitase of yeast. II. Crystallization and general properties of yeast aconitase. J. Biochem. 77,
367^372.
Moody, C.S. and Hassan, H.M. (1984) Anaerobic biosynthesis of the manganese-containing superoxide dismutase in Escherichia coli. J. Biol. Chem. 259, 12821^12825.
McCord, J.M. and Fridovich, I. (1969) Superoxide dismutase:
an enzymic function for erythrocuprein (hemocuprein). J. Biol.
Chem. 244, 6049^6055.
Fridovich, I. (1975) Superoxide dismutases. Annu. Rev. Biochem. 44, 147^159.
Christian, J.R. and Karl, D.M. (1994) Microbial community
structure at the U.S.-Joint Global Ocean Flux Study station
ALOHA : Inverse method for estimating biochemical indicator ratios. J. Geophys. Res. 99, 14269^14276.
Archibald, F.S. (1983) Lactobacillus plantarum, an organism
not requiring iron. FEMS Microbiol. Lett. 19, 29^32.
FEMSEC 997 21-4-99