AMER. ZOOL., 22:611-619 (1982)
Recent Progress in the Study of "Die Ernahrung der Wassertiere
und der Stoffhaushalt der Gewasser"1
GROVER C. STEPHENS
Department of Developmental and Cell Biology, University of California, Irvine,
Irvine, California 92717
SYNOPSIS. It is now possible to provide direct evidence for net removal of significant
amounts of specific amino acids from naturally occurring dissolved organic material by a
marine invertebrate. This demonstration serves to focus attention and invite reconsideration of the large body of less direct evidence supporting the general occurrence of this
capacity among soft-bodied marine invertebrates.
INTRODUCTION
The symposium on the role of uptake of
organic solutes in the nutrition of marine
organisms presented in this issue of the
American Zoologist might appropriately
have carried the title of this contribution.
By quoting the title of August Putter's
monograph (1909), I intend to recognize
and make explicit the connection between
the papers presented here and what has
come to be known as "Putter's hypothesis."
Briefly, Putter argued that dissolved organic substances play an important role in
the nutrition of the great majority of
aquatic animals.
Putter was not the first to suggest that
dissolved organic material (DOM) might
serve as a source of nutrition. More than
a century ago, Haeckel, Sars, and WyvilleThompson invoked DOM to account for
the nutrition of marine animals lacking a
morphologically distinct digestive tract or
for which no obvious source of particulate
food was available (J0rgensen, 1976). Putter's contribution was to formulate an argument and present data to support the
position that DOM was of general significance and played a role in the nutrition of
many or most aquatic animals rather than
being confined to the unusual organisms
that attracted the attention of early naturalists.
The argument is based on an application
of the fundamental relation between food
intake and oxygen consumption first for1
From the Symposium on The Role of Uptake of Organic Solutes in Nutrition of Marine Organisms presented
at the Annual Meeting of the American Society of
Zoologists, 27-30 December 1980, at Seattle. Washington.
mulated by Lavoisier and LaPlace (1780).
The metabolism of animals is based on oxidation of food. When organic material is
oxidized, oxygen is consumed. There is a
well-defined relation between the quantity
of a particular foodstuff oxidized, the volume of oxygen consumed and the energy
derived from that oxidation (heat). Thus
the caloric content of food consumed by
an animal must be at least equivalent to the
caloric value of the oxygen consumed.
Putter used this relation to estimate food
requirements for a variety of aquatic animals by measuring oxygen consumption or
using data from the literature. He compared this with the organic content of
plankton organisms available in the habitat
and calculated the organic content of DOM
present. He then calculated the volume of
water from which the animal would have
to obtain its food from the two sources. His
monograph tabulates such estimates and
concludes that the volume to be processed
by an animal obtaining planktonic food is
unreasonably large whereas relatively small
volumes of water contain adequate organic
material as DOM. He recognized that
aquatic animals might survive by periodic
exploitation of rich food resources. He extended his argument to consider the energy requirements of rapidly growing organisms such as larvae, taking into account
increase in biomass as well as oxidation.
The conclusion was the same. Taken at face
value, his calculations make it very difficult
to understand how many of the aquatic organisms he studied could possibly obtain
particulate food at a rate necessary to survive and grow. August Krogh (1931) provides specific examples from Putter's tables.
611
612
GROVER C. STEPHENS
Putter's monograph was influential and
rapidly attracted criticism. He subsequently agreed that he had initially underestimated plankton availability and overestimated DOM content of natural waters but
maintained that his fundamental position
was correct even after adjustment of his
calculations to meet such criticism. It is apparent that Putter's basic argument for the
importance of DOM is indirect; it is primarily an argument attacking the adequacy of particulate food resources. Putter and
a number of other investigators attempted
to provide direct evidence for the nutritional role of DOM. Although some positive results were reported in efforts to show
that dissolved substrates could effectively
support growth and survival of aquatic organisms, results were not impressive and
experimental procedures were often naive.
The field was summarized and subjected
to a superb critical review by August Krogh
(1931). With some relatively minor reservation, Krogh concluded that "There is no
convincing evidence that any animal takes
up dissolved organic substances from natural water in any significant amount . . . ."
Krogh's review consisted of an extremely
careful consideration of evidence available
at that time and convinced the great majority of biologists that the issue had been
firmly settled; DOM did not play a significant role in the nutrition of aquatic animals barring a few unusual cases. Krogh,
however, retained his interest in DOM for
the remainder of his career until his death.
Taken literally, the conclusion of his review can be interpreted as equivalent to
the Scottish verdict, "not proven." Thus, a
consideration of Krogh's conclusion must
be the first task of this symposium, convened a half century after his brilliant review, to consider if DOM plays a nutritional role in marine organisms.
Readers interested in the history of this
field are referred to J0rgensen (1976) in
addition to Putter's monograph and
Krogh's review.
convincing evidence that dissolved organic
substances are in fact taken up by an animal from natural water in significant
amounts. I take that to involve the following steps:
a. presentation of evidence for net influx
of a specific organic substrate,
b. demonstration of the presence of that
substrate in natural water from the habitat of the animal,
c. demonstration of net influx at significant rates from concentrations which
characterize natural water,
d. direct demonstration of net influx from
natural water of the substrate into the
animal, and, finally,
e. demonstration that the animal is the
major agent in the process.
These steps are interrelated. Thus demonstration of net influx of a substrate is of
little value unless it occurs at concentrations which characterize natural water in
the habitat of the animal. Since DOM is not
uniformly distributed in the marine environment, knowledge of and specific analysis of immediate substrate availability is
crucial. Since DOM is an extremely complex and incompletely characterized mixture of substrates, use of actual samples
from the immediate habitat is necessary to
be sure that net influx actually occurs in
the field.
I will first present briefly evidence for
net influx of free amino acids (FAA) into
the edible marine mussel, Mytilus edulis. I
will then proceed to discuss some of the
issues involved in generalizing this conclusion.
Influx of FAA in Mytilus edulis
Techniques for analysis of free amino
acids have been recently introduced (Lindroth and Mopper, 1979; Gardner and
Miller, 1980) using high-performance liquid chromatography (HPLC) to separate
fluorescent derivatives of these compounds. Sensitivity of detection of specific
amino acids in quantities of the order of
10 IL> moles or less have been achieved.
INFLUX OF ORGANIC: MATERIAL INTO
Separation is accomplished on a reverseMARINE ANIMALS
phase silane column; derivatization emPhrased in the language of Krogh's con- ploys o-phthaldialdehyde and is simple and
clusion, my aim in this paper is to provide rapid; samples of seawater can be analyzed
UPTAKE OF DOM:
without desalting. Specific amino acids are
identified by the elution time of the fluorescent derivatives and quantitative estimates of the presence of each are obtained
by measurement of peak area. We employed this basic procedure in studying influx of FAA in Mytilus edulis (Manahan et
ai, 1982).
Mytilus occurs abundantly in marine embayments. Food intake, waste removal, and
respiratory gas exchange occur by way of
the large volumes of water which the animal draws in along the incurrent margin
of the mantle cavity, passes across the gill,
and extrudes via a well-defined excurrent
siphon. The rate at which water is passed
through the mantle is of the order of 12
liters/ml O2 consumed (J0rgensen, 1966).
As is generally the case in filter feeders,
the water is forcefully ejected from the excurrent siphon so that water passes across
the gill only once.
Mussels were collected, cleaned of adhering organisms and detritus and brought
into the laboratory. On the following day,
water was collected from the immediate vicinity of the mussel bed and filtered
through a 0.2-jU.m Nucleopore filter to remove planktonic organisms including bacteria. Control observations indicated that
no FAA was added during such filtration
of HPLC grade water. An individual Mytilus was placed in a volume of freshly collected and filtered natural seawater. Typically, the animal opened and began to
pump normally within a few minutes.
Samples of water were then taken from the
incurrent margin of the animal and from
the excurrent siphon using a small plastic
cannula positioned with a micromanipulator (see Wright and Stephens, 1978, for a
complete account of the procedure).
Figure 1 is a photograph of the chromatograms obtained by HPLC analysis of
a sample of water from the incurrent margin and excurrent siphon, taken immediately after the initiation of pumping. The
amino acids Asp, Ser and Gly are labeled.
Since the samples were taken essentially simultaneously, the differences in FAA content represent changes occurring during a
single passage of water through the mantle
cavity. At typical pumping rates obtained
613
RECENT PROGRESS
Ser
INCURRENT
Asp
EXCURRENT
Asp
Ser
Gly
FIG. 1. The upper tracing is a photograph of a chromatogram of seawater taken from the incurrent margin of Mytilus edulis. The lower tracing is a photograph of a chromatogram of seawater drawn from
the excurrent siphon of the animal by means of a
cannula. The samples were taken simultaneously. The
seawater is from the immediate habitat of the mussel.
The amino acids Asp, Ser and Gly are indicated. (After Manahan el ai, 1982.)
in our laboratory using this procedure, the
residence time of water in the mantle cavity is a few seconds. In the observations
presented in Figure 1, Asp, Ser and Gly
are decreased by 63%, 84% and 72% respectively.
Approximate concentrations of the three
substrates designated are 0.06, 0.23 and
0.14 /xmoles/liter. These concentrations are
typical of those reported for the FAA content of inshore and marine bay waters. One
can easily work out the amount of oxygen
required for each of the three substrates
to be oxidized to CO2 and H2O plus NH3.
The ml of oxygen required for oxidation
of one micromole each of Asp, Ser and Gly
are respectively 0.067, 0.056 and 0.034 ml
O2. Pumping continuously at a rate of approximately 1.5 liters/hr, an animal will obtain 0.06 /Amoles of Asp, 0.29 /ixmoles of
Ser, and 0.15 /xmoles of Gly removing these
substrates in the percentages observed.
Converting to oxygen required for oxidation of these substrates, 25.1 ix\ of oxygen
is needed. Accepting J0rgensen's rough estimate of 12 liters of water pumped per ml
O2 consumed, the animal providing the
data in Figure 1 would be consuming approximately 125 ju.1 O2/hr. Thus the entry
of the three amino acids from natural water
at the rates observed would supply ap-
614
GROVER C. STEPHENS
proximately 20% of the reduced carbon
necessary to sustain the animal's oxygen
consumption.
With respect to amino nitrogen, the
measured influx provides 0.5 ^tmoles of
amino acid and hence approximately 7 fj.g
of amino nitrogen per hour. Bayne and
Scullard (1977) stress the variability of nitrogen excretion in Mytilus edulis on a seasonal basis as well as variations related to
reproductive state. They also note that
amino nitrogen is seasonally significant as
an excretory product and undetectable at
other times. Manahan et al. (1982) failed
to detect any amino nitrogen in the excurrent water or in water in which animals
had been held for a period of up to 5 hr.
Bayne and Scullard give rates of ammonia
nitrogen loss ranging from 6 /xg/g-hr to 32
jug/g-hr for a one-gram animal which is approximately the weight of the specimens
used by Manahan and co-workers. The data
in Figure 1 were collected using samples
taken on 7 July 1981, which is the season
at which Bayne and Scullard found the
highest rate of ammonia loss. Since Bayne
and Scullard also stress the variability in
phase relations of the seasonal cycle among
different populations, there is really no way
to estimate ammonia loss in our animals.
However, an input of 7 /xg N/hr represents
a significant input throughout the range
reported by Bayne and Scullard.
Neither the preceding estimate concerning reduced carbon input nor the argument concerning amino nitrogen influx are
intended to lead to precise numbers since
key measurements (Vo2, VNH 3 ) for our animal were not made. The burden of the
argument is simply that the measured rate
of influx is significant whether treated as a
reduced carbon input or as a source of
amino nitrogen.
I have already directed attention to the
very brief time which elapses during passage of water through the mantle cavity of
these animals at normal pumping rates. It
is very difficult to imagine a microbial population capable of removing 60—80% of
FAA substrates in a matter of a few seconds. Owen (1974) provides TF.M photographs of the gills of Mytilus edulis which
show no evidence of endosvmbiotic or ad-
herent microorganisms; unpublished observations in our laboratory verify this.
SEM examination of gills (Owen, 1974,
personal observations) do not reveal external microorganisms though the morphological complexity of the gills make it impossible to exclude completely their
presence. Finally, brief exposure to labeled
FAA substrate (see Wright, Fig. 6, 1982)
results in incorporation in the filament epithelium. Thus it is unreasonable to invoke
any agent other than the animal as a major
contributor to the dramatic decrease of
FAA observed in a single very brief transit
of the medium through the mantle cavity.
The observations just cited provide direct analytical evidence for rapid removal
of a naturally occurring substrate from an
unmodified sample of the environmental
medium taken from the immediate habitat
of the common edible mussel. We have just
argued that the circumstantial evidence indicates that the animal is certainly the major and very probably the only significant
agent in this removal. Finally, the rate of
influx is significant whether interpreted as
a contribution to reduced carbon or to nitrogen requirements of the organism. I believe this provides a direct refutation of
Krogh's basic conclusion as stated in his
careful review. I would like to stress very
strongly that this is in no sense to be taken
as any criticism of August Krogh. Quite
the reverse: based on the evidence available at the time, his conclusion was entirely
appropriate.
Other evidence for uptake of
organic materials by marine animals
In the preceding section, I argued that
available evidence now permits us to reject
Krogh's conclusion. At least for Mytilus edulis, convincing evidence can be presented
to demonstrate that the animal takes up
dissolved organic substances from natural
water in significant amounts (see also
Wright, 1982). Mussels are very common
animals, cosmopolitan in distribution, and
have well-defined paniculate feeding
mechanisms which have been studied in
great detail. Thus they cannot be considered at all unusual. The question that now
arises is, to what extent is it likelv that this
UPTAKE OF DOM:
RECENT PROGRESS
615
capacity for removal of organic substrates levels of substrate which were unrealistioccurs more broadly distributed among cally high. Subsequently, Ferguson (1971)
marine invertebrates?
demonstrated net removal of substrate by
In the past two decades, workers inves- starfish at acceptably low concentrations.
tigating this matter have steadily pro- However, techniques for analysis of organgressed in sophistication, both with respect ic substrates were demanding and time
to techniques and interpretation of data. consuming so that detailed investigations
In the decade of the 1960s (and continuing of net exchanges were not practical.
into the present), most of the work which
Stephens (1975) and North (1975) introappeared in the literature employed radio- duced the use of the reagent, fluorescachemically labeled organic substrates. Es- mine, into the study of net exchanges of
sentially all marine animals examined primary amines in marine organisms. The
showed rapid influx of labeled small or- reagent, reported by Udenfriend et al.
ganic substrates into the animal with two (1972), made possible the simple and rapid
major exceptions: arthropods and almost estimation of primary amines in seawater
all vertebrates. This literature is the subject and thus permitted simple comparisons
of a number of reviews, the most recent of between rates of influx of radiochemically
which is Stephens (1981). From the outset, labeled substrates and net rates of removal
careful workers recognized the distinction of known primary amines supplied in the
between influx of labeled substrate and net medium. Such work was reported for anentry of substrate from the environment. nelids (Stephens, 1975), bivalve molluscs
Various indirect methods were employed (J0rgensen, 1976), and echinoderms (Steto attempt to show that the entry of organ- phens et al., 1978). Essentially, initial rates
ic material in labeled form exceeded losses of net entry of known substrates deterby other pathways (exchange diffusion, ex- mined by disappearance of primary amine
cretion, passive exit of the same or related from the medium closely paralleled initial
substrates; e.g., Stephens, 1972).
rates of disappearance of the same labeled
The issue that despite rapid influx of la- substrate as determined radiochemically.
beled organic substrates, animals might well Typically, the rate of net entry of substrate
be losing comparable substrates at similar diverged from the radiochemical data with
or greater rates was sharply drawn by Jo- the passage of time. This was, and continhannes et al. (1969). These investigators ues to be, interpreted as leakage by an unshowed quite clearly that the rate of loss of known pathway of unknown primary
free amino acids by the ectocommensal amine(s). A good deal of work has now apflatworm, Bdelloura, exceeded the rate of peared in the literature supporting this
entry of amino acid at the ambient sub- general conclusion (see Stephens, 1981, for
strate concentration they employed. This review).
was done by measuring both the rate of
Such studies clearly exclude the hypothdisappearance of labeled substrate radio- esis advanced by Johannes et al. (1969) who
chemically and the rate of change of amino proposed that exchange diffusion accountacids in the medium using direct chemical ed for earlier reports of influx of labeled
analysis. By doing this, they raised serious substrates. They correctly pointed out that
questions about the conclusions drawn the kinetics of influx would be the same
from preceding work based solely on radio- for exchange diffusion as those for net acchemical procedures (and much subse- tive influx via a carrier. However, if laquent work as well).
beled amino acids were being exchanged
Prior to publication of the work of Jo- via a carrier for unlabeled amino acids from
hannes et al. (1969) Stephens and Schinske a large internal pool, a direct analysis of
(1961) had demonstrated removal of free changes in the medium should show no
amino acids by a variety of marine inver- change or a net increase in substrate in the
tebrates by direct chemical determination medium. That is not the case in the work
of disappearance of substrate from the cited.
medium. However, it was necessary to use
However, the results of combining ra-
616
GROVER C. STEPHENS
ronment enter these animals at a significant rate (to repeat Krogh's point yet
again). What this body of published work
does do is to provide an increasingly impressive body of evidence which strongly
supports the conclusion that the capacity
for removal of organic material from natural waters at significant rates is in fact a
very common feature found in marine invertebrates much as Putter contended more
than 70 years ago.
How one chooses to interpret this body
of information and the degree of probability assigned to the general occurrence of
net uptake from natural sources is to some
extent open at the present. There are of
course some quite strict conclusions which
can be drawn from this mass of literature.
Small organic compounds of several kinds
do in fact enter marine organisms, often
quite rapidly, when supplied in the medium. Specific known substrates supplied in
the medium are removed from the surrounding water in the sense of net entry
by a variety of marine organisms. In addition, there are many other quite firm
conclusions which can be drawn from this
body of literature unrelated to the question of net entry; I have simply not discussed the range of other features of this
process which have been addressed by various investigators.
The introduction of HPLC analysis of
amino acids offers the possibility of measuring levels of specific organic substrates
in natural waters. The procedure is sufficiently brief and undemanding to permit
CURRENT STATE OF THE FIELD
kinetic studies. There is no addition of any
The argument of the preceding section organic substrate to water samples and
can be summarized as follows. There is now chemical manipulation is minimal. It is thus
quite a large literature reporting influx of an extremely attractive addition to our
radiolabeled organic substrates into ma- technical resources in attacking the probrine invertebrates. There is also a smaller lem of the occurrence and significance of
but substantial body of more recent liter- uptake of organic materials from the very
ature demonstrating net entry of specific low concentrations which characterize
organic amines using fluorescamine as a many marine habitats.
fluorogen reagent. This work also suggests
At the time this symposium was orgaa broad distribution of the capacity for net nized, HPLC analysis in the form deuptake of substrate. However, the limita- scribed had not been introduced. The adtions of these techniques used alone or in vent of this new technique is viewed as
combination prevent us from drawing the permitting a significant advance in our
conclusion without reservation that organ- ability to study this general problem. The
ic materials naturally present in the envi- papers in this volume (Amer. Zool., Vol.
diochemical and fluorometric analysis in
the fashion described remain ambiguous.
Fluorescamine reacts generally with primary amines including free amino acids,
peptides and ammonia as well as other
amine compounds. Specific fluorescence of
the derivatives of different amines differ
widely. Thus quantitative analysis is possible if and only if one independently demonstrates the chemical identity of the amine
compounds being analyzed. This is only
possible upon initial presentation of a solution of known substrate to the animal
studied. With the passage of time, one no
longer knows what amines may be present
in solution as a result of contributions by
the animal. As a consequence of this feature of the technique, it is impossible to
draw any strict conclusions from observations of the disappearance of naturally occurring primary amines (e.g., Stephens,
1975). Efforts to provide an inventory of
amines naturally present (Stephens et al,
1978) are necessarily incomplete given the
complexity of the chemical nature of DOM
in marine waters.
These comments should not be taken to
suggest that radiochemical investigations
or those employing fluorescamine as a
general fluorogenic reagent are without
value at the present. Properly used, they
are powerful tools. However, they do suffer from the limitations just described when
they are employed to demonstrate net entry of organic material from natural waters.
UPTAKE OF DOM:
22, no. 3) presented in the winter of 1980
represent the current state of the field.
Specifically, comparison of these papers
with work published in the two preceding
decades reveals the increased sophistication in data acquisition and interpretation
to which I have already referred.
Wright's contribution discusses the role
of DOM in filter feeding invertebrates with
particular attention to the technical problems involved in the investigations which
lead to our current view. Manahan and
Crisp emphasize the potential importance
of DOM resources for marine larvae and
summarize results of recent work that
strongly supports that view. Southward and
Southward review current evidence for a
role of DOM in the nutrition of pogonophorans and other abyssal forms.
Other contributions focus attention on
particular organisms of special interest by
virtue of their habitat or for other reasons.
Crowe et al. discuss the role of DOM in
ectoparasitic nemerteans whose larvae survive for long periods on the surface of their
decapod host in a habitat unusually rich in
free amino acids. DeLaca reports his studies on amino acid uptake in benthic foraminiferans in the Antarctic. They are of
particular interest since their habitat is quite
similar to abyssal conditions in virtually all
respects except for hydrostatic pressure;
they are thus available models for studies
not currently feasible in the benthos.
Schlichter describes the multiple feeding
strategies of a soft coral and the evidence
for a role of DOM in its nutrition.
Two of the contributions focus on the
question of the mechanism of uptake of
organic solutes from the environment.
Preston addresses the issue of how net
transport against the extremely large concentration gradients which are involved in
entry of organic materials can occur.
Gomme discusses this but from a different
perspective drawing attention to the structure and organization of epithelia across
which organic material is acquired. He also
suggests an alternate functional role for
these transport systems by calling attention
to the possibility that they may provide a
mechanism minimizing losses across the
epidermis in addition to or as an alterna-
RECENT PROGRESS
617
tive to the acquisition of organic substrate
from the environment.
The last contribution by Siebers draws
attention to an issue briefly addressed but
not developed in this introductory contribution but which represents a major concern. He argues that heterotrophic microorganisms in association with marine
animals may account for the majority of
apparent uptake of organic solutes reported for at least some metazoans. I rejected
this in the case ofMytilus edulis for two reasons; the extremely rapid intake of substrate during brief passage of the medium
across the transporting surface in this animal, and the lack of any supporting morphological evidence for the very large microbial population that would be required
to account for this. However, in general
the role of bacteria cannot readily be excluded. Manahan and Crisp present evidence in their paper that bivalve larvae
outcompete natural populations of bacteria for available dissolved resources and
thus argue on different grounds that microorganisms are not an important perturbing element in their data. They also
draw attention to the very exciting prospect of the availability of bacteria-free marine animals as experimental material. Obviously, use of such material will resolve
the importance of the role of bacteria, at
least for those organisms. However, the
problem is a difficult one since marine microorganisms are well known to use DOM
and are virtually impossible to exclude from
experimental systems employing marine
invertebrates by traditional methods. It is
not likely that this issue will be generally
resolved in the short run.
CONCLUSION
I have not considered the question of
the ecological significance of the nutritional role of dissolved organic material in marine organisms. First, as I have said, the
direct evidence for such a role is quite limited although the indirect supporting evidence becomes increasingly impressive.
Second, the usual methods for estimating
the potential contribution of inputs of dissolved carbon or organic nitrogen from
DOM resources are much too simple and
618
GROVER C. STEPHENS
primitive to permit any serious account of nutritional role of DOM in marine inverthe overall role of DOM in the trophic or- tebrates is sufficiently strong to justify diganization of marine communities. For recting attention to this subject. Finally, I
these reasons the potential ecological sig- hope that this symposium will serve to connificance of this field can only be suggested vey my own sense of excitement concerning the important physiological and ecoin quite general remarks.
To the extent that net uptake of organic logical problems in this field which have
materials from the environment does oc- yet to be resolved.
cur, it represents a flow of chemical potential energy which then contributes an unACKNOWLEDGMENTS
known fraction to the energy flow which
Some of the work from our laboratory
results from more traditional feeding reported here was supported by NSF
methods. For purposes of argument, let us Grants PCM 78-09576 and OCE 78-09017.
provisionally accept the view that such net
uptake is a widespread phenomenon in
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vores, detritus feeders, ciliary-mucoid filMollusca). J. Mar. Biol. Assn. U.K. 57:355-369.
ter feeders along with animals which
Ferguson,
J. C. 1971. Uptake and release of free
appear to possess no morphologically wellamino acids by starfishes. Biol. Bull. 141:122-129.
defined feeding mechanisms. The rough Gardner, W. S. and W. H. Miller, III. 1980. Reestimates of the magnitude of DOM inputs
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R. E., S. J. Coward, and K. L. Webb. 1969.
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under more demanding scrutiny in future
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I will conclude with a personal statement. My motive in organizing this symposium was to bring to the attention of a
broad public of biologists contributions that
illustrate the current state of the field. In
doing so, I hope to attract additional investigators to enter the field. I believe that
the advances in marine organic analysis together with such advances as the availability of bacteria-free experimental material
will permit rapid progress in the field. I
also believe that the evidence supporting a
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M. A. Rice. 1982. Transport of dissolved amino
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Putter, A. 1909. Die Erndhrung der Wassertiere und
der Stojfhaushalt der Grwasser. Fischer, Jena.
Stephens. G. (!. 1972. Amino acid accumulation and
assimilation in marine organisms. /// J. W. Camp-
UPTAKE OF DOM:
RECENT PROGRESS
619
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