Download Conceptual models for the biogeochemical role of the photic zone

Progress in Oceanography 44 (1999) 271–286
Conceptual models for the biogeochemical role
of the photic zone microbial food web, with
particular reference to the Mediterranean Sea
T. Frede Thingstad
a
a,*
, Fereidoun Rassoulzadegan
b
Department of Microbiology, University of Bergen, Jahnebakken 5, N-5020 Bergen, Norway
b
Station Zoologique, BP 28, F-06230 Villefranche-sur-Mer, France
Abstract
Observations are reviewed which indicate that not only phytoplankton, but also heterotrophic
bacteria are P-limited during those seasons when there is stratification in the Mediterranean.
It is discussed how these observations fit into a general concept of a size-structured food chain
where the structure is a result of combined top-down control from size-selective predators and
size-dependent competition for phosphate. It is argued that, conceptually, labile DOC and
silicate have symmetrical roles in potentially controlling the flux of P through the ‘microbial’
and ‘classical’ sides, respectively, of this food web. The resulting concept provides a model
linking C, P, and Si fluxes to the size spectrum of biogenic particles in the photic zone. 
1999 Elsevier Science Ltd. All rights reserved.
Contents
1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272
2.
Biomass and growth rate control of bacteria and phytoplankton . . . . . . . . . . . 275
3.
Size spectrum of osmotrophs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284
* Corresponding author.
E-mail address: [email protected] (T..F. Thingstad)
0079-6611/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 7 9 - 6 6 1 1 ( 9 9 ) 0 0 0 2 9 - 4
272
T..F. Thingstad, F. Rassoulzadegan / Progress in Oceanography 44 (1999) 271–286
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284
1. Introduction
In the Mediterranean Sea a combination of factors present a fascinating, although
perhaps somewhat overlooked, combination of challenges and opportunities to modern marine microbiology. Not only is the Mediterranean an area with an interesting
hydrography and biogeochemistry, which is believed to strongly influence the conditions of marine life, but it is also a marine area associated with strong economical,
recreational and conservational interests that is in need of a management plan based
on sound scientific principles.
The general circulation pattern, which is dominated by the negative thermo-haline
circulation drawing nutrient-poor surface waters in through the Strait of Gibraltar,
is supposedly the main mechanism determining the generally oligotrophic status of
the Mediterranean (Redfield, Ketchum & Richards, 1963). this oligotrophic trend
increases towards the eastern region. In some, so-far poorly understood manner, this
circulation is coupled to an unusual peculiar biogeochemistry which leads to N:P
ratios becoming increasingly skewed towards P-deficiency, particularly in the eastern
parts (Krom, Kress, Brenner & Gordon, 1991). With a heavily populated coastal
zone, the stage is also set for strong local gradients to develop where waste water
inputs from human activities meet nutrient poor off-shore waters (Bianchi et al.,
1993; Justic, Rabalais & Turner, 1995; Elbazpoulichet, Garnier, Gaun, Martin &
Thomas, 1996). The Mediterranean is also an area where deep water is formed in
winter (Wüst, 1961; MEDOC Group, 1970; Lacombe, Gascard, Gonella & Bethoux,
1981) creating a situation where the linkages between biological activity and carbon
transport by downwelling occurs under light and temperature conditions that are very
different from those prevailing in areas of deep water formation in, for example, the
North Atlantic. The Mediterranean thus offers a unique set of conditions as a natural
laboratory, for studying the interplay between marine biological, biogeochemical,
and hydrographical processes.
At the basis of a food chain ending in the more conspicuous forms such as economically important fish stocks and marine mammals is a microbial food web consisting of small unicellular phytoplankton, bacteria, protozoa, and viruses, invisible
to the unaided human eye. These tiny life forms are connected by trophic interactions
which together form a complex braid of closely linked processes. Powered by energy
originating (in most cases) from sunlight, the microbial food web drives constant
processes transforming matter between organic and inorganic, dissolved and particulate forms. The function of this food web in terms of material cycling is intimately
linked to the food web structure, i.e., to which functional groups of organisms are
present, the extent to which they interact, and how this structure is a result of external
driving forces such as light, nutrient content, advection etc.
Understanding this system is a challenge very much in its own right, but equally
challenging is confronting the wide range of issues where fundamental knowledge
T..F. Thingstad, F. Rassoulzadegan / Progress in Oceanography 44 (1999) 271–286
273
of the functioning of the photic zone food web is central to a sound, scientifically
based, management of the Mediterranean Sea (Table 1).
With an almost infinite variation in trophic strategies involved in shaping the pelagic food web, extraction of general principles requires some kind of idealisation of
the system. With the obvious opposite danger of ignoring important detail, we want
to focus here on how generalised trophic interactions such as competition and predation, combined with inevitable physical constraints such as diffusion and sinking,
can form a conceptual picture which allows us to understand some of the important
properties of the pelagic Mediterranean ecosystem. Fig. 1 illustrates one way of
summarising the carbon flows through the lower parts of the pelagic food web. The
‘ladder’ like structure in the figure contains (to the right) the linear food web from
relatively large-celled phytoplankton species via copepods to fish, classically thought
to dominate the pelagic food web (e.g., Steele, 1974). As one progresses from this
‘classical’ side towards the ‘microbial’ side (on the left), the cell size of the organisms progressively decreases. The basic principle of this conceptual structure does
not seem to have been critically challenged since its original description (Johannes,
1965; Sheldon, Prakash & Sutcliffe, 1972; Pomeroy, 1974; Williams, 1981; Azam,
Fenchel, Field, Gray, Meyer-Reil & Thingstad, 1983), and variations of this theme
form the basis of most present conceptualisations in the field. Fig. 1 emphasises two
classes of trophic strategies, osmotrophy, i.e. feeding by organisms taking up dissolved nutrients through the membrane, and phagotrophy, here used for organisms
feeding by eating particulate matter. Osmotrophs include both the heterotrophic bacteria and the autotrophic phytoplankton, while the predatory food chain includes both
the phagotrophic protozoa, mesozooplankton, and higher predators, all heterotrophs.
Carbon fixed in photosynthesis is, through predatory processes, channelled ‘upwards’
in the food web towards larger organisms, but is also lost through a multitude of
processes to the pool of dissolved organic-C (DOC) which can then, to a smaller or
larger extent, be re-incorporated in the particulate food web. This cycle via DOC of
Table 1
Some of the areas where a correct understanding of the mechanisms regulating the pelagic microbial food
web is suggested to be of importance to a scientifically based management of marine areas
Stoichiometric coupling of biogeochemical cycles of C, N, P
Relevant to:
Carbon sequestration from the atmosphere/climate
Food production for commercially important fish stocks
Fate and effect of organic and inorganic waste disposal
Size spectrum and pigmentation of biogenic particles
Relevant to:
Adsorption area for surface active pollutants (heavy metals, ship paints,
hydrocarbons etc.)
Sinking flux of particulate material and vertical transport of pollutants
Optical and aesthetic qualities of coastal waters
Interpretation of remote sensing data
Diversity controlling mechanisms
Relevant to:
Biodiversity, ecosystem resilience
Ecosystem conservation
Survival of pathogens in recreational areas
274
T..F. Thingstad, F. Rassoulzadegan / Progress in Oceanography 44 (1999) 271–286
Fig. 1. Idealised view of carbon flow through the pelagic photic zone food web (modified from Fenchel, 1987).
organic material, otherwise ‘lost’ from the particulate food web is often referred to
as the ‘microbial loop’ (Azam, Fenchel, Field, Gray, Meyer-Reil & Thingstad, 1983).
Organic matter produced in the photic zone may be exported not only via the ‘classical’ side, by the production of particles large enough to sink at a significant rate
(Miquel, Fowler, Larosa & Buatmenard, 1994), but also as DOC, which if present
at a sufficiently high concentration may be expected by diffusion or transported
downwards during downwelling events and thus result in a quantitatively important
T..F. Thingstad, F. Rassoulzadegan / Progress in Oceanography 44 (1999) 271–286
275
flux (e.g. Copin-Montegut & Avril, 1993; Miquel, Fowler, Larosa & Buatmenard,
1994).
2. Biomass and growth rate control of bacteria and phytoplankton
Views on population regulation in natural systems are generally dominated by two
alternative schools, leaving the impression that there is either a so-called ‘top-down’
control by predators, or a ‘bottom-up’ control through resource availability. In ladder-like structures of the type shown in Fig. 1, however, this dichotomy in analysis
may rapidly become confusing. It seems fruitful to recognise the fact that bacterial
production is the product of growth rate and biomass (e.g., Thingstad, Hagstrom &
Rassoulzadegan, 1997a). This allows the analysis to be separated into those factors
affecting biomass (top-down), growth rate (bottom-up), and also diversity
(Thingstad & Lignell, 1997b). For the heterotrophic bacteria in the lower left corner
of Fig. 1, the main controlling mechanisms usually considered are: the availability
of organic substrates (carbon and energy), available forms of nitrogen (N) or phosphorus (P), predation by (mainly) protozoans, and lysis by viruses (Fig. 2A). From
simple steady state models one would expect protozoan grazing to control the bacterial biomass, and viruses mainly to affect diversity, while growth rates may be
limited either by organic carbon, by mineral forms of N or P, or by a combination
of inorganic and organic forms of N and P (Thingstad & Lignell, 1997b).
Bacterial predation by protozoa has been a well studied phenomenon in the Mediterranean for many years (Rivier, Brownlee, Sheldon & Rassoulzadegan, 1985; Rassoulzadegan & Sheldon, 1986; Rassoulzadegan, Laval-Peuto & Sheldon, 1988; Sherr,
Rassoulzadegan & Sherr, 1989; Bernard & Rassoulzadegan, 1990; Wikner, Razzoulzadegan & Hagström, 1990; Rassoulzadegan, 1993) and is believed to be the mechanism that keeps bacterial numbers relatively constant. If heterotrophic flagellates
are assumed to be non-selective (any bacterium is a good bacterium) and to be the
only predators on bacteria, then the steady state requirement that loss of heterotrophic
flagellate biomass (H) equals growth gives the relationship:
YHaHBH⫽dHH,
where aH is the clearance rate of heterotrophic flagellates for bacteria (B) and dH is
the specific loss rate. Solving for bacterial abundance B, this gives:
B⫽
dH
.
YHaH
If we assume a turnover of the flagellate population once per day, dH=1.0 d ⫺1
(0.04 h⫺1), a ‘typical’ flagellate that needs in the order of 200 bacteria to divide
(YH=0.005), and have a maximum clearance rate of 10⫺5 ml h⫺1 (Fenchel, 1982),
insertion gives an estimated steady state population of bacteria of ⬇8 105 ml⫺1 ),
which is a typical value for the Mediterranean photic zone. On such a general level,
this theory thus seems consistent with observations.
276
T..F. Thingstad, F. Rassoulzadegan / Progress in Oceanography 44 (1999) 271–286
Fig. 2. Illustration of how (A) heterotrophic bacteria may be controlled ‘from below’ by the lack of
either labile organic-C (L-DOC) or by available forms of nitrogen (N) or phosphorus (P). If, for simplicity,
protozoan predators are assumed to be non-selective among the bacterial prey, total bacterial abundance
at steady state is controlled ‘from above’ by the protozoa. If viruses are host-specific, the main effect of
viruses at steady state may be on diversity inside the bacterial community (Thingstad & Lignell, 1997b).
If both bacterial and phytoplankton growth rate is phosphate limited in the Mediterranean, the simple
model in (B) suggests that that there is no stable algal-bacterial coexistence. Introducing selective predation as in (C), however, solves this theoretical problem.
Bacterial growth rate is often assumed to be C-limited (e.g. Tusseau, Lancelot,
Martin & Tassin, 1997). If this should be the case, it would greatly complicate further
analyses since bacterial production would be regulated by a set of many complex
and poorly quantified processes such as viral lysis (Middelboe, Jorgensen & Kroer,
1996), sloppy feeding (Jumars, Penry, Baross, Perry & Frost, 1989), and phytoplankton excretion (Myklestad, Holm-Hansen, Vorum & Volcani, 1989). If, however, bacterial growth rate is limited by the same mineral nutrients as phytoplankton growth
rate, then it is the competition with phytoplankton for mineral nutrient that deter-
T..F. Thingstad, F. Rassoulzadegan / Progress in Oceanography 44 (1999) 271–286
277
mines bacterial growth rate. Not only the biogeochemical evidence of skewed nitrate:phosphate ratios, but also direct evidence from flow cytometric analysis (Vaulot,
Lebot, Marie & Fukai, 1996), and bioassays (Fiala, Cahet, Jacques, Neveau & Panouse, 1976; Justic, Rabalais & Turner, 1995) have suggested P as the most important
limiting element for phytoplankton growth in Mediterranean stratified summer situations. Indications from both the north-western (Thingstad, Zweifel & Rassoulzadegan, 1998) and the eastern Mediterranean (Zohary & Robarts, 1998) indicate strongly
that the growth rate of not only phytoplankton, but also of heterotrophic bacteria is Plimited during stratification. P-limited bacteria, and a resulting competition between
bacteria and photosynthetic organisms, has also been suggested in Mediterranean
benthic seagrass communities (López, Duarte, Vallespinós, Romero & Alcoverro,
1995). Interestingly, P-limited bacterial growth rates have recently been reported also
in other truly marine pelagic environments (Pomeroy, Sheldon, Sheldon & Peters,
1995; Cotner, Ammerman, Peele & Bentzen, 1997), as well as in fresh water influenced coastal areas (Thingstad, Skjoldal & Bohne, 1993; Zweifel, Norrman & Hagstrom, 1993), suggesting that P-limited growth of marine bacteria is probably not a
phenomenon exclusive to the Mediterranean.
If both bacteria and phytoplankton growth rates are P-limited (Fig. 2B), this leads
us into a theoretical problem which actually can be seen as extending Hutchinson’s
classical ‘paradox of the plankton’ (Hutchinson, 1961) also to include heterotrophic
bacteria. Since bacteria with their superior surface:volume ratio are usually assumed
to be the best competitors at low orthophosphate concentrations, the problem can be
illustrated by the following variation of the theme of such an extended Hutchinson’s paradox:
Why do not the P-limited bacteria outcompete phytoplankton until phytoplankton
biomass is reduced to a level where the systems’ production of organic-C is so
low that bacteria become C-limited?
Probably the simplest theoretical solution to this paradox is to include predation
in the models. Competing species can coexist stably on one common resource if
there is a mechanism such as predation, that selectively kills the winner (Fig. 2C).
A higher predation pressure on the superior competitor thus solves the theoretical
problem (Thingstad, Hagstrom & Rassoulzadegan, 1997a) without the need to infer
variations in time and space to explain the coexistence. A consequence of such a
model is that it allows for a system where heterotrophic bacteria, being ‘sandwiched’
between predation and competition, do not respond to increased concentrations of
free organic carbon, either in their growth rate or in thir biomass. The theoretical
consequence is that labile DOC may accumulate and thus potentially be subject to
(photo)chemical processes changing its availability as bacterial substrate (Benner &
Biddanda, 1998).
Although not altering the general conclusion of the need for a mechanism selectively killing the winner, some experimental evidence may, however, shed doubt on
the traditional assumption that heterotrophic bacteria invariably are such superior
competitors. The ability of an osmotrophic organism to compete at permanently low
278
T..F. Thingstad, F. Rassoulzadegan / Progress in Oceanography 44 (1999) 271–286
substrate concentrations is given by its maximum specific affinity (a). This is the
slope of the uptake-versus-substrate curve as substrate concentration approaches zero.
The interpretation of specific affinity is the volume of water cleared for substrate
per unit biomass per unit time, and specific affinity is thus analogous to the clearance
rate of a phagotrophic organism. Few experimental data on bacterial affinity seem
to be available, but recently published investigations on cultured isolates led Vadstein
(1997) to suggest that the difference between phytoplankton and bacterial affinity
may not be as clear as hitherto assumed. We can estimate the specific affinity in
natural populations from experimental data following the procedure suggested in
Table 2. Using typical values from Villefranche Bay (north-west Mediterranean) for
bacterial counts and chlorophyll as biomass measures, the resulting estimates for
bacterial and phytoplankton specific affinities are comparable to those obtained by
Vadstein (1997) for cultured bacteria. Values for phytoplankton and heterotrophic
bacteria are comparable (Table 2), supporting the suggestion that bacterial superiority
should be viewed with some caution. Estimates such as these are, however, subject
to large uncertainties. A major uncertainty stems from the recent proposition that
only a minor portion of the bacteria seen in the epifluorescence microscope are active
(Zweifel & Hagström, 1995). Any reduction in the biomass of active bacteria used
in the calculations would increase the a-estimate for bacteria correspondingly.
The numerical value of the affinity estimates in Table 2 deserves another comment.
Based on the assumption that the cell is diffusion limited, i.e. that the cell’s uptake
system is so efficient (and the bulk nutrient concentration so low) that all substrate
molecules hitting the cell surface are captured, it is possible to derive a theoretical
expression for maximum affinity for a spherical cell of radius r (Thingstad & Lignell, 1997b):
a⫽
3D
,
sr2
(1)
where D is the diffusion constant for the substrate molecules, and s is the volume
specific content of the element in question. Inserting D⬇10⫺5 cm2 s⫺1 for a small
molecule such as orthophosphate, and assuming a phytoplankton cell to have a density of 1.2 g cm⫺3, 50% of wet weight to be dry weight, 50% of carbon to be dry
weight, and a molar C:P-ratio of 106 in phytoplankton biomass, we get s⬇0.24
µmol-P cm⫺3. The estimated phytoplankton affinity of 0.016 l nmol-P⫺1 h⫺1 in Table
2 then corresponds to a diffusion limited cell of radius of about 1.7 µm. For a cell
of radius 1 µm, corresponding approximately to that of unicellular cyanobacteria in
the Mediterranean, we get an estimated a1µm⬇0.046 l nmol-P⫺1 h⫺1. Although
imprecise, these rough estimates suggest that phytoplankton cells larger than about
2–3 µm may be diffusion limited in Villefranche Bay summer surface waters.
Chemical determination of bioavailable orthophosphate Sn in P-deficient waters is
difficult, both because values may be close to the detection limit, and because of
uncertainties as to whether the measured values of SRP (Soluble Reactive
Phosphorus), when obtained, really represent Sn. To overcome this problem, we have
used a somewhat intricate way of estimating Sn by combining measured loss rate of
P from the bacterial size-fraction, turnover time measured for radioactively labelled
T..F. Thingstad, F. Rassoulzadegan / Progress in Oceanography 44 (1999) 271–286
279
Table 2
Procedure suggested for estimating affinity constants from experimental data
Uptake is usually described as a hyperbolic function of substrate concentration. When substrate
concentration is sufficiently low, the initial part of the curve can be approximated by a linear
relationship, e.g. where uptake is proportional to substrate concentration. In the case of balanced,
substrate limited growth, bacterial uptake must equal growth so that mB=aBSn where mB is specific
growth rate, aB is the bacterial affinity, and Sn is the natural concentration of substrate in the water.
Multiplying both sides with the biomass B and solving for aB we get aB=[(mB/Sn)/B]. If B is in
units of bacterial biomass-P, mB is the bacterial incorporation rate of P. If f is the fraction of total
incorporation going into bacteria, mB/f is the total incorporation rate of phosphate, and mBB/fSn=1/Tt
where Tt is the turnover-time for orthophosphate in the natural environment. We thus get:
f
aB =
TtB
Since f and Tt can be estimated from size-fractionated uptake of 33P or 32P labelled orthophosphate
(Dolan et al., 1995), and B can be estimated from microscopic determination of bacterial biomass or
size-fractionation of particulate-P, this allows estimation of an ‘apparent’ aB. If substrate
concentration is beyond the range of linear uptake, this ‘apparent’ aB should be smaller than the
‘true’ value. ‘Apparent’ affinities estimated through this procedure should therefore provide lower
limits for the true values.
Similarly:
(1−f)
aA=
TtA
where aA is the affinity of phytoplankton, A is P in algal biomass, and only phytoplankton and
bacteria have been assumed to incorporate P from orthophosphate. If an estimate for A is available
from e.g. a conversion from chlorophyll, also aA can be obtained.
Although f and Tt in our experience can vary widely, ‘typical’ value for Villefranche Bay (northwest Mediterranean) could be 0.5 and 1.6 h, respectively (Dolan et al., 1995). A ‘typical’ bacterial
abundance of 0.8 106 bacteria ml⫺1 corresponds to B⬇27 nmol-P l⫺1 if a carbon content of 20 fg C
cell⫺1 and a molar C:P ratio of 50:1 is assumed. This fits well with particulate-P measured in the
size fraction 1–0.2 µm (Dolan et al., 1995). The corresponding ‘apparent’ aB is thus in the order of
aB⬇0.012 l nmol-P⫺1h⫺1.
Using 0.5 µg Chl l⫺1 as a ‘typical’ chlorophyll concentration (Dolan et al., 1995). A Chl:C ratio of
50 and a molar Redfield C:P ratio of 106, gives an estimated A⬇20 nmol-P l⫺1. This is about 50% of
the particulate-P ⬎1 µm reported by Dolan et al. (1995), which again gives a reasonable agreement
since parts of the measured particulate-P in this size fraction would be expected to be in protozoa.
The corresponding aA would be in the order of aA⬇0.016 l nmol-P⫺1 h⫺1.
orthophosphate, and the fraction of this label estimated to go into heterotrophic bacteria. In Villefranche Bay (Thingstad, Dolan & Fuhrman, 1996) as well as in the
brackish layer of a Norwegian fjord (Thingstad, Skjoldal & Bohne, 1993), this procedure led to estimates of Sn⬇0.8 nmol-P l⫺1. To put this number into perspective,
a water volume 1000 times that of a bacterium would, at this concentration, contain
in the order of 30 free phosphate molecules. If we combine this concentration with
the estimated affinity a1µm⬇0.046 l nmol-P⫺1 h⫺1, the corresponding growth rate
becomes mA=aASn⬇0.88 d ⫺1. This is close to the growth rate of 0.95 d ⫺1 determined
280
T..F. Thingstad, F. Rassoulzadegan / Progress in Oceanography 44 (1999) 271–286
from flow cytometric analysis for the native Synechococcus population of Villefranche Bay (Jacquet, Lennon, Marie & Vaulot, 1999). As a conclusion from the present
state of knowledge, it seems that the suggestion of diffusion limited phytoplankton
growth, the low estimates of bioavailable orthophosphate concentration, and the relatively short measured generation times for the small phytoplankton species, together
form a consistent picture.
Iron-containing dust from Sahara has been suggested to scavenge phosphate from
the Mediterranean pelagic system (Krom, Kress, Brenner & Gordon, 1991). Using
electron microscope X-ray analysis Mostajir, Fagerbakke, Heldal, Thingstad and Rassoulzadegan (1998) found, however, little P in inorganic particles from the NW Mediterranean. Possibly, the effect of Saharan dust may be slightly more complicated
than suggested so far. The phosphate it originally carries may be exchanged with
the high-affinity microbes in the surface waters, locally stimulating biological activity
in the surface layer. This possibility remains, however, that the dust particles may
scavenge phosphate when sinking deeper into the aphotic zone, and thus in effect
move the nutricline downwards.
3. Size spectrum of osmotrophs
The phosphate-bacteria-algae-protozoa system of Fig. 2(C) can be generalised a
step further by adding more size classes of phytoplankton and their corresponding
predators (Fig. 3). Comparison of Figs. 1 and 3 should reveal the emerging similarity
of our theoretical construction to the more generally accepted ideas of carbon flow
in food webs. The steady states of structures such as that in Fig. 3 can be analysed
as functions of the nutrient richness of the system, i.e. of the total amount ST of the
limiting element available for sharing between all particulate and dissolved pools in
the food web. It then turns out that the concentration of free limiting nutrients Sn,
as a general trend, will increase with increasing ST (Thingstad & Sakshaug, 1990).
Fig. 3. Generalisation of the food web in Fig. 2(C), to include mesozooplankton top-predators and two
size classes of phytoplankton and of protozoa. Solid arrows represents P-flows (recycling pathways omitted for simplicity). The dotted arrows illustrate the needed input of labile organic matter and silicate, and
thus their symmetric role in potentially limiting the P-flow through the left ‘microbial’, and right ‘classical’
sides of the food web, respectively.
T..F. Thingstad, F. Rassoulzadegan / Progress in Oceanography 44 (1999) 271–286
281
With higher substrate concentrations Sn, new, larger phytoplankton species, although
having lower a than those already present, may potentially become established
towards the ‘classical’ side of the food web. In apparent agreement with such a view,
the distribution of 32PO4-uptake within the phytoplankton size-range (⬎1 mm) was
found to shift towards larger organisms in parallel with the destabilisation of the
water column occurring during autumn cooling of the top layer in Villefranche Bay
(Dolan, Thingstad & Rassoulzadegan, 1995). The presently available evidence thus
suggest that the Mediterranean system fits into the series of observations supporting
the theory that nutrient enrichment stimulates the ‘classical’ right side of Fig. 3 (e.g.
Kiørboe, 1993).
The present impression from experimental observations is that, in oligotrophic
areas, biomass of heterotrophic bacteria becomes increasingly dominant relative to
phytoplankton (Fuhrman, Sleeter, Carlson & Proctor, 1989; Cho & Azam, 1990). A
steady state analysis of the idealised food web in Fig. 2(C) gives such a decrease
in the bacteria:phytoplankton biomass ratio with decreasing total nutrient content ST,
as long as bacterial growth rate is mineral nutrient limited (Thingstad, Hagstrom &
Rassoulzadegan, 1997a). In the Mediterranean, the expected pattern emerging from
such a theory would be an increasing relative dominance of heterotrophic bacteria
in the eastern parts. In accordance with such expectations, Van Wambeke, Christaki,
Bianchi, Psarra and Tselepides (2000) reports such dominance of heterotrophic bacterial biomass, with the exception of a coastal station in spring. Robarts, Zohary,
Waiser and Yacobi (1996), however, found the effect to be less pronounced with
bacterial biomass about 50% of phytoplankton biomass.
The establishment of new, larger species on the ‘classical’ side of the food web,
depends upon the shift in balance between loss and growth processes. The positive
effect of reduced loss resulting from reduced predation must overcome the negative
effects of reduced a and of increased sinking loss for the large cells. For a nonswimming osmotrophic cell with density higher than seawater, an expression for the
minimum concentration SC of limiting nutrient at which it can maintain a stable
population, can be derived from the requirement that diffusion limited growth must
balance losses by sinking. Since Stoke’s law gives sinking rate as the second power
of cell radius, combination of Stoke’s law with the expression in Eq. (1) above,
gives an expression for this minimum concentration which goes as the fourth power
of cell radius (Thingstad, 1998):
2g(r−r)s 4
S C⫽
r.
27hhD
(2)
The proportionality factor depends on the gravitational constant (g), the viscosity of
water (h), the depth of the mixed layer (h) and the diffusion constant for the substrate
(D), the net buoyancy (r⫺rW), and the volume-specific content of the limiting
element. Since Eq. (2) ignores the effect of increased nutrient supply on large sinking
cells, the fourth power relationship is in effect a kind of ‘worst case’ estimate. From
such considerations, however, it is not surprising that oligotrophic areas like the
Mediterranean have phytoplankton populations that are dominated by small phytoplankton species. The more interesting question is how larger species such as diatoms
282
T..F. Thingstad, F. Rassoulzadegan / Progress in Oceanography 44 (1999) 271–286
ever become established. In the Mediterranean, a typical habitat for diatoms when
they are present, is in the deep chlorophyll maximum situated on the boundary
between where there is sufficient available light in the upper part of the water column, and the upper boundary of available free nutrients in the deeper, aphotic zone
(Estrada, 1985). Here they presumably benefit from diffusion and from intermittent
hydrographic phenomena that inject nutrient rich deep water up into the base of the
photic zone (Estrada, 1996). With the strong penalty for being large suggested above,
one may speculate whether nutrient enrichment alone is a sufficient mechanism to
explain the occurrence of these larger phytoplankton species. If a 1 µm cell needs
a concentration around 1 nmol-P l⫺1, a 10 µm cell would, in the extreme case, need
a concentration of 10 µmol l⫺1 which is much higher than observed in Mediterranean
deep waters (e.g., Redfield, Ketchum & Richards, 1963). It is, therefore, tempting
to speculate that larger phytoplankton species must have special strategies to counteract the proposed size-penalty. In the proportionality factor of Eq. (2), the only two
factors readily available for phytoplankton adaptation strategies are their buoyancy
(r), and their volume-specific nutrient content (s). A particularly important feature
of diatoms in this context is their vacuole (Egge, 1998), which means that the cytoplasm is concentrated along their silicate wall. This is probably the reason for diatoms
having a lower volume-specific carbon content than flagellates, and a carbon content
that is correlated better with plasma volume than with total cell volume (Smayda,
1978). Assuming P only to be in the cytoplasm, and a constant cytoplasm thickness
of ⬇1 µm, the relative advantage of diatoms over flagellates of the same size would,
theoretically, increase with cell radius (Fig. 4). Adding the possibility that the vacuole
may play a role in buoyancy regulation and perhaps also in storage of P during rapid
luxury uptake, it would seem that diatoms may have a means to ‘look large’ to
predators while still being efficient in terms of nutrient uptake, combined with a
means to reduce sinking loss rate. Thus it may be speculated that diatoms may have
evolved a strategy particularly well suited for rapid exploitation of ephemeral situations where nutrient enrichment opens for the possibility for larger phytoplankton
species to become established. The price for this seems to be the need for silicate.
However, in an environment such as the north-west Mediterranean, where the nutricline for silicate does not seem to be situated deeper than that for phosphate (e.g.,
Estrada, 1985), natural P-enrichment episodes would generally be expected also to
supply silicate. Mesocosm experiments in Norwegian waters have suggested that Pdeficiency may reduce the competitive abilities of diatoms, relative to that in Ndeficient systems (Egge, 1998). The observed occurrence of diatoms (Estrada, 1985),
however, does not suggest that such an effect prevents diatom establishment in Mediterranean deep chlorophyll maxima. Based on pigment signatures Claustre (1994)
found both diatom and dinoflagellate pigments to increase linearly with chlorophyll,
whereas other pigments did not correlate with chlorophyll. Within our theoretical
framework, autotrophic dinoflagellates would be able to reduce the fourth power size
penalty by swimming. For large cells, swimming can not only compensate for sinking, but will also increase nutrient flux to the cell (Lazier & Mann, 1989). Dinoflagellates are, however, usually (though not always (Chang & Carpenter, 1994))
T..F. Thingstad, F. Rassoulzadegan / Progress in Oceanography 44 (1999) 271–286
283
Fig. 4. Theoretical reduction in maximum specific affinity with cell size when the difference between
diatoms and flagellates is the assumption of diatoms having a vacuole free of P, and a fixed cytoplasm
thickness of 1 µm. Since the y-axis is logarithmic, the distance between the curves gives the ratio between
the two affinities.
assumed to have low maximum growth rates, and may need time to fill niches temporarily opened to the ‘classical’ side of the food web.
Conceptually, these arguments lead us to a view of the pelagic food web where
L-DOC and Si have somewhat symmetric roles in regulation of the P-flow through
the food web. For the system in Fig. 3, there is one steady state of the P-flow through
the system when both L-DOC and Si are in excess of the need of heterotrophic
bacteria and diatoms, respectively. If L-DOC supply sinks below this, P-flow through
the left, ‘microbial’ side is restricted, if Si-supply is insufficient, P-flow through the
right ‘classical’ side is restrained (at least until this niche can be filled by
dinoflagellates). The size selective grazing central to this concept, resembles that of
Sheldon’s theory for the particle size-spectrum (Sheldon, Prakash & Sutcliffe, 1972).
In Sheldon’s analysis, growth rate is, however a property determined by cell size.
In the concept suggested here, the size spectrum is much less fixed because growth
rate is a function of food availability and its potential modification by L-DOC and
silicate. The size spectrum of mineral nutrient uptake should, from theoretical con-
284
T..F. Thingstad, F. Rassoulzadegan / Progress in Oceanography 44 (1999) 271–286
siderations, greatly influence the amount of carbon reaching trophic levels of potential importance as food for fish (Suttle, Fuhrman & Capone, 1990). Since the mechanisms for production, import, and loss for L-DOC and Si are very different, this
picture allows for large variations in the structure of the photic zone food web and
its coupling to higher trophic levels, as Si and phosphate is imported and lost through
the seasons and with upwelling events.
Acknowledgements
This work was financed by the EU MAST-III programme through contract MAS3CT95-0016 ‘MEDEA’.
References
Azam, F., Fenchel, T., Field, J. G., Gray, J. S., Meyer-Reil, L. A., & Thingstad, T. F. (1983). The
ecological role of water-column microbes in the sea. Marine Ecology Progress Series, 10, 257–263.
Benner, R., & Biddanda, B. (1998). Photochemical transformations of surface and deep marine dissolved
organic matter: Effects on bacterial growth. Limnology and Oceanography, 43, 1373–1378.
Bernard, C., & Rassoulzadegan, F. (1990). Bacteria or microflagellates as a major source for marine
ciliates: possible implications for the microzooplankton. Marine Ecology Progress Series, 64, 147–155.
Bianchi, M., Gaudy, R., Goutx, M., Leveau, M., Lochet, F., Pagano, M., Sotomercado, Y., & Videau,
C. (1993). Trophic relationships between pelagic components of a mesoscale front—the Rhone river
plume front ecosystem. Annales De l’Institut Oceanographique, 69, 57–62.
Chang, J., & Carpenter, E. J. (1994). Active growth of the oceanic dinoflagellate Ceratium-teres in the
Caribbean and Sargasso seas estimated by cell-cycle analysis. Journal of Phycology, 30, 375–381.
Cho, B. C., & Azam, F. (1990). Biogeochemical significance of bacterial biomass in the ocean’s euphotic
zone. Marine Ecology Progress Series, 63, 253–259.
Claustre, H. (1994). The trophic status of various oceanic provinces as revealed by phytoplankton pigment
signatures. Limnology and Oceanography, 39, 1206–1210.
Copin-Montegut, G., & Avril, B. (1993). Vertical distribution and temporal variation of dissolved organic
carbon in the North-Western Mediterranean Sea. Deep-Sea Research I, 40, 1963–1972.
Cotner, J. B., Ammerman, J. W., Peele, E. R., & Bentzen, E. (1997). Phosphorus-limited bacterioplankton
growth in the Sargasso Sea. Aquatic Microbiology Ecology, 13, 141–149.
Dolan, J., Thingstad, T. F., & Rassoulzadegan, F. (1995). Phosphate transfer between size fractions in
Villefranche Bay (N.W. Mediterranean Sea), France, in Autumn 1992. Ophelia, 41, 71–85.
Egge, J. (1998). Are diatoms poor competitors at low phosphate concentrations? Journal of Marine Systems, 16, 191–198.
Elbazpoulichet, F., Garnier, J. M., Guan, D. M., Martin, J. M., & Thomas, A. J. (1996). The conservative
behaviour of trace metals (Cd, Cu, Ni and Pb) and As in the surface plume of stratified estuaries:
Example of the Rhone River (France). Estuarine Coastal and Shelf Science, 42, 289–310.
Estrada, M. (1985). Deep phytoplankton and chlorophyll maxima in the western Mediterranean. In M.
Moraitou-Apostolopoulou, & V. Kiortsis, Marine Mediterranean ecosystems (pp. 247–277). New
York: Plenum Press.
Estrada, M. (1996). Primary production in the northwestern Mediterranean. Scientia Marina, 60, 55–64.
Fenchel, T. (1982). Ecology of heterotrophic microflagellates. II Bioenergetics and growth. Marine Ecology Progress Series, 8, 223–225.
Fenchel, T. (1987). Ecology—potentials and limitations. Oldendorf/Luhe, Ecology Institute.
Fiala, M., Cahet, G., Jacques, G., Neveux, J., & Panouse, M. (1976). Fertilization de communautés phyto-
T..F. Thingstad, F. Rassoulzadegan / Progress in Oceanography 44 (1999) 271–286
285
planctoniques. I. Cas d’un milieu oligotrophique: Mediterranée nord-occidentale. Journal of Experimental Marine Biology and Ecology, 24, 151–163.
Fuhrman, J. A., Sleeter, T. D., Carlson, C. A., & Proctor, L. M. (1989). Dominance of bacterial biomass
in the Sargasso Sea and its ecological implications. Marine Ecology Progress Series, 57, 207–217.
Hutchinson, G. E. (1961). The paradox of the plankton. American Naturalist, 95, 137–145.
Jacquet, S., Lennon, J.-F., Marie, D., & Vaulot, D. (1999). Picoplankton population dynamics in coastal
waters of the N.W. Mediterranean Sea. Limnology and Oceanography, 43, 1916–1931.
Johannes, R. E. (1965). Influence of marine protozoa on nutrient regeneration. Limnology and Oceanography, 10, 434–442.
Jumars, P. A., Penry, D. L., Baross, J. A., Perry, M. J., & Frost, B. W. (1989). Closing the microbial
loop: Dissolved carbon pathway to heterotrophic bacteria from incomplete ingestion, digestion and
absorption in animals. Deep-Sea Research, 36, 483–495.
Justic, D., Rabalais, N. N., & Turner, R. E. (1995). Stoichiometric nutrient balance and origin of coastal
eutrophication. Marine Pollution Bulletin, 30, 41–46.
Kiørboe, T. (1993). Turbulence, phytoplankton cell size, and the structure of pelagic food webs. Advances
in Marine Biology, 29, 1–71.
Krom, M. D., Kress, N., Brenner, S., & Gordon, L. I. (1991). Phosphorus limitation of primary productivity in the eastern Mediterranean Sea. Limnology and Oceanography, 36, 424–432.
Lacombe, H., Gascard, J. C., Gonella, J., & Bethoux, J. P. (1981). Response of the Mediterranean to the
water and energy fluxes across its surface, on seasonal and interannual scales. Oceanologia Acta, 4,
247–255.
Lazier, J. R. N., & Mann, K. H. (1989). Turbulence and the diffusive layers around small organisms.
Deep-Sea Research, 36, 1721–1733.
López, N. I., Duarte, C. M., Vallespinós, F., Romero, J., & Alcoverro, T. (1995). Bacterial activity in
NW Mediterranean seagrass (Posidonia oceanica) sediments. Journal of Experimental Marine Biology
and Ecology, 187, 39–49.
MEDOC Group (1970). Observation of formation of deep water in the Mediterranean Sea. Nature, London, 277, 1037–1040.
Middelboe, M., Jorgensen, N. O. G., & Kroer, N. (1996). Effects of viruses on nutrient turnover and
growth efficiency of noninfected marine bacterioplankton. Applied Environmental Microbiology, 62,
1991–1997.
Miquel, J. C., Fowler, S. W., Larosa, J., & Buatmenard, P. (1994). Dynamics of the downward flux of
particles and carbon in the open northwestern Mediterranean-sea. Deep-Sea Research I, 41, 243–261.
Mostajir, B., Fagerbakke, K. M., Heldal, M., Thingstad, T. F., & Rassoulzadegan, F. (1998). Elemental
composition of individual pico- and nano-sized marine detrital particles in the North Western Mediterranean Sea, using transmission electron microscopy and X-ray microanalysis. Oceanologica Acta, 21,
589–596.
Myklestad, S., Holm-Hansen, O., Vørum, K. M., & Volcani, B. E. (1989). Rate of release of extracellular
amino acids and carbohydrates from the marine diatom Chaetocheros affinis. Journal of Plankton
Research, 11, 763–773.
Pomeroy, L. R. (1974). The ocean’s food web: A changing paradigm. BioScience, 24, 499–504.
Pomeroy, L. R., Sheldon, J. E., Sheldon, W. M. J., & Peters, F. (1995). Limits to growth and respiration
of bacterioplankton in the Gulf of Mexico. Marine Ecology Progress Series, 117, 259–268.
Rassoulzadegan, F. (1993). Protozoan patterns in the Azam–Ammerman’s bacteria–phytoplankton mutualism. In R. Guerrero, & C. Pedros-Alio, Trends in Microbial Ecology (pp. 435–439). Spanish Society
for Microbiology.
Rassoulzadegan, F., Laval-Peuto, M., & Sheldon, R. W. (1988). Partitioning of the food ration of marine
ciliates between pico- and nanoplankton. Hydrobiologia, 159, 75–88.
Rassoulzadegan, F., & Sheldon, R. W. (1986). Predator–prey interactions of nanozooplankton and bacteria in an oligotrophic marine environment. Limnology and Oceanography, 31, 1010–1021.
Redfield, A. C., Ketchum, B. H., & Richards, F. A. (1963). The influence of organisms on the composition
of seawater. In M. N. Hill, The sea 2 (pp. 26–77). Chichester: John Wiley and Sons.
Rivier, A., Brownlee, D. C., Sheldon, R. W., & Rassoulzadegan, F. (1985). Growth of microzooplankton:
a comparative study of bactivorous zooflagellates and ciliates. Marine Microbial Food Webs, 1, 51–60.
286
T..F. Thingstad, F. Rassoulzadegan / Progress in Oceanography 44 (1999) 271–286
Robarts, R. D., Zohary, T., Waiser, M. J., & Yacobi, Y. Z. (1996). Bacterial abundance, biomass, and
production in relation to phytoplankton biomass in the Levantine Basin of the southeastern Mediterranean Sea. Marine Ecology Progress Series, 137, 273–281.
Sheldon, R. W., Prakash, A., & Sutcliffe, W. H. (1972). The size distribution of particles in the ocean.
Limnology and Oceanography, 17, 327–340.
Sherr, E. B., Rassoulzadegan, F., & Sherr, B. (1989). Bacterivory by pelagic choreotrichous ciliates in
coastal waters of the NW Mediterranean Sea. Marine Ecology Progress Series, 55, 235–240.
Smayda, T. (1978). From phytoplankton to biomass. In A. Sournia, Phytoplankton manual (pp. 273–279).
Paris: UNESCO.
Steele, J. H. (1974). The structure of marine ecosystems. Cambridge, MA: Harvard University Press.
Suttle, C., Fuhrman, J., & Capone, D. (1990). Rapid ammonium cycling and concentration-dependent
partitioning of ammonium and phosphate: Implications for carbon transfer in planktonic communities.
Limnology and Oceanography, 35, 424–433.
Thingstad, T. F. (1998). A theoretical approach to structuring mechanisms in the pelagic food chain.
Archiv. für Hydrobiologie, 363, 59–72.
Thingstad, T. F., Dolan, J. R., & Fuhrman, J. A. (1996). Loss rate of an oligotrophic bacterial assemblage
as measured by H-3-thymidine and (PO4)-P-32: Good agreement and near-balance with production.
Aquatic Microbial Ecology, 10, 29–36.
Thingstad, T. F., Hagstrøm, Å., & Rassoulzadegan, F. (1997a). Export of degradable DOC from oligotrophic surface waters: caused by a malfunctioning microbial loop? Limnology and Oceanography,
42, 398–404.
Thingstad, T. F., & Lignell, R. (1997b). A theoretical approach to the question of how trophic interactions
control carbon demand, growth rate, abundance, and diversity. Aquatic Microbiology and Ecology,
113, 19–27.
Thingstad, T. F., & Sakshaug, E. (1990). Control of phytoplankton growth in nutrient recycling ecosystems. Theory and terminology. Marine Ecology Progress Series, 63, 261–272.
Thingstad, T. F., Skjoldal, E. F., & Bohne, R. A. (1993). Phosphorus cycling and algal–bacterial competition in Sandsfjord, western Norway. Marine Ecology Progress Series, 99, 239–259.
Thingstad, T. F., Zweifel, U. L., & Rassoulzadegan, F. (1998). P-limitation of both phytoplankton and
heterotrophic bacteria in NW Mediterranean summer surface waters. Limnology and Oceanography,
43, 88–94.
Tusseau, M.-H., Lancelot, C., Martin, J.-M., & Tassin, B. (1997). 1-D coupled physical–biological model
of the northwestern Mediterranean Sea. Deep-Sea Research II, 44, 851–880.
Vadstein, O. (1997). Evaluation of competitive ability of two heterotrophic planktonic bacteria under
phosphorus limitation. Aquatic Microbiology and Ecology, 14, 119–127.
Van Wambeke, F., Christaki, U., Bianchi, M., Psarra, S., Tselepides, A. (2000). Heterotrophic bacterial
production in the south Aegean Sea (NE Mediterranean). Progress in Oceanography, (in press).
Vaulot, D., Lebot, N., Marie, D., & Fukai, E. (1996). Effect of phosphorus on the Synechococcus cell cycle
in surface Mediterranean waters during summer. Applied Environmental Microbiology, 62, 2527–2533.
Wikner, J., Razzoulzadegan, F., & Hagström, Å. (1990). Periodic bacterivore activity balances bacterial
growth in the marine environment. Limnology and Oceanography, 35, 313–325.
Williams, P. J. L. (1981). Incorporation of microheterotrophic processes into the classical paradigm of
the planktonic food web. Kieler Meeresforsch, 5, 1–28.
Wüst, G. (1961). On the vertical circulation of the Mediterranean Sea. Journal of Geophysical Research,
66, 3261–3271.
Zohary, T., & Robarts, R. D. (1998). Experimental study of microbial P limitation in the eastern Mediterranean. Limnology and Oceanography, 43, 387–395.
Zweifel, U. L., & Hagström, Å. (1995). Total counts of marine bacteria include a large fraction of nonnucleoid containing ‘ghosts’. Applied Environmental Microbiology, 61, 2180–2185.
Zweifel, U. L., Norrman, B., & Hagstrøm, Å. (1993). Consumption of dissolved organic carbon by marine
bacteria and demand for inorganic nutrients. Marine Ecology Progress Series, 101, 23–32.