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International audience; Inspired by agroecology, ecological aquaculture proposes an alternative model that uses ecology as a paradigm to develop innovative, more eco-friendly aquaculture with environmental, economic and social benefits. Integrated multi-trophic aquaculture (IMTA) is one application of this principle. Inspired by the natural trophic chain, it associates primary producers with primary or secondary consumers, providing a new source of biomass without requiring supplementary feed by recycling inorganic and organic wastes. Of these systems, land-based IMTAs demonstrate several advantages, especially easier control of nutrient flows, contaminants and/or predators. This study focused on a land-based marine IMTA, combining a recirculating aquaculture system for fish consecutively with a natural marine polyculture of microalgae and oyster cultivation. The objective was to assess the ability of the microalgal polyculture both to bioremediate fish nutrients and to sustain oyster growth. For the first time in a Mediterranean climate, we confirmed the feasibility of developing a microalgae community of interest for oysters maintained by fish effluent. Despite strong variability in microalgae production, this IMTA system resulted in significant oyster growth over the experimental period of one month, with growth results of the same order of magnitude as natural juvenile growth. In the conditions tested, this IMTA with reduced human intervention allowed a gain in recoverable biomass of 3.7 g of oyster produced per kg of fish feed distributed. By transforming waste into additional biomass, IMTAs offer a more promising, ecological avenue for aquaculture, based on a circular economy, which may in turn increase the social acceptability of fish farming.

International audience; Light is a key environmental factor for the growth of micro-algae, and optimizing the capture of light is critical for high efficiency production systems. As the density of the population of micro-algae increases, the availability of light decreases, leading to a reduction in the growth rate because of mutual shading, while other effects, such as photo-inhibition, might be especially frequent when the population density is low. Several models in the literature have been developed to take into account light phenomena and predict micro-algal growth, particularly in a mono-culture. With the help of a simple expression for the attenuation of the light, we propose and justify a new growth function that incorporates both photo-inhibition and photolimitation. In agreement with the experimental data, this new formulation describes the micro-algal response to a wide range of situations of excessive or insufficient light intensities through an explicit dependence on both the incident light and the biomass concentration. While simple, the proposed expression can be satisfactorily applied to practical cases under nutrient replete conditions in photo-bioreactors with different sizes and geometries. It extends naturally to the growth of different species, providing a dynamic model which can simulate experiments in a mono-culture as well as in polycultures. The investigation of the competition for light-limited growth shows that the model predicts competitive exclusion, which has also been experimentally demonstrated. This leads to new perspectives for the control and optimization of mixed micro-algal cultures.

International audience; The productivity of most marine ecosystems is limited by the availability of dissolved nitrogen (N) and phosphorus (P). Nutrient recycling is therefore a key process for ecosystem functioning. Fish recycle nutrients through the excretion of ammonia and phosphate and can influence the abundance and community structure of primary producers such as phytoplankton. However, the effect of fish on other plankton compartments, and whether all fish species have similar effects, is largely unknown. We used a tank experiment to test how 2 Mediterranean fish species, gilthead seabream Sparus aurata and golden mullet Chelon auratus, with distinctly different N and P excretion rates, can affect the abundance and community structure of 3 plankton compartments: phytoplankton, bacterioplankton, and microzooplankton. We found that the nutrients released by seabream (whose excreta had an N:P molar ratio greater than the Redfield ratio of 16:1) induced a substantial increase in the abundance of all plankton compartments. In addition, with seabream, the relative abundance of diatoms in the phytoplankton communities increased. However, no significant change was observed with mullet, which had a low excreta N:P molar ratio, suggesting that the growth of microbial plankton was limited by the availability of N. Our results demonstrate that nutrient excretion by fish affects the microbial food web through a species-specific bottom-up effect on the total abundance and community structure of the phytoplankton, bacterioplankton, and microzooplankton communities.

International audience; Recently, CO2 recycling for the production of valuable microalgae has acquired substantial interest. Most studies investigating CO2 conversion efficiency in algal cultures were based on single species, although a stabilising effect of algal diversity on biomass production was recently highlighted. However, addition of CO2 into polyalgal cultures requires a careful control of pH; performance of CO2 conversion, growth and carbon biomass production are affected by pH differently, depending on the species of microalgae. This study investigates the efficiency of CO2 conversion by natural marine algal assemblage cultivated in open, land-based raceways (4.5 m3, 10 m2), working as high rate algal ponds (HRAP). Ponds were enriched with nitrogen and phosphate, pure CO2 was added and algal cultures were grown under three different fixed pH levels: pH 6, 7 and 8. The highest conversion of photosynthetically fixed CO2 into carbon biomass (40%) was reached at pH 7, an intermediate level, due to the partial CO2 asphyxiation of algal predators (copepods, ciliates), while being under the suboptimal conditions for the development of marine amoebae. Under this pH, the theoretical maximal biological conversion of available CO2 into carbon biomass was estimated to be 60% in naturally inoculated open ponds.

International audience; We address the problem of determining functional framing from experimental data points in view of robust time-varying predictions, which is of crucial importance in bioprocess monitoring. We propose a method that provides guaranteed functional bounds, instead of sets of parameters values for growth functions such as the classical Monod or Haldane functions commonly used in bioprocess modeling. We illustrate the applicability of the method with bioreactor simulations in batch and continuous mode, as well as on real data. We also present two extensions of the method adding flexibility in its application, and discuss its efficiency in providing guaranteed state estimations.

International audience; Dziani Dzaha is a maar the age of which is close to 4000 years. While its water is thought to have originated from seawater it is now considered as an extreme environment due to its hypersaline and alkaline characteristics. Those extreme features have led to the simplification of the trophic network. Cyanobacteria account for up to 95% of the photosynthetic biomass. The main biogeochemical processes, i.e. photosynthesis, bacterial sulfate reduction and methanogenesis could explain the current water composition. As far as we know, this ecosystem could be unique on Earth, extending the nature and chemical limits of aquatic inland ecosystems.

The Proterozoic Era records two periods of abundant positive carbon isotope excursions (CIEs), conventionally interpreted as resulting from increased organic carbon burial and leading to Earth’s surface oxygenation. As strong spatial variations in the amplitude and duration of these excursions are uncovered, this interpretation is challenged. Here, by studying the carbon cycle in the Dziani Dzaha Lake, we propose that they could be due to regionally variable methane emissions to the atmosphere. This lake presents carbon isotope signatures deviated by ~ + 12‰ compared to the modern ocean and shares a unique combination of analogies with putative Proterozoic lakes, interior seas or restricted epireic seas. A simple box model of its Carbon cycle demonstrates that its current isotopic signatures are due to high primary productivity, efficiently mineralized by methanogenesis, and to subsequent methane emissions to the atmosphere. By analogy, these results might allow the reinterpretation of some positive CIEs as at least partly due to regionally large methane emissions. This supports the view that methane may have been a major greenhouse gas during the Proterozoic Era, keeping the Earth from major glaciations, especially during periods of positive CIEs, when increased organic carbon burial would have drowned down atmospheric CO2.

Most older methods of fish farming, or aquaculture, have focused on growing only a single species of sea life, for example, salmon. Modern aquaculture systems involve the cultivation of two or more species together, based on what happens normally in the food chain, so that one species can provide a source of food for another species in the farm. This article describes the results of an experiment combining fish culture with algae culture and oyster culture. We show that algae can grow using fish waste, and oysters can eat algae to produce good-quality, healthy food, which reduces the pollution generated by aquaculture.

Author Correction: Demonstration of facilitation between microalgae to face environmental stress. In Scientific Reports, vol; 10, n°1, Article : 3557 ; DOI: 10.1038/s41598-020-59860-0 ; WOS:000563252500001 Author Correction: Demonstration of facilitation between microalgae to face environmental stress. In Scientific Reports, vol; 10, n°1, Article : 3557 ; DOI: 10.1038/s41598-020-59860-0 ; WOS:000563252500001The original version of this Article omitted an affiliation for Emna Krichen and the Acknowledgements section contained typographical errors and was incomplete. This erratum is available as an annex file.; International audience; Positive interactions such as facilitation play an important role during the biological colonization and species succession in harsh or changing environments. However, the direct evidence of such ecological interaction in microbial communities remains rare. Using common freshwater microalgae isolated from a High Rate Algal Pond HRAP treating wastewaters, we investigated with both experimental and modeling approaches the direct facilitation between two algal strains during the colonization phase. Our results demonstrate that the first colonization by microalgae under a severe chemical condition arose from the rapid growth of pioneer species such as Chlorella sorokiniana, which facilitated the subsequent colonization of low growth specialists such as Scenedesmus pectinatus. The pioneer species rapidly depleted the total available ammonia nitrogen favoring the specialist species initially inhibited by free ammonia toxicity. This latter species ultimately dominated the algal community through competitive exclusion under low nutrient conditions. We show that microbial successions are not only regulated by climatic conditions but also by interactions between species based on the ability to modify their growth conditions. We suggest that facilitation within the aquatic microbial communities is a widespread ecological interaction under a vast range of environmental stress.

Phytoplankton and bacterioplankton are the key components of the organic matter cycle in aquatic ecosystems, and their interactions can impact the transfer of carbon and ecosystem functioning. The aim of this work was to assess the consequences of chemical contamination on the coupling between phytoplankton and bacterioplankton in two contrasting marine coastal ecosystems: lagoon waters and offshore waters. Bacterial carbon demand was sustained by primary carbon production in the offshore situation, suggesting a tight coupling between both compartments. In contrast, in lagoon waters, due to a higher nutrient and organic matter availability, bacteria could rely on allochthonous carbon sources to sustain their carbon requirements, decreasing so the coupling between both compartments. Exposure to chemical contaminants, pesticides and metal trace elements, resulted in a significant inhibition of the metabolic activities (primary production and bacterial carbon demand) involved in the carbon cycle, especially in offshore waters during spring and fall, inducing a significant decrease of the coupling between primary producers and heterotrophs. This coupling loss was even more evident upon sediment resuspension for both ecosystems due to the important release of nutrients and organic matter. Resulting enrichment alleviated the toxic effects of contaminants as indicated by the stimulation of phytoplankton biomass and carbon production, and modified the composition of the phytoplankton community, impacting so the interactions between phytoplankton and bacterioplankton.

International audience; Several studies focused on wastewater treatment in High Rate Algal Ponds (HRAP) suggest that highly variable climatic conditions cause large variations of microalgal biomass productivity. In the present study, we show that similar carbon, nitrogen and phosphorus removal efficiencies were reached in different HRAPs treating urban wastewaters located in two different temperate (Mediterranean and oceanic) climates. Furthermore, similar ecological successions were observed in these HRAPs. During the start-up phase, the consumption of organic matter by detritivores, already present in the wastewater, appears to be necessary for the microalgae to grow within two weeks in spring. The growth of the rapid-growing species, Chlorella sp., followed by the grazing-resilient species, Scenedesmus sp., combined with nitrifying and denitrifying bacterial activity, removed most the ammonia. The resulting exhaustion of ammonia would limit the complete removal of dissolved COD by bacteria and phosphate by microalgae in the HRAPs. This study shows that similar biological and environmental constraints were applied on the HRAPs, making the process efficiency highly reproducible under different temperate latitudes.

International audience; Integrated multi-trophic aquaculture (IMTA) systems are a promising solution for sustainable aquaculture combining nutrient recycling with increased biomass production. An innovative land-based recirculating aquaculture system (RAS) was studied in France for a 60-day experiment. It combined a European sea bass (Dicentrarchus labrax) RAS with two other production systems: high rate algal ponds (HRAP) with natural marine polyspecific algal assemblages, and oysters in separate open tanks. The objective was the assessment of: 1) the efficiency and the stability of the microalgae bioremediation of the effluent from a fish RAS in spring and summer, 2) the abundance and the diversity patterns of the microalgae biomass for consumption in the oyster compartment of the IMTA. Silicate was added every week after the beginning of the experiment for maintaining a Si:N:P molar ratio of 10:5:1 in the HRAP to encourage the growth of diatoms. The HRAP have an overall removal efficiency of 98.6 ± 0.2% for NO3-N, 98.0 ± 0.4% for NO2-N, 97.3 ± 0.7% for NH4-N and 96.1 ± 0.6% for PO4-P, with removal rates of 335.8 ± 0.8, 23.6 ± 0.2, 30.9 ± 0.2, and 22.3 ± 0.2 mgm−2 d−1, respectively. The concentration of total suspended solid (TSS) and chlorophyll a (chl a) increased during the experiment and reached maximum values on day 46 (135.3 ± 34.7 mg TSS l−1 and 0.42 ± 0.03 mg chl a l−1) after which the microalgae collapsed due to a CO2 limitation (pH ca. 10). Sequencing analysis revealed that the microalgae community was dominated by Tetraselmis sp. from day 1 to day 16 (45.7% to 73.8% relative abundance). From day 30 to day 43 the culture was dominated by diatoms, Phaeodactylum sp. (83.4% to 98.1% relative abundance). Although the stable carbon isotope signatures confirmed that the microalgae were consumed, oysters' growth was limited in the RAS-IMTA, suggesting that oysters were under stress or not fed enough

When microalgae are exposed to contaminants, the role of associated bacteria within the phycosphere, the microenvironment surrounding algal cells, remains largely unknown. The present study investigated the importance of algae-associated bacteria on the responses of microalgae growth to metallic and organic toxicant exposure. The effects of a polluted sediment elutriate, and of metal or pesticide mixtures at environmentally relevant concentrations (

Recent observations in polar marine waters have shown that a large fraction of primary production may be lost to respiration by planktonic bacteria due to very low bacterial growth efficiencies in cold waters. Here we report that sea temperature may be a key factor (but not the only one) influencing the interaction between bacteria and primary production in North Atlantic and Arctic waters, suggesting that low primary production rates could not sustain bacterial carbon demand in the coldest Arctic waters. The use of freshly produced phytoplankton exudate by bacteria in early- and mid-summer was assessed, together with the bacterial uptake of dissolved inorganic nitrogen (DIN = nitrate and ammonium), in surface waters along a latitudinal gradient from the North Sea to the Arctic sea ice. Bacterial production was independent of the low primary production measured in the coldest waters. Under these conditions, heterotrophic bacteria can consume a large fraction of DIN and N-rich organic matter, making them strong contributors to N fluxes in these waters.

International audience; This study investigated the influence of light and temperature on Chlorella sorokiniana grown on a mixture of acetate and butyrate, two of the volatile fatty acids produced by dark fermentation. Exposure to light caused autotrophic biomass production (56% of the final biomass) and reduced the time to reach butyrate exhaustion to 7 days at 25°C from 10 days in the dark. For growth on acetate at the optimum temperature (35°C), the presence of butyrate reduced the growth rate (by 46%) and the carbon yield (by 36%). For successful microalgae growth on dark fermentation effluent, butyrate inhibition may be reduced by setting the temperature to 30°C and providing light.

International audience; Coupling dark fermentation (DF), which produces hydrogen from diverse effluents or solid waste, and heterotrophic cultivation of microalgae, which produces lipids, carbohydrates and proteins, is a promising and innovative solution for developing sustainable biorefineries. The use of a raw DF effluent, containing acetate and butyrate, to support the heterotrophic growth of Chlorella sorokiniana was investigated. All the acetate in sterilized and unsterilized DF effluent was exhausted in less than three days of heterotrophic cultivation, whereas butyrate was not used by the microalgae. The microalgae biomass reached 0.33 g L− 1 with a carbon yield on acetate of 55%. The algal yield was higher than previously reported for synthetic DF effluent. It was concluded that compounds other than volatile fatty acids were present in the DF effluent and these could be consumed by the microalgae. After the acetate had been exhausted, butyrate was consumed by facultative and strict aerobic bacteria originating from the DF effluent. The concentration of the bacterial community increased during the experiment but did not have any significant impact on heterotrophic microalgae growth. A high microalgal biomass yield was achieved without requiring the DF effluent to be sterilized.

International audience; The impact of fish and oysters on components of the pelagic microbial food web (MFW) was studied in a 10 d mesocosm experiment using Mediterranean coastal waters. Two mesocosms contained natural water only (‘Controls’), 2 contained natural water with Crassostrea gigas (‘Oyster’), and 2 contained natural water with Atherina spp. (‘Fish’). Abundances and biomasses of microorganisms (viruses, bacteria, phytoplankton, heterotrophic flagellates, and ciliates) were measured to estimate their contribution to the total microbial carbon biomass. Two MFW indices, the microbial autotroph:heterotroph C biomass ratio (A:H) structural index and the gross primary production:respiration ratio (GPP:R) functional index, were defined. In the Fish mesocosms, selective predation on zooplankton led to a trophic cascade with 51% higher phyto- plankton C biomass and consequently higher A:H and GPP:R than in the Controls. By the end of the experiment, the Oyster mesocosms had a bacterial C biomass 87% higher and phytoplankton C biomass 93% lower than the Controls, giving significantly lower A:H and GPP:R (1), whereas oyster activities made the MFW more hetero- trophic (both indices

International audience; Rivers are known to be major contributors to eutrophication in marine coastal waters, but little is known on the short-term impact of freshwater surges on the structure and functioning of the marine plankton community. The effect of adding river water, reducing the salinity by 15 and 30%, on an autumn plankton community in a Mediterranean coastal lagoon (Thau Lagoon, France) was determined during a 6-day mesocosm experiment. Adding river water brought not only nutrients but also chlorophyceans that did not survive in the brackish mesocosm waters. The addition of water led to initial increases (days 1-2) in bacterial production as well as increases in the abundances of bacterioplankton and picoeukaryotes. After day 3, the increases were more significant for diatoms and dinoflagellates that were already present in the Thau Lagoon water (mainly Pseudo-nitzschia spp. group delicatissima and Prorocentrum triestinum) and other larger organisms (tintinnids, rotifers). At the same time, the abundances of bacterioplankton, cyanobacteria, and picoeukaryote fell, some nutrients (NH4 (+), SiO4 (3-)) returned to pre-input levels, and the plankton structure moved from a trophic food web based on secondary production to the accumulation of primary producers in the mesocosms with added river water. Our results also show that, after freshwater inputs, there is rapid emergence of plankton species that are potentially harmful to living organisms. This suggests that flash flood events may lead to sanitary issues, other than pathogens, in exploited marine areas.

We describe a new transportable floating mesocosm platform with autonomous sensors. The platform has 9 separate units that can be transported by medium-sized research vessels and positioned in coastal waters. The in situ mesocosms are equipped with a set of sensors for measuring water temperature, conductivity, chlorophyll a fluorescence (Chl a), and dissolved oxygen concentration. It can take measurements every 2 min, store these measurements, and transmit them in real time. Each mesocosm has a pump with regulated flow to mix the water column. One of the floating units is used as an in situ observatory to monitor the water temperature and Chl a in the water around the mesocosms as well as weather data and the incident light. The main data logger on the platform sends all the data collected to a remote PC computer. This floating mesocosm platform was successfully deployed in 2010 and 2011 in Mediterranean coastal waters (Thau lagoon and Cretan Sea, respectively). Simultaneous, automatic, high temporal resolution monitoring of physical, chemical, and biological parameters in the mesocosms proved to be a powerful, noninvasive, and effective approach for i) monitoring the variations in physical and chemical parameters in real time and ii) assessing the short-term variations in Chl a and the pelagic food web metabolism (e.g., the community respiration, gross primary production, and net community production) in the mesocosms without any manipulation of water samples.

International audience; This work describes a method to model the optical properties over the (400-750 nm) spectral range of a dense microalgal culture using the chemical and physical properties of the algal cells. The method was based on a specific program called AlgaSim coupled with the adding-doubling method: at the individual cell scale, AlgaSim simulates the spectral properties of one model, three-layer spherical algal cell from its size and chemical composition. As a second step, the adding-doubling method makes it possible to retrieve the total transmittance of the algal medium from the optical properties of the individual algal cells. The method was tested by comparing the simulated total transmittance spectra for dense marine microalgal cultures of Isochrysis galbana (small flagellates) and Phaeodactylum tricornutum (diatoms) to spectra measured using an experimental spectrophotometric setup. Our study revealed that the total transmittance spectra simulated for the quasi-spherical cells of Isochrysis galbana were in good agreement with the measured spectra over the whole spectral range. For Phaeodactylum tricornutum, large differences between simulated and measured spectra were observed over the blue part of the transmittance spectra, probably due to non-spherical shape of the algal cells. Prediction of the algal cell density, mean size and pigment composition from the total transmittance spectra measured on algal samples was also investigated using the reversal of the method. Mean cell size was successfully predicted for both species. The cell density was also successfully predicted for spherical Isochrysis galbana, with a relative error below 7%, but not for elongated Phaeodactylum tricornutum with a relative error up to 26%. The pigments total quantity and composition, the carotenoids:chlorophyll ratio in particular, were also successfully predicted for Isochrysis galbana with a relative error below 8%. However, the pigment predictions and measurements for Phaeodactylum tricornutum showed large discrepancies, with a relative error up to 88%. These results give strong support for the development of a promising tool providing rapid and accurate estimations of biomass and physiological status of a dense microalgal culture based on only light transmittance properties.

Different calculations of bacterial carbon demand (BCD) were applied to bacterioplanktonic production datasets from oceanic euphotic waters and compared with estimates of dissolved and total phytoplanktonic production estimates. All calculations showed that the BCD in low productive waters was not supported by contemporary dissolved phytoplanktonic production or by the whole phytoplanktonic production when empirically modeled bacterial respiration and growth efficiency rates were used. This supports the new ecological concept of bacterial independence on phytoplankton production at the time scales (days, weeks) relevant to the microbial generation times observed in the oceans and used in biological and ecological models.

For several decades, prokaryotic and eukaryotic inhibitors have been used to exclude bacteria from microalgal cultures and for investigating prey-predator relationships. Recently there has been considerable interest in using specific inhibitors for studying the interactions between bacteria and phytoplankton, by selective repression of either organism’s activity. The effectiveness of chemical inhibitors must be tested before applying them to natural communities to partition metabolic activities between functional groups. Six different antibiotics selected from the most commonly reported in the literature were tested, at concentrations varying from 12.5 to 100 mg L −1 , for their effect on bacterial growth and functional diversity of natural communities from Mediterranean coastal waters. Penicillin and streptomycin each at a final concentration of 100 mg L −1 significantly reduced bacterial growth within 2 h. There was a greater impact on bacterial functional diversity when both antibiotics were mixed together. This mixture did not have any significant effect on the growth of selected cultured phytoplankton strains, whereas the eukaryote inhibitor cycloheximide at 100 mg L −1 reduced growth within 2 h of incubation. The penicillin–streptomycin mixture and cycloheximide alone successfully partitioned NH 4 + and NO 3 − uptake between bacteria and phytoplankton bi-weekly sampled in a coastal lagoon in Autumn, where bacterial contribution to total NH 4 + and NO 3 − uptake averaged 46 and 41%, respectively. The use of specific inhibitors may be a valuable method for studying interactions, such as competition and mutualism, or lack of interaction between the different components of microbial communities and could be used to study their relative importance in biogeochemical fluxes.

Positive relationships between heterotrophic bacteria and particulate phytoplankton production (respectively, BP and PPP) have been reported for several areas, suggesting that material produced by phytoplankton was a major substrate for bacterial growth. Since then, thousands of simultaneous measurements of both PPP and BP have been performed. A review of these data showed that BP may exceed PPP considerably (median ranged between 132% and 484%) in all aquatic systems with the lowest PPP. In oceanic waters, BP did not seem to be temporally synchronized with PPP and the median BP : PPP ratio is 15% with moderate PPP, but the immediate bacterial carbon (C) demand (including bacterial respiration) was greater than the corresponding total primary production (i.e. dissolved and particulate primary production) for >80% of both volumetric and areal datasets. In freshwaters, the strong covariation observed between BP and PPP seemed mainly due to a common response to sudden nutrient inputs into enclosed systems, leading to a similar range of production rates and temporal synchronicities. Indeed, phytoplankton exudates contributed directly to only 32% (median) of BP when C-tracking experiments were performed in freshwaters. Therefore, because direct C dependency of bacteria on phytoplankton is questionable, other C sources might be more significant for bacterial growth.

In recent years, coupling bacterial dark fermentation (DF) and heterotrophic cultivation of microalgae (HCM) has been pointed out as a promising sustainable approach for producing both gaseous and liquid biofuels. Complex organic waste and effluents that are not susceptible to be directly degraded by microalgae are first converted into volatile fatty acids (VFAs) and hydrogen by DF. In this work, the feasibility of using DF effluents to sustain has been thoroughly reviewed and evaluated. Promising perspectives in terms of microalgae biomass and lipids production are proposed and can be extended as guidelines to promote HCM whatever the organic waste used. Abiotic and biotic factors from DF effluents that promote or inhibit microalgae growth are discussed as well as the use of unsterile DF effluents. Overall, the microalgae growth is favored on effluents containing high acetate concentration (> 3 g L−1), with a high acetate:butyrate ratio (> 2.5), and when pH is strictly controlled. At a low acetate:butyrate ratio (10 g L−1), a low substrate:microalgae ratio and the presence of light appear to enhance microalgae growth. Butyrate content appears to be a key factor when coupling DF/HCM since high butyrate concentration inhibits the microalgae growth.

The effect of light and nutrients on a temperate water planktonic microbial community was investigated via experiments conducted in Scottish coastal waters during summer. Experimental water was collected from the depth of maximum chlorophyll concentration in the Clyde Sea on the 7th and 9th of August 2001. For each experiment, the microbial community was incubated for 36 h under one of two differing natural light regimes: continuous 3% solar irradiation or alternating 1-hour periods of 80% solar irradiation and darkness. Water incubated with the alternating light regime was also subjected to three initial nutrient manipulations: added nitrate, added nitrate and phosphate, or no nutrient addition. The four treatments were designed to simulate stratified or mixed water column conditions; the latter with or without nutrient entrainment. During the first experiment, the photosynthetic capacity and efficiency of phytoplankton were significantly reduced after 36 h incubation under the alternating light regime, regardless of nutrient addition; however phytoplankton in the second experiment showed not such response. Higher incident light intensities, recorded towards the end of the first experiment and throughout the second, may have resulted in photoinhibition of phytoplankton during the last hours of the first experiment, with cells acclimatised by the end of the second experiment, despite low nitrogen availability. In general, the four treatments had relatively little effect on the taxonomic composition and abundance of the microbial community; however, significant differences were observed in the abundance and activity of some microbial groups and species. Bacterial production increased during incubation across all treatments and a positive relationship was observed between bacterial specific growth and photosynthetic capacity. This suggests strong coupling between phytoplankton and heterotrophic bacteria via dissolved organic material allowing bacteria and phytoplankton to sustain their growth under fluctuating environmental conditions. By contrast, the dinoflagellates Alexandrium sp. exhibited highest abundance when exposed to the alternating light regime but only during the first experiment. Reduced competition for ammonium, due to photoinhibition of low-light adapted diatoms, may explain this observation suggesting that growth rate of potentially toxic dinoflagellates, such as Alexandrium spp., can be limited in temperate waters by ammonium availability rather than irradiance.

A nutrient-manipulation mesocosm study was conducted in Norwegian coastal waters to determine the effect on a nitrogen-limited microbial food web of changes in the relative concentrations of dissolved organic and inorganic nitrogen (N). Four replicated treatments were studied: no N addition, inorganic N, organic N, or a 50 : 50 mix of inorganic and organic N. Comprehensive abundance, biomass, and rate measurements were made over the 20-d experiment. The form of N available influenced species composition, succession, and the efficiency of carbon (C) incorporation. Inorganic N generated a bloom of the diatom Leptocylindrus danicus, even though silicon concentrations were below 1 mmol L 21 . The c-proteobacteria initially competed effectively with diatoms for inorganic N, exhibiting high productivity and escaping grazing control. Organic N, when available alone, was utilized more slowly by the microbial community, with diatom growth, fueled by regenerated N, achieving a lower peak abundance. A more diverse diatom species assemblage was also evident. The c-proteobacteria were less prevalent and net bacterial productivity was initially lower when the N source was organic and, in contrast to the response following inorganic N addition, the bacterial community was initially grazer controlled. A rapid succession of heterotrophic grazers occurred but with differences in species and their contribution to biomass between treatments. The efficiency of C-biomass production was lower when the N source was purely organic. However, when both inorganic and organic N were available, biomass yield was greater than the mean of the two single N source treatments.

We estimated rates of heterotrophic bacterial and phytoplankton uptake of nitrate, ammonium, and urea using 15 N-labelled nitrogen and specific metabolic inhibitors of prokaryote and eukaryote nitrogen metabolism in the surface waters of the North Water (northern Baffin Bay) during autumn that were charactenzed by the absence of cyanobactena (comprising prochlonphytes). The percentage of nitrate + ammonium uptake by heterotrophic bacteria ranged between 44 and 78% of the measured total uptake and was the highest when the phytoplankton biomass was relatively low (

The influence of dissolved inorganic nitrogen (DIN) enrichments on cell-normalized carbon uptake rate, chlorophyll a content, and apparent cell size of a picoeukaryote (1 microm) ( Ostreococcus tauri, the smallest eukaryotic cell) from a natural summer phytoplanktonic assemblage (200 microm) in a northern Mediterranean Lagoon (Thau Lagoon) was studied in 20-L enclosures in June 1995. The natural planktonic community was incubated in situ for 24 h with initial ammonium and nitrate enrichments and compared to a control without enrichment. O. tauri cell-normalized productivity was estimated from the combination of flow cytometric (FCM) enumeration and 2-h (radioactive) carbonate incorporation measured on post-incubation size fractions (1microm). No difference between the effects of the two DIN sources of enrichment on the studied biological parameters was measured during this experiment. Growth of natural O. tauri was perturbed by the low DIN availability in the control with drastic changes in cell productivity, chlorophyll content, and cell cycle (from the variations in apparent cell size) as compared to the DIN sufficiency conditions. On the other hand, a very high specific growth rate for natural O. tauri, up to 8 day(-1) under DIN enrichments, has been estimated from production and abundance data obtained during this experiment. This supports values measured in culture and suggests that the yearly high contribution of picophytoplankton to the total primary production in Thau Lagoon is likely to be due to their high growth rate rather than the previously suggested lack of grazing pressure.

Regions of Restricted Exchange (RREs) are an important feature of the European coastline. They are historically preferred sites for human settlement and aquaculture and their ecosystems, and consequent human use, may be at risk from eutrophication. The OAERRE project (EVK3-CT1999-0002) concerns 'Oceanographic Applications to Eutrophication in Regions of Restricted Exchange'. It began in July 2000, and studies six sites. Four of these sites are fjords: Kongsfjorden (west coast of Spitzbergen); Gullmaren (Skagerrak coast of Sweden); Himmerfjarden (Baltic coast of Sweden); and the Firth of Clyde (west coast of Scotland). Two are bays sheltered by sand bars: Golfe de Fos (French Mediterranean); and Ria Formosa (Portuguese Algarve). Together they exemplify a range of hydrographic and enrichment conditions. The project aims to understand the physical, biogeochemical and biological processes, and their interactions, that determine the trophic status of these coastal marine RRE through the development of simple screening models to define, predict and assess eutrophication. This paper introduces the sites and describes the component parts of a basic screening model and its application to each site using historical data. The model forms the starting point for the OAERRE project and views an RRE as a well-mixed box, exchanging with the sea at a daily rate E determined by physical processes, and converting nutrient to phytoplankton chlorophyll at a fixed yield q. It thus uses nutrient levels to estimate maximum biomass; these preliminary results are discussed in relation to objective criteria used to assess trophic status. The influence of factors such as grazing and vertical mixing on key parameters in the screening model are further studied using simulations of a complex 'research' model for the Firth of Clyde. The future development of screening models in general and within OAERRE in particular is discussed. In addition, the paper looks ahead with a broad discussion of progress in the scientific understanding of eutrophication and the legal and socioeconomic issues that need to be taken into account in managing the trophic status of RREs.

We investigated the combined effects of mixing regimes and ultraviolet-B radiation (UVBR: 280‐320 nm) on the uptake of nitrate, ammonium, and urea by an estuarine phytoplankton community during a 10-d mesocosm experiment in July 1997. The experiment was conducted in eight mesocosms (1,500 liters each) subjected to varying conditions of UVBR and mixing: (1) natural UVBR and fast mixing, (2) enhanced UVBR and fast mixing, (3) natural UVBR and slow mixing, and (4) enhanced UVBR and slow mixing. The study was carried out during a typical midsummer period of low nutrient concentrations. Throughout the experiment, the phytoplankton community was dominated by a mixed community composed mainly of centric diatoms, cyanobacteria, and prymnesiophytes, with larger algal cells (.5 mm) representing .70% of the total chlorophyll a biomass. The phytoplankton nutrient status changed over the experiment. Algal cells were nitrate-deficient until the occurrence of a nitrification event on day 7, and then became silicic acid‐deficient. Mesocosms with different mixing rates showed significant differences in ambient nutrient concentrations, nitrogen uptake rates, and phytoplankton species composition. In general, no significant difference was detected between the N transport rates from samples incubated close to the surface and close to the bottom of the mesocosms, and no difference was detected between the UVBR treatments on all the other studied biological and chemical variables measured during the experiment. It is only under the slow mixing regime that the enhanced UVBR treatments significantly depress both the transport and the chlorophyll a‐ specific transport rates of dissolved organic nitrogen (urea). The UVBR effects observed only in the slow mixing regime could be a consequence of an inherent difference in the uptake capacity of the different communities under the two mixing regimes. Both nutrient conditions and mixing regime influenced the natural phytoplankton physiology and composition toward a community more sensitive to UVBR enhancement under a slower mixing regime.

The importance of heterotrophic bacteria relative to phytoplankton in the uptake of ammonium and nitrate was studied in Mediterranean coastal waters (Thau Lagoon) during autumn, when the Mediterranean Sea received the greatest allochthonous nutrient loads. Specific inhibitors and size-fractionation methods were used in combination with isotopic 15N tracers. NO3 − and NH4 + uptake was dominated by phytoplankton (60 % on average) during the study period, which included a flood event. Despite lower biomass specific NH4 + and NO3 uptake rates, free-living heterotrophic bacteria contributed significantly (>30 %) to total microbial NH4 + and NO3 − uptake rates in low chlorophyll waters. Under these conditions, heterotrophic bacteria may be responsible for more than 50 % of primary production, using very little freshly produced phytoplankton exudates. In low chlorophyll coastal waters as reported during the present 3-month study, the heterotrophic bacteria seemed to depend to a greater extent on allochthonous N and C substrates than on autochthonous substrates derived from phytoplankton.

International audience; The growth of two lipid-producing Chlorella species on fermentative end-products acetate, butyrate and lactate, was investigated using a kinetic modeling approach. Chlorella sorokiniana and Auxenochlorella protothecoides were grown on synthetic media with various (acetate:butyrate:lactate) ratios. Both species assimilated efficiently acetate and butyrate with yields between 0.4 and 0.5 g carbon of biomass/g carbon of substrate, but did not use lactate. The highest growth rate on acetate, 2.23 d−1, was observed for C. sorokiniana, and on butyrate, 0.22 d−1, for A. protothecoides. Butyrate removal started after complete acetate exhaustion (diauxic effect). However, butyrate consumption may be favored by the increase of biomass concentration induced by the initial use of acetate. A model combining Monod and Haldane functions was then built and fitted the experimental data well for both species. Butyrate concentration and (acetate:butyrate) ratios were identified as key parameters for heterotrophic growth of microalgae on fermentative metabolites.

The composition of the phytoplanktonic communities in the surface waters of the La Reunion-Kerguelen transect (from 38°36S to 46°33S) has been investigated under spring conditions (Antares 3 cruise, France-JGOFS, 28 September–8 November, 1995). The study, conducted at six stations in the subtropical frontal zone, involved size fractionations (threshold: 2 μm). The large variations in the overall biomass and autotrophic carbon fixation, calculated via Rubisco activity measurements and expressed respectively in terms of μg chlorophyll (a + b + c) per liter and nmol fixed carbon dioxide per liter and per hour, were attributable only to phytoplanktonic cells of >2 μm, with a peak observed in the frontal zone. The picophytoplankton (

Small wickertraps used to catch Macrobrachium (Crustacea: Decapoda) in the rivers of Polynesia are described. The traps were used in Nuku-Hiva (Marquesas Islands) to sample Macrobrachium lar Fabricius and Macrobrachium australe Guerin–Meneville. Effects of setting direction, colour, bait, and ballast were studied. The orientation of the trap aperture to the current was shown to have a significant effect.

Coupling algal biomass production and anaerobic digestion is one of the most promising bioprocesses for economically viable algal production. This study assesses the production rates of some native microalgae growing in media supplemented with algal digestate, urban wastewater or digested sludge. Native microalgal populations isolated from temperate freshwaters ( Scenedesmus spp.) and marine ecosystems ( Nannochloris spp.) had the highest potential production rates (about 100 mg DW L −1 d −1 ) with algal digestate at about 20% loading ratio. However, no growth was measured for Nannochloris spp., when the ammonium concentration exceeded 100 mg L −1 although Scenedesmus spp. appeared to be tolerant to higher NH 4 + concentrations. Very low production rates, or no growth, were measured when microalgae isolated from high salinity waters ( Dunaliella salina , Lyngbya aestuarii ) were used, suggesting that populations well adapted to extreme environmental conditions are not suitable candidates for growing on wastewater or anaerobic digestate.

International audience; The effects of grazing pressure and inorganic nutrient availability on the direct carbon transfer from freshly produced phytoplankton exudates to heterotrophic bacteria biomass production were studied in Mediterranean coastal waters. The short-term incorporation of 13C (H13CO3) in phytoplankton and bacterial lipid biomarkers was measured as well as the total bacterial carbon production (BP), viral lysis and the microbial community structure under three experimental conditions: (1) High inorganic Nutrient and High Grazing (HN + HG), (2) High inorganic Nutrient and Low Grazing (HN + LG) and (3) under natural in situ conditions with Low inorganic Nutrient and High Grazing (LN + HG) during spring. Under phytoplankton bloom conditions (HN + LG), the bacterial use of freshly produced phytoplankton exudates as a source of carbon, estimated from 13C enrichment of bacterial lipids, contributed more than half of the total bacterial production. However, under conditions of high grazing pressure on phytoplankton with or without the addition of inorganic nutrients (HN + HG and LN + HG), the 13C enrichment of bacterial lipids was low compared with the high total bacterial production. BP therefore seems to depend mainly on freshly produced phytoplankton exudates during the early phase of phytoplankton bloom period. However, BP seems mainly relying on recycled carbon from viral lysis and predators under high grazing pressure.

International audience; Few studies on the consequences of future global warming and increased ultraviolet-B radiation (UVBR, 280-320nm) have been carried out at the food web scale and very few have considered the effect of the combination of these two climatic stressors. The single and combined effect of 3°C warming and 20% increased UVBR on planktonic carbon and nitrogen dynamics was investigated during a 10-day mesocosm experiment in coastal Mediterranean waters. Planktonic organisms from Thau Lagoon were sampled and incubated into eight in situ moored 2000L mesocosms and exposed to 4 different temperature and light conditions in duplicates; 1) natural light and temperature conditions; 2) +3°C above in situ water temperature; 3) +20% above incident UVBR, and 4) simultaneous +3°C and +20% above natural conditions. Compared to the natural in situ temperature and light conditions, increasing UVBR alone reduced bacterial carbon production and biomass by an average of 20%, with some reduction in ammonium net production. Warming alone significantly reduced bacterial carbon production by about 40% as a consequence of a trophic cascade effect on bacterial biomass. Warming also increased phytoplankton carbon production and associated nitrate uptake rates by an average of 90% and 200%, respectively. Thus, the results showed a direct positive effect of temperature increase on phytoplankton metabolism and greater nutrient availability owing to the reduction in potential bacterial competition. The combination of temperature and increased UVBR significantly increased the autotrophic production but without accumulation of freshly produced particulate organic matter. This suggests an accumulation of dissolved organic matter under these climatic conditions.

Over half of the total nitrogen, phosphorus, silicate and dissolved organic carbon (DOC) loading was discharged from the Wile River into the Thau Lagoon waters within the first five days of what was considered to be the autumn flash flood period. Such loads represented about 8% and 3% of the yearly averaged total nitrogen and phosphorus load in Thau Lagoon, respectively. Although this event affected >20% of the total lagoon volume, river trace metal loads contributed apparently only weakly to the increase in labile trace metal concentrations in the lagoon surface waters. Differences between theoretical dilution values and observed values were also noticed for phosphate, silicate and dissolved organic carbon (DOC) concentrations. DOC losses (10-50%) mainly through flocculation, together with the substantial increases in some metallic contaminants such as Zn (from 6 to >30 mu g L-1) observed during the flash flood in saline lagoon waters, may have limited the carbon production of bacterial communities. The potential osmotic shock and the increase in turbidity may mainly explain the low phytoplankton C turnover rates (average of 0.02 h(-1)) measured in brackish waters (

Background Small size eukaryotes play a fundamental role in the functioning of coastal ecosystems, however, the way in which these micro-organisms respond to combined effects of water temperature, UVB radiations (UVBR) and nutrient availability is still poorly investigated. Results We coupled molecular tools (18S rRNA gene sequencing and fingerprinting) with microscope-based identification and counting to experimentally investigate the short-term responses of small eukaryotes ( Conclusions This multifactorial experiment highlights the potential impacts, over short time scales, of changing environmental factors on the structure of various functional groups like small primary producers, parasites and saprotrophs which, in response, can modify energy flow in the planktonic food webs.

Over the past 20 years, plants have been used to anincreasing extent in various environments for mitigat-ing pollutant concentrations in contaminated sites(soils or waters) and for producing biodiesel. How-ever, because plant growth rates and biomass yieldsare low and because a considerable amount of waterand large surface area are required, intensive cultiva-tion of microalgae has been proposed as an alternativemethod for phycoremediation and producing bio-energy (Dismukes et al. 2008; Rawat et al. 2011).However, mass cultivation of microalgae in opensystems (and, to a certain extent, in closed systems) issubject to strong competition (from local microalgalcommunities when single species microalgal biopro-cesses are considered) and predation and are partic-ularly sensitive to sudden changes in environmentalconditions (light, temperature and nutrient availabil-ity). Such factors may contribute to the rapid collapseof the microalgae culture. The use of indigenousmicroalgae species for local intensive production,better adapted to local climatic conditions, may reducepotential competition to some extent without risk ofthe culture becoming noxious or invasive, as claimedrecently by Wilkie et al. (2011). However, the stabilityof microalgal productivity in a single species culture isdoubtful when cultures are supplied with heteroge-neous wastewaters containing various organic andinorganic compounds, some of which are toxic,together with their own microbial communities feed-ing on the waste.The application of ecological observations ofnatural ecosystems to large-scale microalgal cultiva-tion might help to ensure successful, intensive, stablemass algal production by increasing the efficiency ofmultiple resource use and by reducing potentialcompetition and predation.1 Exploiting microalgal diversityRecent studies have shown positive relationshipsbetween microalgae diversity and resource (phospho-rus, nitrate) use efficiency in freshwater and brackishcommunities (Ptacnik et al. 2008; Cardinale 2011).Algal communities with greater species richness makebetter use of niche opportunities in an environment,allowing them to capture a higher proportion ofavailableresources(Cardinale2011).Thegreatvarietyof nutrition modes displayed by microalgae speciesfor acquiring carbon or nutrients helps to extendtheir niche opportunities in changing environments.Depending on the species, their physiology, cell sizeand biotic and abiotic environmental growth condi-tions, microalgae can use various forms of nitro-gen (elemental nitrogen, ammonium, nitrate, organic

Association of microalgae culture and anaerobic digestion seems a promising technology for sustainable algal biomass and biogas production. The use of digestates for sustaining the growth of microalgae reduces the costs and the environmental impacts associated with the substantial algal nutrient requirements. A natural marine algae–bacteria consortium was selected by growing on a medium containing macro nutrients (ammonia, phosphate and acetate) specific of a digestate, and was submitted to a factorial experimental design with different levels of temperature, light and pH. The microalgal consortium reached a maximum C conversion efficiency (i.e. ratio between carbon content produced and carbon supplied through light photosynthetic C conversion and acetate) of 3.6%. The presence of bacteria increased this maximum C conversion efficiency up to 6.3%. The associated bacterial community was considered beneficial to the total biomass production by recycling the carbon lost during photosynthesis and assimilating organic by-products from anaerobic digestion. Highlights ► Carbon conversion efficiency of marine microalgae growing on a synthetic anaerobic digestate was 4%. ► Carbon conversion efficiency increased to 6.3% when bacteria were included into the carbon budget. ► Bacteria recycled the carbon lost during photosynthesis and originated from anaerobic digestion.

Episodic river flash floods, characteristic of Mediterranean climates, are suspected to greatly affect the functioning of microbial food webs. For the first time, the abundance, biomass and diversities of microbial food web components were studied before and during 4 consecutive days after a flash flood that occurred in November 2008, in the surface waters of five stations along a salinity gradient from 20 to 36 in the Thau lagoon. Eukaryotic pico- and nanophytoplankton were discharged from the river into the lagoon and increased by 30- and 70-fold, respectively. Bacteria increased by only 2-fold in the lagoon, from around 4–8 × 106 cells ml−1, probably benefiting from river nutrient input. Chlorophyll a increased 4-fold, and pigment biomarkers showed that the dinophyceae, prasinophyceae and prymnesiophyceae were sensitive to the flood perturbation, whereas the bacillariophyceae, cryptophyceae and chlorophyceae were resistant and/or transported to the lagoon from the river. Predator responses were more complex as total heterotrophic flagellate abundance decreased slightly, whereas those of specific naked ciliates increased, particularly for Uronema sp. The flood also induced a specific change in diversity, from a community dominated by Strobilidium spiralis to a community dominated by Uronema sp. The tintinnid community was particularly sensitive to the flood event as the abundance of all species decreased greatly. The high increases in biomass, mainly brought by the river during the flood, could have eventually sedimented to the benthic layer and/or been transported further into the lagoon, supporting the pelagic food web, or have even been exported to the Mediterranean Sea.

Positive relationships between heterotrophic bacteria and particulate phytoplankton production (respectively, BP and PPP) have been reported for several areas, suggesting that material produced by phytoplankton was a major substrate for bacterial growth. Since then, thousands of simultaneous measurements of both PPP and BP have been performed. A review of these data showed that BP may exceed PPP considerably (median ranged between 132% and 484%) in all aquatic systems with the lowest PPP. In oceanic waters, BP did not seem to be temporally synchronized with PPP and the median BP : PPP ratio is 15% with moderate PPP, but the immediate bacterial carbon (C) demand (including bacterial respiration) was greater than the corresponding total primary production (i.e. dissolved and particulate primary production) for80% of both volumetric and areal datasets. In freshwaters, the strong covariation observed between BP and PPP seemed mainly due to a common response to sudden nutrient inputs into enclosed systems, leading to a similar range of production rates and temporal synchronicities. Indeed, phytoplankton exudates contributed directly to only 32% (median) of BP when C-tracking experiments were performed in freshwaters. Therefore, because direct C dependency of bacteria on phytoplankton is questionable, other C sources might be more significant for bacterial growth.

New and regenerated production were estimated from nitrate, ammonium and urea uptake in the highly productive North Water region (NOW, northern Baffin Bay) during the late summer bloom and its decline in early autumn 1999. Nitrogen uptake rates decreased due to a significant 4-fold reduction of the average irradiance in the euphotic zone between these 2 periods. Nitrate availability also regulated the relative uptake of nitrate (ratio of nitrate uptake to total nitrogen uptake, or f-ratio) over spatial scales in early autumn. Stations located along the Greenland coast, which had relatively warm (-0.5°C) and saline (32.5) waters, had higher nitrate concentrations (153 vs. 51 mmol N m -2 ) and f-ratios (0.36 vs. 0.16) than those of stations located on the Canadian side, where the euphotic zone was relatively colder (-1.4°C) and less saline (30.6). The f-ratios were also corrected for nitrogen uptake by heterotrophic bacteria using the results from experiments that employed specific metabolic inhibitors. The corrected f-ratio range of 0.24 to 0.41 considered only the fraction of the nitrogen uptake due to phytoplankton in order to obtain a more accurate estimate of new production. Based on this range, new production varied from 203 to 346 and 85 to 145 mg C m -2 d -1 in late summer and early autumn, respectively. For the entire sampling period, averaged new production was 132 to 228 mg C m -2 d -1 ; however, when the dissolved form was included, we estimated that new production may double to between 262 and 452 mg C m -2 d -1 . This estimate illustrates that dissolved forms can be significant and need to be considered. When combining our results with those from the literature, new production in the NOW in 1998 - 99 amounted to between 143 and 152 g C m -2 yr -1 , suggesting that 60% of the total annual primary production may potentially be exported from the euphotic zone. This study completes the first annual time series of new production in a high-latitude ecosystem.

The influence of dissolved inorganic and organic nitrogen (DIN and DON) enrichments on pools of enzymes responsible for CO2 fixation by the Calvin-Benson (Rubisco) and beta-carboxylation pathways (beta-carboxylases) were studied in a natural plankton assemblage. The plankton community from a coastal Mediterranean lagoon were incubated in situ for 24 h with initially ammonium, nitrate and DON (taurine) enrichments and compared to a control without any enrichment. An increase of small picophytoplankton and diatom biomass was observed in the enriched inorganic nitrogen treatments 7 - 10 h after the initial enrichment. Phytoplankton biomass decreased in the control and under taurine enrichment suggesting an inorganic nitrogen limitation of phytoplankton growth.

During the ANTARES 3 cruise in the Indian sector of the Southern Ocean in October-November 1995, the surface waters of Kerguelen Islands plume, and the surface and deeper waters (30-60 m) along a transect on 62 degrees E from 48 degrees 36'8 to the ice edge (58 degrees 50'S), were sampled. The phytoplankton community was size-fractionated (2 mu m) and cell numbers, chlorophyll biomass and carbon assimilation, through Rubisco and beta-carboxylase activities, were characterized. The highest contribution of 2 mu m. The mixotrophic capacity of these small cells is proposed. From sampling stations of the Kerguelen plume, a relationship was observed between the Rubisco activity per picophytoplankton cell and apparent cell size, which varied with the sampled water masses. Moreover, a depth-dependent photoperiodicity of Rubisco activity per cell for