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International audience; The environmental fate and behavior of nanoplastics (NPs) and their toxicity against aquatic organisms are under current investigation. In this work, relevant physicochemical characterizations were provided to analyze the ecotoxicological risk of NPs in the aquatic compartment. For this purpose, heteroaggregates of 50 nm polystyrene nanospheres and natural organic matter were prepared and characterized. The kinetic of aggregation was assimilated to a reaction-limited colloid aggregation mode and led to the formation of heteroaggregates in the range of 100–500 nm. Toxicities of these heteroaggregates and polystyrene nanospheres (50 and 350 nm) were assessed for a large range of concentrations using four benthic and one planktonic algal species, in regards to particle states in the media. Heteroaggregates and nanospheres were shown to be stable in the exposure media during the ecotoxity tests. The algal species exhibited very low sensitivity (growth and photosynthetic activity), with the noteworthy exception of the planktonic alga, whose growth increased by more than 150% with the heteroaggregates at 1 µg L−1. Despite the lack of a strong direct effect of the NPs, they may still impair the functioning of aquatic ecosystems by destabilizing the competitive interactions between species. Moreover, further work should assess the toxicity of NPs associated with other substances (adsorbed pollutants or additives) that could enhance the NP effects

A species new to science, Germainiella clandestina, was discovered during experiments performed in an artificial freshwater channel. This species is described based on valve morphology typical for the genus Germainiella Lange-Bertalot et Metzeltin. The major features conforming to Germainiella are the valve face as well as a part of the mantle covered by a conopeum and the presence of very small apertures corresponding to a subconopeum canal system close to the raphe fissure. These characters clearly differentiate this taxon from the two more related genera, namely Fallacia Stickle et Mann and Pseudofallacia occulta (Krasske) Y. Liu, J.P. Kociolek et Q.X. Wang. Moreover, two chloroplasts could be observed for the first time in Germainiella, providing an additional feature to separate Germainiella and both genera cited above. Some information is given on its ecology.

As a way to prevent resource depletion by other species, many phototrophic aquatic microorganisms produce inhibitory compounds. This process, known as allelopathy, has been widely studied in planktonic environments, where it is recognized as being a driving force of planktonic communities. However, in benthic environments, biofilms provide very particular micro-environments. The present review focuses on allelopathic interactions involving benthic phototrophic prokaryotes and micro-eukaryotes ('microalgae'), which generally form biofilms, and includes any interaction involving benthic microalgae either as the emitter or as the target in both marine and freshwater habitats. To support our hypothesis on the importance of allelopathy in biofilms due to the particularities of biofilms, we show that (i) reported allelopathic species and compounds are diverse and numerous in the three major groups of benthic phototrophic microorganisms, (ii) allelopathic benthic species could affect community composition, (iii) allelopathy in biofilms is currently underestimated because of the lack of suitable methods. As benthic primary producers represent an important source of organic carbon in some streams and littoral areas, these interactions could impact the whole ecosystem in these areas, probably more than in areas dominated by planktonic communities.

International audience; Many organisms produce chemical compounds, generally referred as secondary metabolites, to defend against predators and competitors (allelopathic compounds). Several hypotheses have been proposed to explain the interaction between environmental factors and secondary metabolites production. However, microalgae commonly use simple metabolites having a role in primary metabolism as allelopathic compounds. The aim of this study was to determine whether classical theories of plant chemical defences could be applied to microalgae producing allelochemicals derived from the primary metabolism. Our study was designed to investigate how growth phase, algal population density, nutrient limitation and carbon assimilation affect the production and release of allelopathic free fatty acids (FFAs) among other FFAs. The model species used was Uronema confervicolum, a benthic filamentous green alga that produces two allelopathic FFAs (linoleic and α-linolenic acids) inhibiting diatom growth. FFAs have been quantified in algal biomass and in culture medium. Our results were analysed according to two classical plant defence theories: the growth-differentiation balance hypothesis (GDBH) and the optimal defence theory (ODT), based on the metabolic capacities for defence production and on the need for defence, respectively. While a higher production of allelopathic compounds under increased light conditions supports the use of GDBH with this microalga, the observation of a negative feedback mechanism mostly supports ODT. Therefore, both theories were insufficient to explain all the observed effects of environmental factors on the production of these allelochemicals. This highlights the needs of new theories and models to better describe chemical interactions of microalgae.

International audience; Autochthonous primary production is generally much reduced in forested headwater streams. Several hypotheses have been proposed for explaining this observation, among them, the low light intensity, or the strong constraints exerted by stream current. Allelopathic inhibition of competitors is a common ecological process in aquatic environments. Aquatic hyphomycetes are known to chemically inhibit bacteria and other fungi (including other aquatic hyphomycetes) but a possible allelopathic effect of aquatic hyphomycetes on primary producers has never been tested. The inhibitory effect of twelve aquatic hyphomycete species was tested on three diatom species. Nine aquatic hyphomycete species exhibited anti-diatom activity. Up to 100% diatom growth inhibition was observed. Our study reveals that such allelopathic interactions might be common in streams and probably involve an array of fungal compounds. We propose that the generally reduced primary production observed in forested headwater streams is, among other factors, due to the inhibition of primary producers by allelopathic compounds released by aquatic hyphomycetes.

In the context of climate change, with an increase in the incidence of seasonally dry streambeds, we examined the impact of drought, and its duration, on the structure of diatom communities and the consequences of this on diatom index values (Biological Diatom Index; Specific Polluosensitivity Index) in the Mediterranean river Maureillas. The effects of several durations of droughts or low flow events (1, 2 and 4 weeks) on diatom-dominated biofilms were studied by removing pebbles from the river and comparing the growing communities after rewetting with those of control biofilms (pebbles left in the streambed) and with those of newly formed biofilms (cleaned-up pebbles). Sampling was performed six times during the 28-day rewetting phase. We determined the specific composition of diatom communities, the mortality rate after rewetting, and the Biological Diatom Index and Specific Polluosensitivity Index scores for each sample, distinguishing between potentially living diatoms (full frustules with chloroplasts) and dead diatoms (empty frustules). Our results showed a notable and durable impact of even short drought on diatom communities. The different trajectories of biofilm communities observed between the types of pebbles showed the importance of the biofilm history for the community structure. This should be taken into account to improve the sampling practices used for monitoring. We also observed a stable mortality rate whatever the drought duration was. Diatom index values were stable despite the observed impact of treatment on diatom communities. These results emphasize the reliability of the diatom index to evaluate biological quality according to European requirements. In addition, the distinction of empty and full frustules did not induce significant differences, but it may reduce biases in interpretation. Copyright © 2014 John Wiley & Sons, Ltd.

Marine waters in general and coastal areas in particular suffer globally from stressing events. These events can be largely attributed to higher levels of human-induced eutrophication, contamination by pollutant, and siltation. The European Union and other countries have adopted legislation to guide monitoring efforts and set goals for good quality coastal environments. Few biological quality elements were identified by legislators for their ability to detect and identify sources of ecosystem perturbations. Research programs have developed indices calculation methods for benthic invertebrate fauna, phytoplankton, macroalgae and angiosperms. Most of these indices are multimetric, based on the richness, abundance and biomass of the community. Such indices determine the quality at one site after observational comparison with a reference situation. Functional indices, however, are based on species ecological preferenda and autoecology. Since species are generally response-specific, functional index allows the discernment of the stressing factor involved in the ecosystem perturbation. The final index value provides a direct appreciation of the ecological status of the study site. At the present time, there are few functional indices that can be used on a routine basis for monitoring coastal waters. Because biological quality elements cited below are not available in all areas, the range of monitoring is restricted to particular environments. This review paper suggests that marine benthic diatoms (Bacillariophyceae) may result in a functional index. Arguments for the use of these organisms as bio-indicators include knowledge from the past and present use of diatoms in freshwater environmental analysis. Marine benthic diatoms are abundant and ubiquitous in terms of ecosystems and water chemistry and are sensitive to nutrient variation, as revealed by two studies conducted in mesotrophic and oligotrophic areas. This review is focused on benthic diatoms and reveals that no study has yet been conducted on the use of these organisms as bio-indicators. Aspects of practical sampling and sample treatment for marine diatoms are considered. As the bio-indication role of diatoms is little explored in marine environments, the dataset on ecological indicator values of species is scarce. Future work that relates to taxonomic determination and physico-chemical description of sites are needed to better define species ecological preferenda, prior to the development of a marine diatom biotic index.

International audience; Sedimentation is a key process in lake functioning, and plays an important role in nutrient and carbon cycles at both regional and global scales. Several biological processes have been shown as quantitatively affecting sedimentation, but very few works have tried to relate the structure of aquatic communities and the quality of sinking organic matter. We tested in a mesocosm study how food-web structure affects quantitatively and qualitatively sedimentation in eutrophic systems. We carried out a long-term experiment (14 months) in large replicated enclosures either dominated by planktivorous fish or fishless. Food-web structure modified the specific composition of zooplankton communities and phytoplankton biomass, as expected by the trophic cascade theory. Planktivorous fish had a strong positive effect on gross sedimentation rate, but the fraction of suspended particulate material that sank only slightly differed between treatments. The density of transparent exopolymer particles (TEP) was a good descriptor of sedimentation rate, highlighting the key role of these polysaccharidic particles in sinking processes. In fish enclosures, sediment elemental ratios were positively related to seston elemental ratios, suggesting the dominance of algal sedimentation. In fishless mesocosms, N/P ratios of sinking material and of zooplankton showed a strong negative relationship, indicative of a major contribution of zooplankton-egested material to sinking particles. Analyses of free lipids in sediments confirmed the distinct origins of sinking material. Despite the absence of clear elemental composition distinctions between the two types of sediment, food-web structure strongly modified sediment biochemical composition.

Within biofilms, microorganisms are exposed to a wide range of chemicals released by phototrophic organisms. Those chemicals are likely to influence the dynamics and functioning of biofilms. 2E,4E-decadienal (DD) is a polyunsaturated aldehyde produced by diatoms which is known to induce adverse effects in many aquatic organisms . It has been shown to inhibit the adhesion and motility of one benthic diatom. The aim of this article was to determine if the effects of DD on diatom adhesion were widespread and if it could affect biofilm formation and functioning. The adhesion of 5 of 10 benthic diatom strains was strongly inhibited at 2.5 μg ml −1 DD. This indicates a high variability in diatom sensitivity to DD. Several experiments in microcosms showed that the presence of DD diffusing from a substrate decreased biofilm formation. This effect was dose-dependent and persisted for 72 h, though the molecule is highly volatile. Using a PHYTO-PAM fluorometer, we also showed that the effective quantum efficiency of charge separation of PSII of biofilms exposed to DD was negatively affected. This indicates a decrease in the efficiency of the photochemical processes. All these results suggest that the presence of DD-producing strains may have a significant impact on the composition and physiology of biofilms.

The effects of food web structure on the quantity and biochemical composition of seston, zooplankton and recently deposited sediment in experimental freshwater mesocosms were examined. Food web structure was manipulated by addition of zooplanktivorous fish. Biochemical characterisations were carried out using lipid biomarkers (sterols, fatty acids, chlorophyll-derived compounds and long-chain alkanediols). Fish addition decreased zooplankton biomass and increased seston biomass and deposited sediment through a trophic cascade. Fish presence strongly influenced the biochemical characteristics of seston and sediment. In contrast, food web structure had a minor impact on the lipid biomarker composition of zooplankton. Although the relative abundance of sterols in the different compartments did not differ strongly between treatments, sterol profiles in seston and sediment depended on food web structure. The predominance of Δ7-sterols in seston and sediment in the fish treatment indicated a major contribution of Chlorophyceae. In contrast, the distribution of sterols in seston and sediment in the fishless treatment, dominated by cholesterol, indicated a major zooplanktonic input. The distribution of fatty acids and the relative abundance of chlorophyll-derived compounds and long-chain alkanediols agreed with the predominant contribution of phytoplankton or zooplankton to seston and sediment in the two treatments. The relative abundance of bacterial biomarkers suggested that the contribution of bacteria was rather low. The high relative abundance of polyunsaturated fatty acids (PUFAs) and the absence of stanols in sediments suggested low microbial reworking of organic matter in the recently accumulated sediments. The trophic cascade, generated by the addition of fish, increased the relative abundance of PUFAs in deposited organic matter, thus enhancing sediment quality and potential degradability.

Lors d'une premiere etude de la flore diatomique epilithique de l'ile de La Reunion (France), quatre nouvelles especes appartenant au genre Geissleria Lange-Bertalot et Metzeltin ont ete decouvertes: Geissleria bourbonensis, G. mascarenicensis, G. creolorum et G. mafatensis. Une etude detaillee de leur morphologie a ete effectuee a l'aide du microscope photonique (MP) et du microscope electronique a balayage (MEB). Les nouvelles especes sont compareees aux autres especes du genre. Au MP, le seul critere qui permet de s'orienter vers le genre Geissleria est la presence d'un stigma central. Au MEB, la structure interne des areoles et notamment celles de type «annuloide» est tres rare dans le genre. De plus, pour la premiere fois dans le genre, les plaques occluant les areoles sont montrees etre similaires a un crible avec une seule rangee de minuscules pores. Les differents assemblages diatomiques incluant le genre Geissleria, bases sur l'abondance relative des taxons, sont detailles. Les especes nouvelles ...

Temporal patterns in biomass and in the production of inhibitory compounds were assessed for a diatom- dominated epilithic biofi lm of a shallow temperate river. Biomass showed a typical seasonal pattern with the de- velopment of two peaks of biomass parallel with species successions. Analysis with self-organizing maps (SOM), a non-supervised neural network, indicated that species composition of phototrophic assemblages was completely explained by environmental parameters during the low-biomass period. When epilithic biomass was high, these fac- tors were not enough to explain species composition. This suggests that as the biomass increased, intrinsic param- eters took over from extrinsic factors in the regulation of biofi lm composition. Over the studied period, 70 % of the biofi lm samples showed inhibitory activity against one or several of the four indicator phototrophic strains tested in the bioassays. The presence of inhibitory compounds was not correlated to biofi lm biomass. The occurrence of several patterns of inhibitory activity suggested the implication of different compounds or mix of compounds, certainly linked to various compositions of algal assemblages. The SOM analysis underlined putative relationships between the occurrence of certain species and the presence of an activity inhibiting one of the indicator strains. This indicates potential targets or producers of the implied compounds. To our knowledge, this study followed for the fi rst time the temporal dynamics of inhibitory activity in a biofi lm. The frequent presence of inhibitory compounds is in agreement with the hypothesis that biofi lms are auspicious systems for the development of allelopathic interac- tions, and provides new insights for further research on allelopathic interactions in benthic phototrophic systems.

In biofilms, the competition between microorganisms for light, nutrients and space is extreme. Moreover, planktonic algae can be considered as competitors insofar as they decrease the available light for the benthic algae. One of the strategies employed by microorganisms to eliminate competitors is the release of inhibiting compounds, a process known as allelopathy. Here we demonstrate that a benthic/epiphytic alga, Uronema confervicolum, produces allelopathic compounds that induce oxidative stress and growth inhibition in the planktonic Desmodesmus quadrispina. Some of these compounds can also trigger the formation of colony in D. quadrispina. As colonies have higher sedimentation rates than unicells, their induction by U. confervicolum might decrease shading. This study is the first report of colony induction in the context of alga-alga interaction. Our results also suggest the implication of mitogen-activated protein (MAP) kinases in the transduction of the signal leading to the formation of reactive oxygen species in the cells. A comparison with allelochemicals from another planktonic green alga, Monoraphidium aff. dybowski, emphasizes the specificity of colony induction by U. confervicolum, in contrast with oxidative stress which is induced by several compounds. The reciprocal production of inhibiting compounds by D. quadrispina makes this interaction an interesting example of co-evolution between two microorganisms belonging to different compartments of the ecosystem.

International audience; The role of chemical interactions in shaping microbial communities has raised increasing interest over the last decade. Many benthic microorganisms are known to develop chemical strategies to overcome competitors, but the real importance of chemical interactions within freshwater biofilm remains unknown. This study focused on the biological and chemical mechanisms of an interaction involving two benthic microorganisms, an allelopathic filamentous green alga, Uronema confervicolum, and a common diatom, Fistulifera saprophila. Our results showed that functions critical for benthic phototrophic microorganisms were inhibited by U. confervicolum extracts. Growth, cell motility, adhesion, and photosynthetic activity were impaired at extract concentrations ranging between 5 and 20 μg ml(-1). The adhesion inhibition was mediated by intracellular nitric oxide (NO) induction. A bioassay-guided fractionation of the extract with HPLC helped to identify two C18 fatty acids present in the growth-inhibiting fractions: linoleic (LA) and α-linolenic (LNA) acids. These compounds represented 77% of the total free fatty acids of U. confervicolum and were present in the culture medium (1.45 μg l(-1) in total). Both could inhibit the diatom growth at concentrations higher than 0.25 μg ml(-1), but had no effect on cell adhesion. The discrepancy between the effective concentrations of fatty acids and the concentration found in culture medium may be explained by the presence of high-concentration microenvironments. The compounds involved in adhesion inhibition remain to be identified. Though further experiments with complex biofilms are needed, our results suggest that U. confervicolum may participate to the control of biofilm composition by inhibiting diatom adhesion.

International audience; Autotrophic biofilms are complex and fundamental biological compartments of many aquatic ecosystems. Since microbial species differ in their sensitivity to stressors, biofilms have long been proposed for assessing the quality of aquatic ecosystems. Among the many stressors impacting aquatic ecosystems, eutrophication and metal pollution are certainly the most common. Despite that these stressors often occur together, their effects on biofilms have been far much studied separately than interactively. In this study, we evaluated the interactive effects of silver (Ag), a reemerging contaminant, and phosphorus (P), a nutrient often associated with freshwater eutrophication, on the structure and functioning of two types of autotrophic biofilms, one dominated by diatoms and another one dominated by cyanobacteria. We hypothesized that P would alleviate the toxic effects of Ag, either directly, through the contribution of P in metal detoxification processes, or indirectly, through P-mediated shifts in biofilm community compositions and associated divergences in metal tolerance. Results showed that Ag impacted biofilm community structure and functioning but only at unrealistic concentrations (50 mu g/L). P availability led to significant shifts in biofilm community composition, these changes being more pronounced in diatom- than those in cyanobacteria-dominated biofilm. In addition, P tended to reduce the impact of Ag but only for the cyanobacteria-dominated biofilm. More generally, our results highlight the preponderant role of the initial community structure and nutrient level on biofilm response to metallic pollutants.

The use of community-level physiological profiles obtained with Biolog microplates is widely employed to consider the functional diversity of bacterial communities. Biolog produces a great amount of data which analysis has been the subject of many studies. In most cases, after some transformations, these data were investigated with classical multivariate analyses. Here we provided an alternative to this method, that is the use of an artificial intelligence technique, the Self-Organizing Maps (SOM, unsupervised neural network). We used data from a microcosm study of algae-associated bacterial communities placed in various nutritive conditions. Analyses were carried out on the net absorbances at two incubation times for each substrates and on the chemical guild categorization of the total bacterial activity. Compared to Principal Components Analysis and cluster analysis, SOM appeared as a valuable tool for community classification, and to establish clear relationships between clusters of bacterial communities and sole-carbon sources utilization. Specifically, SOM offered a clear bidimensional projection of a relatively large volume of data and were easier to interpret than plots commonly obtained with multivariate analyses. They would be recommended to pattern the temporal evolution of communities' functional diversity.

Benthic dinoflagellates of the genus Ostreopsis Schmidt are common in tropical and subtropical water, and some species produce toxins potentially involved in human intoxication events. A benthic bloom of Ostreopsis mascarenensis Quod was observed near Rodrigues Island during a survey of benthic dinoflagellates in the southwestern Indian Ocean. The morphology of O. mascarenensis was studied by LM and SEM. Preliminary screening of a crude extract of an O. mascarenensis bloom revealed neurotoxicity in mice similar to that induced by palytoxin. After partition of the crude extract, the highest toxicity was retained in the butanol-soluble fraction, which retained hemolytic activity suggestive of palytoxin analogues. Two new toxins, mascarenotoxin-A and -B, were resolved from this fraction by HPLC coupled to a diode array detector. The closed mass spectrum profile and fragmentation pattern obtained by advanced nano-electrospray ionization quadrupole time-of-flight mass spectrometry between purified toxins and a reference palytoxin confirmed the mascarenotoxins as palytoxin analogues. These results were confirmed by tandem mass spectrometry with the identification of specific fragment ion m/z 327. An on-line liquid chromatography protocol coupled to tandem mass spectrometry was developed for detection of these palytoxin analogues. The present study describes the first purification, chemical, and toxicological characterization of new palytoxin analogues isolated from a benthic bloom of O. mascarenensis. These results suggest that O. mascarenensis, which is largely distributed in the southwestern Indian Ocean, could be a source of palytoxin poisoning in this tropical area.

International audience; We studied a group of monoraphid diatom species (Bacillariophyceae, Achnanthidiaceae) found in the marine coastal environment of tropical islands of the Caribbean Sea, western Atlantic Ocean, Indian Ocean, South Pacific Ocean, the Adriatic Sea and the Black Sea. Based on light and electron microscope examination, Madinithidium was formally described as a genus new to science with the generitype defined as Madinithidium undulatum. The characteristic features of the valve structure are a strongly developed sternum and raphe sternum, transapical striae formed by a single areola (macroareolae) positioned in small depressions, elevated virgae and coaxial internal central raphe endings. Furthermore, the striae of both raphe and sternum valves are closed by finely perforated hymenes. Madinithidium and Achnanthidium species are difficult to identify correctly with light microscopy since their valves are small and finely structured. Four species recently described from the western Indian Ocean and assigned to Achnanthidium sensu lato – Achnanthidium capitatum, A. flexuistriatum, A. pseudodelicatissimum and A. scalariforme – possessed morphological features permitting their placement into Madinithidium gen. nov. The above-mentioned species were formally transferred to the new genus.

The response of microbial communities to disturbance is a major concern for microbial ecologists since potential modifications in their composition and functioning may affect ecosystems to a larger extent. Microbial ecosystems may be resistant (not affected) or may present engineering (return to initial state) or ecological resilience. In the latter case, when the disturbance is released, the ecosystem evolves towards a new equilibrium state. The aim of this study was to determine if variations in the magnitude of a disturbance could induce either engineering or ecological resilience. We used phototrophic biofilms grown in mesocosms as a model of microbial ecosystem and increasing drought duration (1–8 weeks) as a range of disturbances. Biofilm composition (algal and prokaryotic), photosynthetic activity (PhytoPAM), and potential functional diversity (Biolog) were determined at the end of dry phase and after a 2-week rewetting phase in individual aquaria. We only observed an ecological resilience of the biofilm, with a resistance of phototrophic component for the weakest disturbance. After rewetting, the biofilm could fulfill the same functions, but its species composition was highly modified. We observed a shift from cyanobacteria dominance towards diatom dominance. The disturbance caused a transition towards a new steady state of the biofilm. We also observed a positive effect of stress duration on biofilm productivity after resilience.

A new benthic dinoflagellate, Coolia areolata Ten-Hage, Turquet, Quod & Coute sp. nov., was isolated from sandy substrata in La Possession Bay, La Reunion, and at various other sites in the western Indian Ocean. This new species is described and illustrated by light and scanning electron micrographs and drawings. The plate formula of C. areolata is P03′7″ 6c 6?s 5′″ 2″″. Cell size ranges are 30–36.5 μm long and 28–36.5 μm wide. Cells are spherical, areolate, and covered with scattered round pores. The surface of the apical plate 1′ is smooth. The apical pore is 9–10 μm long. Cells of C. areolata are distinguished from C. monotis and C.tropicalis by thecal surface morphology, thecal plate size and shape, and apical pore length.

Okadaic acid was isolated from a strain of Prorocentrum arenarium Faust (Prorocentrales, Dinophyceae) collected from Europa Island (40 degrees 22'E, 22 degrees 20'S, SW Indian Ocean). The presence of okadaic acid in the algal extract was suspected after cytotoxicity and phosphatase 2A inhibition testing. It was confirmed by ADAM derivatization, immunoaffinity extraction and liquid chromatography with fluorimetric detection analysis as well as by liquid microchromatography with mass spectrometric detection. Results indicate that the P. arenarium strain was toxinogenic and could be potentially involved in the toxin production associated with the human diseases, diarrhetic shellfish poisoning and possibly ciguatera fish poisoning in the SW Indian Ocean area.

A new toxic benthic marine dinoflagellate, Prorocentrum borbonicum Ten-Hage, Turquet, Quod, Puiseux-Dao & Coute sp. nov., isolated from the fringing coral reef of St Leu from La Reunion Island (France, southwest Indian Ocean) is described from light and scanning micrographs. The species was identified based on shape, size, surface micromorphology, ornamentation of thecal plates, and architecture of the periflagellar area and intercalary band. Prorocentrum borbonicum is a small species (18–24 μm long and 16–20 μm wide) with a broadly ovate shape. Thecal depressions are numerous (950–1020 per valve). Two sizes of valve pores were recognized: large pores, which are scattered on the valve surface, except in the central area, and smaller pores, which are localized chiefly in the centre of the valve and at the periphery, along the intercalary band. The periflagellar area is V-shaped and accommodates a flagellar pore and an auxiliary pore, the latter being the smaller of the two. Prorocentrum borbonicum...

A new benthic marine dinoflagellate, Sinophysis canaliculata Quod, Ten-Hage, Turquet, Mascarell et Coute sp. nov., was isolated from coral and sediment samples from sites in the western Indian Ocean. This new species is described and illustrated by drawings and light and scanning electron micrographs. Members of Sinophysis Nie et Wang are generally laterally flattened with circular outlines. The new species differ from others of the genus in the following characters: cell shape and size and features of thecal plates (plate shapes, size and number of valve pores, presence of a narrow thecal cut in the left hypothecal plate). Sinophysis canaliculata is heterotrophic, lacking any chloroplasts.

A preliminary screening of toxins of the crude extract of Ostreopsis mascarenensis Quod, a benthic dinoflagellate from the south-western Indian Ocean, revealed a neurotoxicity in mice similar to that induced by palytoxin, one of the most potent marine toxins involved in seafood poisoning. The highest toxicity was retained in the butanol-soluble fraction, revealing a strong haemolytic activity, as found for palytoxin. Analyses using high-performance liquid chromatography with ultraviolet detection allowed the detection of two new toxins named mascarenotoxins (McTXs). Their mass spectrum profile and fragmentation pattern, obtained by advanced nano-ESI-Q-TOF mass spectrometry of purified McTXs, were similar to those of a reference palytoxin. They were identified as palytoxin analogues, with the characterisation of a specific fragment ion m/z 327. O. mascarenensis is a species not previously known to produce palytoxin analogues.

International audience; Habitat stability is an important driver of ecological community composition and development. River epilithic biofilms are particularly unstable habitats for the establishment of benthic communities because they are regularly disturbed by floods. Our aim was to determine the influence of habitat instability on meiobenthic organisms. We hypothesized that hydrologic variables are the most important predictors of meiofauna distribution. We monitored epilithic communities (meiofauna and microalgae) with a high sampling frequency during 2 sampling periods with contrasting hydrodynamic patterns in a temperate river (the Garonne, France). Nematodes and rotifers dominated meiofaunal assemblages. The critical flow velocity threshold for their maintenance in the biofilm was ,30 cm/s, a result suggesting that meiofauna can resist higher flow velocity within the biofilm than within sediments. Nematode distribution was primarily influenced by the duration of undisturbed periods, whereas rotifer distribution was also correlated with the thickness of the biofilm. During the periods after floods, rotifers were faster colonizers than nematodes. Collectively, our results show that flow regime was an essential driver for biofilm community development.

International audience; In their natural environment, the structure and functioning of microbial communities from river phototrophic biofilms are driven by biotic and abiotic factors. An understanding of the mechanisms that mediate the community structure, its dynamics and the biological succession processes during phototrophic biofilm development can be gained using laboratory-scale systems operating with controlled parameters. For this purpose, we present the design and description of a new prototype of a rotating annular bioreactor (RAB) (TayloreCouette type flow, liquid working volume of 5.04 L) specifically adapted for the cultivation and investigation of phototrophic biofilms. The innovation lies in the presence of a modular source of light inside of the system, with the biofilm colonization and development taking place on the stationary outer cylinder (onto 32 removable polyethylene plates). The biofilm cultures were investigated under controlled turbulent flowing conditions and nutrients were provided using a synthetic medium (tap water supplemented with nitrate, phosphate and silica) to favour the biofilm growth. The hydrodynamic features of the water flow were characterized using a tracer method, showing behaviour corresponding to a completely mixed reactor. Shear stress forces on the surface of plates were also quantified by computer simulations and correlated with the rotational speed of the inner cylinder. Two phototrophic biofilm development experiments were performed for periods of 6.7 and 7 weeks with different inoculation procedures and illumination intensities. For both experiments, biofilm biomasses exhibited linear growth kinetics and produced 4.2 and 2.4 mg cm-2 of ash-free dry matter. Algal and bacterial community structures were assessed by microscopy and T-RFLP, respectively, and the two experiments were different but revealed similar temporal dynamics. Our study confirmed the performance and multipurpose nature of such an innovative photosynthetic bioreactor for phototrophic biofilm investigations.

International audience; Lotic epilithic biofilms are submitted to seasonal disturbances (e.g. flood events, self-detachment), which influence the biomass, diversity and viability of their algal and bacterial communities. The objective of this study is to examine whether (1) biofilm-dwelling nematodes respond to such seasonal changes in terms of diversity and community structure, (2) nematode species and feeding-types distribution respond to the varied trophic situations within the biofilm, since variations in biofilm microalgal composition may represent a variation in available food. The biofilm-dwelling nematode community was monitored in a temperate river over an 18 month period with a high sampling frequency. These data were linked to environmental abiotic and biofilm biotic factors. Nematode density was positively correlated to biofilm and microalgal biomass, but was dampened by floods. A clear seasonal pattern of the community was detected (summer shift), so that two nematode groups stand out: (1) the epistrate-feeders Chromadorina bioculata (Schultze in Carus, 1857) and Chromadorina viridis (Linstow, 1876) were primarily related to diatom availability, and dominated the nematode assemblage most of the time, (2) seven species from various feeding types (deposit-feeders, suction-feeders, and chewers) grew mainly under summer conditions concomitantly to a change of biofilm trophic status and microalgal composition. Overall, the results suggested that, in addition to abiotic disturbances, the availability of potential preys in the biofilm might represent an important driver of nematode community patterns.

Phototrophic biofilm formation and dynamics result from the interaction between several parameters, including chemical interactions. Some of the secondary metabolites released by microalgae can influence the composition of benthic communities. We determined the effects of decadienal (DD), a polyunsaturated aldehyde produced by diatoms, on a benthic diatom, Fistulifera saprophila. At 5 μg ml(-1), DD reduced cell motility by 88% and cell adhesion to the substrate by 91%. The effects occurred in less than 30 min. Using a fluorescent probe, we showed that DD could induce nitric oxide (NO) accumulation in F. saprophila cells. Cells exposed to a NO donor presented reduced adhesion and motility, which suggests the involvement of this cellular messenger in the mode of action of DD. Short-term experiments in microcosms showed that the presence of DD on a substrate strongly inhibited biofilm formation. Moreover, when the biofilm was bispecific, DD modified the proportion of the two species present. This indicates that the presence of DD-producing diatoms in a biofilm may favor the presence of certain microalgae at the expense of others. In addition to the effects on adhesion and motility, DD induced the formation of aggregates of F. saprophila cells. Aggregation was independent of NO production. Complementary experiments were performed with two other benthic diatoms, Nitzschia palea and Mayamea atomus. They showed that the effects of DD on adhesion and aggregation were species-dependent.

Chemicals produced by aquatic organisms, and especially micro-organisms, have received increasing attention in the last decade for their role in shaping interactions and communities. Several cases emphasize the fact that chemical signals or defence may modulate interspecific interactions. Notably, it has been shown that diatoms, unicellular algae and key primary producers in aquatic ecosystems produce a wide range of bioactive metabolites. Among these compounds, polyunsaturated short-chain aldehydes in vitro strongly impair the reproduction of various potential grazers. In the field, the relationship between aldehyde production and reproductive failure in copepods remains unclear. Recent studies have suggested that these putative defence compounds may also be involved in intercellular communication and in interactions with competitors. Potential effects of the aldehyde precursors on various organisms have also been described. This review presents an overview of various results obtained in the last decade that could help us to understand the role of polyunsaturated aldehydes and their precursors in the ecology of diatoms. It is focused on the dichotomy between freshwater and marine environments. Indeed, most of the results on anti-proliferative aldehydes concern marine planktonic diatoms, whereas they are also known to be produced by benthic and freshwater species.

Algal and cyanobacterial composition in biofilms is controlled by several factors, including light, nutrients, substratum, hydrodynamics, and grazing. Another potential determinant rarely taken into account is allelopathy, i.e., negative chemical interactions between competitors. We examined the interaction between abiotic variables (light, nutrients, and salt stress) and allelopathy by analyzing the effects of culture conditions on production of allelochemicals by 2 green algal species (Uronema confervicolum and Desmodesmus quadrispina). Production of allelochemicals was greater during the exponential growth phase than during other growth phases and was affected by light, nutrients, and salt concentrations, albeit differently for the 2 species. We then exposed 3 target species (D. quadrispina, Nitzschia palea, and Nostoc PCC 7120) to compounds produced by D. quadrispina, Monoraphidium aff. dybowski, or U. confervicolum. Nutrient-depleted conditions decreased sensitivity, whereas elevated salt con...

Studies on the effects of biodiversity on ecosystem functioning have generally revealed a positive asymptotic relationship between biodiversity and single functions, suggesting species redundancy with respect to these functions. However, most research was performed on specific processes and did not consider ecosystem 'multifunctionality'. There is also little information on the relationship between genetic and functional diversity. To analyze this relationship, we performed a microcosm experiment on a complex lake assemblage of decomposers, in the presence of the green alga Scenedesmus obliquus, which acted as carbon source for decomposers. By manipulating nutrient enrichment and the N : P input ratio, we observed that the structures of particle-associated and free bacterial assemblages were highly predictable in response to stoichiometric constraints. For a given treatment, the taxonomic compositions of free and particle-associated bacterial communities appeared close to each other only when phosphorus was not depleted. A coinertia analysis revealed a clear coupling between the genetic diversity of the microbial community, assessed using PCR-denatured gradient gel electrophoresis, and its potential functional diversity, studied with Biolog Ecoplates. This suggests that an ecologically relevant fraction of bacterial communities is characterized by lower level of redundancy than frequently thought, highlighting the necessity of exploring further the role of biodiversity in multifunctionality within ecosystems.

A wide range of chemicals are exuded by microalgae or released by grazed cells. In biofilms, these bioactive compounds may potentially exert a large effect on growing cells, since the cellular density is high and intercellular distances are short. We studied whether such compounds affect the functioning of the biofilm and, therefore, we tested several compounds. These included eicosapentaenoic acid (EPA) and 2E,4E-decadienal (DD), which are produced during grazing on diatoms. In addition, we obtained a mixture of compounds by methanolic extraction of the allelo- pathic green alga Uronema confervicolum. We studied how the net oxygen production rates of an artificial diatom biofilm were affected by the different compounds. The extracts of U. confervicolum significantly reduced these rates. Unexpectedly, EPA had a strong effect on the oxygen production rates in the biofilm. A toxic effect was also observed in ecotoxicological assays with the diatom Nitzschia palea. However, the inhibitory effect of DD on the biofilm was weaker than that of EPA, while DD was shown to be more toxic in liquid culture. This differential effect on liquid culture and biofilms indicates that the organization of benthic algae in biofilm should be considered when studying chemical interactions between these organisms.

Copper is commonly used as an algicide and plant herbicide in the aquatic environment. Despite of its frequent application in rivers, ponds and lakes, few studies have been performed at the ecosystem level on primary producers' communities. A long-term lotic mesocosms study was carried on, in 20m long channels, under environmentally realistic concentrations of copper (0, 5, 25 and 75 microgL(-1)) delivered continuously for 18 months. Results showed significant effects at the 25 and 75 microgL(-1) copper treatment levels on the phytoplankton, periphyton and macrophyte community structure. Effects on biomass and abundance were dissimilar depending on the studied community. Phytoplankton showed no abundance variation with treatment, whereas periphyton had higher biomass and macrophytes had lower coverage at 25 and 75 microgL(-1). Taxa richness was significantly affected for all primary producers communities at 25 and 75 microgL(-1). Some taxa seemed to be sensitive to copper, i.e. Cocconeis spp., Epithemia sp., Gomphonema spp., Lemna minor, Nasturtium officinale and filamentous macroalgae. Other taxa appeared to be tolerant or facilitated by indirect effects, i.e. Leptolyngbya sp., Mougeotia sp., Nitzschia palea, Pseudanabaena sp. and Ulothrix sp. and therefore increased in the 25 and 75 microgL(-1) treatments. Important indirect factors influenced the community responses, such as trophic interactions like decrease of grazing pressure, competition for light and nutrients and temperature favouring blue-green algae development. After 18 months of copper exposure, the no observed effect concentration at community level (NOEC(community)) for primary producers was set up at 5 or 4mugL(-1), if we consider the average effective concentration, with a lowest observed effect concentration (LOEC(community)) at 25 or 20 microgL(-1) (AEC).

In aquatic ecosystems, phytoplanktonic organisms are the major primary producers and bacteria the major decomposers. The interactions between phytoplankton and bacteria may be dependent on nutrient resources. Anthropogenic inputs, by modifying nutrient status and stoichiometry of lakes, might induce changes in these interactions, and thus, could have many consequences on some ecological processes such as primary production or importance of microbial recycling activity. To test this hypothesis, we grew an axenic strain of a green alga, Scenedesmus obliquus, in a range of stoichiometric situations, in absence and in presence of a natural bacterial community. Here, we show that different phytoplankton limiting factors can generate between algae and bacteria either competition for nutrients in phosphorus-limited conditions, commensalism in nitrogen-limited conditions, or mutualism in eutrophic nutrient-unlimited conditions. Causes of these different interaction types are discussed, in particular the hypothesis that in very eutrophic systems with high primary production, mutualism between algae and bacteria could be due to CO2 supply by heterotrophic respiration to inorganic carbon limited algae. Some probable consequences for aquatic ecosystems functioning are proposed.

Summary 1. The photoautotrophic micro-organisms collectively termed ‘micro-algae’ (including micro-eukaryotes and cyanobacteria) are known to produce a wide range of secondary metabolites with various biological actions. A small subset of these compounds has been identified. Some of them, termed allelopathic compounds, have been shown to play a role in allelopathy, defined here as inhibitory effects of secondary metabolites against either competitors or predators. Freshwater cyanobacteria also produce some secondary metabolites, termed toxins, which are highly toxic for animals. 2. While allelopathic compounds play a role in the interactions between the emitter organisms and their direct competitors or predators, toxins are categorised according to their toxic effect on several organisms, including some that may not be present in their immediate environment. However, these two definitions are not mutually exclusive. This review considers the evolutionary, ecological and physiological aspects of the production of allelopathic compounds by micro-algae in freshwaters, and compares the characteristics of allelopathic compounds with those of toxins. 3. Allelopathic compounds include alkaloids, cyclic peptides, terpens and volatile organic compounds. Toxins include alkaloids, cyclic peptides and lipopolysaccharides. No allelopathic compound type is associated with a particular phylogenetic group of algae. In contrast, freshwater toxins are only produced by cyanobacteria belonging to a restricted number of genera. Allelopathic compounds have various modes of action, from inhibition of photosynthesis to oxidative stress or cellular paralysis. Toxins are often enzyme inhibitors, or interfere with cell membrane receptors. 4. The ecological roles of allelopathic compounds have been well identified in several cases, but those of toxins are still debated. In the light of descriptions of negative effects of toxins on both micro-invertebrates and photoautotrophic organisms, we suggest that at least some toxins should actually be considered as allelopathic compounds. Further research on toxic secondary metabolites in freshwaters is now needed, with emphasis on the ecological effects of the compounds in the immediate environment of the emitter algae.

The aim of the present study was to examine the effects of biofilm-dwelling nematode activity on the oxygen turnover of cultured diatom biofilms. Oxygen microprofiles were simultane- ously measured using the microelectrode technique in biofilms incubated with and without nema- todes during short-term (24 h) and medium-term (6 d) experiments. During 24 h nycthemeral incuba- tions, maximum oxygen concentrations and oxygen peak depths of microprofiles, as well as net oxygen production rates, were significantly higher in the diatom biofilms with nematodes. These effects might be dependent on the densities of nematodes. In the present study, significant effects were observed at nematode densities exceeding 100 ind. cm -2 , but not at a density

Reliability of bacterial diversity assessment using polymerase chain reaction (PCR) denaturing gradient gel electrophoresis (DGGE) analysis of 16S rDNA fragments was evaluated for a particular complex microbial assemblage: river epilithic biofilm. By comparing 3 routine protocols on replicates of one river biofilm sample, we found that common DNA extraction procedures gave comparable diversity (from 28.0 to 30.7 bands detected) and community composition (> 75% of homology) despite differences in the total amount of extracted DNA (from 0.9 to 4.2 microg). Therefore methodological improvements only concerned electrophoretic separation of DNA fragments (range of denaturing gradient from 35% to 70% and migration time=18h) and standardisation of DNA amounts used (PCR-template=50 ng, gel loading=700 ng). Using such a standardised methodology we found a good reproducibility of all steps of the procedure. When an Escherichia coli strain was introduced as a contaminant in a biofilm sample, we were able to recover ribotypes from the strain. As concerns fields sampling, a satisfactory repeatability of banding patterns from neighbouring pebbles (sampling point) allowed discriminating between the biofilm intrasite variability (various points from a cross-profile). These trials confirmed that PCR-DGGE is suitable to assess a reliable genetic fingerprint of epilithic biofilms in the river. Phylogenetic analysis of 40 partial sequences of 16S rDNA from DGGE gels of two sets of river biofilms samples proved evidences for the retrieval of DNA fragments related to phototroph Eukarya. However, in both cases plastidial 16S rDNA represented less than 25% of the analysed operational taxonomic units. Taking into account that Cyanobacteria, as members of the Bacteria, were also detected, sequence analysis of relevant bands from the pattern is required to target "bacteria", i.e. the functional group of prokaryotic microorganisms to which one commonly refers as a key component in sustaining the nutrient turnover.

Benthic dinoflagellates of the genus Prorocentrum are common in tropical and subtropical water and several species produce phycotoxins potentially involved in human toxic outbreaks. The toxic dinoflagellate Prorocentrum borbonicum collected at La Reunion Island (France) was cultured in laboratory. A crude extract of the organism displayed significant toxicity in mice characterized by progressive limb paralysis, severe dyspnea, and death, and the toxicity was retained, after partition, in the extract's butanol-soluble fraction (BSF). Electrophysiological experiments characterizing the fraction's effect on isolated vertebrate neuromuscular preparations revealed that it depolarizes the muscle membrane and reduces the driving force for endplate potentials (EPPs) evoked by nerve stimulation, blocking directly- and indirectly-elicited muscle twitches. The depolarization induced by P. borbonicum BSF was not due to Na(+) influx through voltage-dependent Na(+) channels, since tetrodotoxin neither prevented nor suppressed the depolarization. However, ouabain, a specific ligand of the Na/K ATPase, reduced the depolarization. These results suggest the presence of palytoxin-like compounds in the fraction. HPLC-MS and MS/MS analysis showed the presence of several toxins having identical UV absorbance, among which two new isomeric toxins, borbotoxin-A and -B, of molecular mass of 1037.6 Da were isolated. The purified borbotoxin-A, had no effect on the resting membrane potential of muscle fibers and did not affect directly-elicited muscle twitches. However, the toxin reduced nerve-evoked muscle twitches, in a dose-dependent manner, reduced EPPs' amplitudes and completely blocked miniature endplate potentials. These observations suggest that the main action of borbotoxin-A is to block post-synaptic nicotinic ACh receptors.

A new benthic marine dino¯agellate, Bysmatrum granulosum Ten-Hage, Turquet, Quod & Couté, sp. nov., was obtained from sediment and coral samples from sites of La Réunion Island (SW Indian Ocean). This new species is described and illustrated by light and scanning electron micrographs. Cells are 40-50 µm long and 40-46 µm wide. The epitheca is conical and smaller than the hypotheca, which is trapezoidal with convex sides. Plate tabulation is typical for the genus (Pₒ, X, 4', 3a, 7'', 6c, 5s, 5''', 2''''); epithecal plates 3’ and 4’’ separate the intercalary plates 2a and 3a. Thecal plates are perforated by pores and covered by both small and minute wart-like projections, linearly arranged, radiating from the apical pore. This new species differs from the three others of the genus Bysmatrum in the following characters: cell shape and size, size of the apical pore complex and features of thecal plates (plate shapes and ornamentations)

International audience; A new freshwater diatom species belonging to the genus Achnanthidium Kütz. has been found in several French rivers namely those of the Languedoc-Roussillon and Rhin-Meuse regions. Achnanthidium delmontii sp. nov. is described based on light and scanning electron microscopy observations. This species belongs to the group of Achnanthidium taxa with terminal raphe fissures curved to the same side of the valve and it is compared to the most related species: Achnanthidium pfisteri Lange-Bertalot, Achnanthidium pyrenaicum (Husdedt) Kobayasi, Achnanthidium temniskovae Ivanov et Ector and Achnanthidium zhakovschikovii Potapova. The assessment of its ecology was made through physicochemical analysis carried out on sites which are part of water quality monitoring networks. Some comments are made on the rapid expansion of Achnanthidium delmontii sp. nov.

Allelopathic interactions are defined as the release, by an organism, of chemicals that inhibit competitors. In aquatic environments, they are recognised as a driver of phytoplankton community dynamics. Despite the favourable conditions created by the organisation of microorganisms in biofilms, allelopathy has been much less studied in benthic phototrophic microorganism communities. The objective of this thesis was to increase our comprehension of allelopathic interactions amongst phototrophic microorganisms in biofilms by analysing several aspects of a specific allelopathic interaction. The model chosen was the allelopathic effect of Uronema confervicolum Lagerheim, a filamentous green alga isolated from river biofilms, on diatoms from the same environment. The main hypothesis was that, at least in specific environmental conditions, U. confervicolum produce and release sufficiently large amounts of allelopathic compounds to inhibit significantly the development of epiphytic diatoms. Three questions arised from this hypothesis: (i) what are the allelopathic compounds implied, (ii) do their production and release depend on environmental factors and (iii) what is the effect of these compounds on epiphytic diatoms? This interaction has been studied using multiple approaches. Activity of algal biomass extracts and culture filtrates have been tested on diatom growth, photosynthesis, adhesion and motility. Bioassay-guided fractionation and metabolomic profiling have been used to identify the allelopathic compounds produced. The effect of environmental factors (nutrient limitation, light and gas supply) and growth phase on the production and release of the different allelopathic compounds have also been studied. In-depth study of the effect of anti-adhesion allelochemicals on diatom by combining metabolomic profiling, transcriptomics and scanning electron microscopy have been carried out. The extracts of U. confervicolum exhibited negative effects on diatoms growth, photosynthesis, adhesion and motility in bioassays. Two polyunsaturated fatty acids (linoleic and a-linolenic acids) have been identified as allelopathic compounds affecting growth and photosynthesis but not adhesion ability of diatoms. The inhibition of diatom adhesion was caused by other allelopathic compounds, unidentified so far. Increasing light intensity enhanced the production of U. confervicolum allelopathic compounds as predicted by plant defence theories, but the regulation of allelopathic activity was not completely explainable by any classic plant defence theory. Finally, beside an inhibition of diatom adhesion U. confervicolum filtrate was also found to inhibit the formation of diatom extracellular polymeric substances matrix, affect energy metabolism and induce global modification of diatom transcriptome and metabolome. Some of the genes and metabolites implied in the response of diatoms to adhesion inhibition have been identified. These results indicate the strong potential of the filamentous green alga U. confervicolum to impact biofilm community by its allelopathic activity. Results on the allelopathic activity of this alga and the effect on diatoms in laboratory experiments provide the basis for further experiments to access the role of allelopathy in complex biofilms. These findings will encourage researchers working on biofilms to consider allelopathy when phototrophic biofilm ecology is studied.; Les interactions allélopathiques sont définies comme la libération par un organisme de composés chimiques inhibant les compétiteurs. Dans les milieux aquatiques, elles sont reconnues comme un facteur de la dynamique des communautés de phytoplancton. Malgré les conditions favorables créées par l'organisation des microorganismes en biofilms, l'allélopathie a été beaucoup moins étudiée dans les communautés de microorganismes phototrophes benthiques. L'objectif de cette thèse était d'étudier des interactions allélopathiques, au travers d'un cas particulier, entre les microorganismes dans les biofilms phototrophes d'eau douce. Ainsi le travail s'est focalisé sur les effets allélopathiques d'une algue verte filamenteuse, Uronema confervicolum Lagerheim, sur des diatomées benthiques. L'hypothèse principale est que, dans certaines conditions, U. confervicolum libère suffisamment de composés allélopathiques pour inhiber significativement le développement des diatomées épiphytes. Cette hypothèse soulève trois questions (i) quels sont les composés allélopathiques, (ii) leur production dépend-elle des conditions environnementales et (iii) quel est l'effet des composés allélopathiques produits sur les diatomées ? Cette interaction a été étudiée au travers de multiples approches. L'activité d'extraits de biomasse algale et de filtrat de culture a été testée sur la croissance, la photosynthèse, l'adhésion et la motilité des diatomées. Les techniques de fractionnement guidé par bioessais et de profilage métabolomique ont été utilisées pour identifier les composés allélopathiques produits. Les effets des facteurs environnementaux (limitations en nutriment, apports de lumière et de gaz) et de la phase de croissance sur la production et la libération de différents composés allélopathiques ont également été étudiés. Une étude approfondie de l'effet sur les diatomées des composés allélopathiques anti-adhésion a été réalisée en combinant le profilage métabolomique, la transcriptomique et la microscopie électronique à balayage. Les extraits d'U. confervicolum ont montré un effet négatif sur la croissance, la photosynthèse, l'adhésion et la motilité des diatomées. Deux acides gras polyinsaturés (acides linoléique et a-linolénique) ont été identifiés comme des composés allélopathiques inhibant la croissance et la photosynthèse des diatomées. L'inhibition de l'adhésion des diatomées est, par contre, causée par d'autres composés allélopathiques non identifiés. L'augmentation de l'intensité lumineuse a accru la production de composés allélopathiques, à l'instar des composés défensifs des plantes. Cependant, aucune des théories classiques sur les défenses chimiques des plantes n'a permis d'expliquer l'ensemble des résultats obtenus sur les questions de régulation. En plus de l'inhibition de l'adhésion, le filtrat d'U. confervicolum inhibe la formation de la matrice constituée de substances polymériques extracellulaires, impacte le métabolisme énergétique et induit une modification globale du transcriptome et du métabolome des diatomées. Une partie des gènes et métabolites les plus fortement impliqués dans la réponse des diatomées à l'inhibition de l'adhésion ont été identifiés. Ce travail a permis de mieux évaluer l'activité allélopathique de l'algue verte filamenteuse U. confervicolum et son impact potentiel sur la structure et le fonctionnement des biofilms phototrophes. D'autres approches seront nécessaires, à l'avenir, pour mieux comprendre le rôle global de l'allélopathie dans des biofilms phototrophes complexes.