SUMMARY1. 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.

In many ecosystems, detritus is the dominant source of energy and the driver of ecosystem functioning. In particular, in forested headwater streams, allochthonous detritus (e.g. leaf litter, dead wood) constitute the main energy source for detritivores and living primary producers contribute marginally to ecosystem metabolism and energy flows. We hypothesised that a low consumption of benthic diatoms, a high‐quality resource, could be of major importance for the growth of detritivores. In particular, these resources might represent an essential source of polyunsaturated fatty acids (PUFAs). In a microcosm experiment, three food resources were manipulated: alder (Alnus glutinosa: Betulaceae) leaf litter, fungal mycelium and a common benthic diatom. They were offered to juveniles of Gammarus fossarum (Crustacea: Amphipoda) as food resources, either alone or in combination, with each resource type being enclosed in agarose pellets. Juveniles were fed for 5 weeks in controlled conditions. Survival, feeding and growth rates were monitored. The fatty acids content of food resources and gammarids were also quantified. Our results showed that detritus alone permits survival, but not the significant growth of detritivores. The presence of diatoms in food resources was necessary to ensure a significantly positive mass growth of detritivores over the 5‐week experiment. More importantly, detritivores that did not receive algae in their food were generally unable to maintain their PUFA levels when compared to juveniles collected in the field. Gut‐content analysis of field‐collected G. fossarum showed that low amounts of benthic algae were always visible, indicating that most individuals fed at least for a small part on benthic algae. These minor, but high quality, food sources might therefore be essential for ensuring the growth and survival of detritivores. Our results clearly highlight the need to consider the functional importance of such minor food sources.

International audienceHabitat 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

Summary 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.

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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.

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1. Phototrophic microbes, also known as micro-algae, display a high abundance in many terrestrial surface soils. They contribute to atmospheric carbon dioxide fluxes through their photosynthesis, and thus regulate climate similar to plants.However, microbial photosynthesis remains overlooked in most terrestrial ecosystems. Here, we hypothesise that phototrophic microbes significantly contribute to peatland C uptake, unless environmental conditions limit their development and their photosynthetic activity.2. To test our hypothesis, we studied phototrophic microbial communities in five peatlands distributed along a latitudinal gradient in Europe. By means of metabarcoding, microscopy and cytometry analyses, as well as measures of photosynthesis, we investigated the diversity, absolute abundance and photosynthetic rates of the phototrophic microbial communities.3. We identified 351 photosynthetic prokaryotic and eukaryotic operational taxonomic units (OTUs) across the five peatlands. We found that water availability and plant composition were important determinants of the composition and the structure of phototrophic microbial communities. Despite environmental shifts in community structure and composition, we showed that microbial C fixation rates remained similar along the latitudinal gradient. Our results further revealed that phototrophic microbes accounted for approximately 10% of peatland C uptake. 4. Synthesis. Our findings show that phototrophic microbes are extremely diverse and abundant in peatlands. While species turnover with environmental conditions, microbial photosynthesis similarly contributed to peatland C uptake at all latitudes. We estimate that phototrophic microbes take up around 75 MT CO 2 per year in northern peatlands. This amount roughly equals the magnitude of | 3425