In situ resistance, not immigration, supports invertebrate community resilience to drought intensification in a Neotropical ecosystem.

While future climate scenarios predict declines in precipitations in many regions of the world, little is known of the mechanisms underlying community resilience to prolonged dry seasons, especially in 'naïve' Neotropical rainforests. Predictions of community resilience to intensifying drought are complicated by the fact that the underlying mechanisms are mediated by species' tolerance and resistance traits, as well as rescue through dispersal from source patches. We examined the contribution of in situ tolerance-resistance and immigration to community resilience, following drought events that ranged from the ambient norm to IPCC scenarios and extreme events. We used rainshelters above rainwater-filled bromeliads of French Guiana to emulate a gradient of drought intensity (from 1 to 3.6 times the current number of consecutive days without rainfall), and we analysed the post-drought dynamics of the taxonomic and functional community structure of aquatic invertebrates to these treatments when immigration is excluded (by netting bromeliads) or permitted (no nets). Drought intensity negatively affected invertebrate community resistance, but had a positive influence on community recovery during the post-drought phase. After droughts of 1 to 1.4 times the current intensities, the overall invertebrate abundance recovered within invertebrate life cycle durations (up to 2 months). Shifts in taxonomic composition were more important after longer droughts, but overall, community composition showed recovery towards baseline states. The non-random patterns of changes in functional community structure indicated that deterministic processes like environmental filtering of traits drive community re-assembly patterns after a drought event. Community resilience mostly relied on in situ tolerance-resistance traits. A rescue effect of immigration after a drought event was weak and mostly apparent under extreme droughts. Under climate change scenarios of drought intensification in Neotropical regions, community and ecosystem resilience could primarily depend on the persistence of suitable habitats and on the resistance traits of species, while metacommunity dynamics could make a minor contribution to ecosystem recovery. Climate change adaptation should thus aim at identifying and preserving local conditions that foster in situ resistance and the buffering effects of habitat features.

Regulation of Fatty Acid Production and Release in Benthic Algae: Could Parallel Allelopathy Be Explained with Plant Defence Theories?

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.

Comparison of extraction methods for the characterization of extracellular polymeric substances from aggregates of three biofilm-forming phototrophic microorganisms.

This paper aims to define a robust procedure to extract extracellular polymeric substances (EPS) from aggregates of three benthic phototrophic microorganisms: the cyanobacterium Phormidium autumnale, the diatom Nitzschia palea, and the green alga Uronema confervicolum. This study focuses on the extraction efficiency of polysaccharide and protein EPS by using two physical methods (sonication, cation exchange resin) and three chemical methods (formamide, EDTA, Tween 20) with minimum cell lysis. Cell lysis was evaluated by monitoring chlorophyll a release. The results indicated that sonication or incubation of the algae aggregates with 0.25% Tween 20 induced a high level of cell lysis. A combined extraction approach, with an initial dispersing pretreatment (Ultra-Turrax, 13 500 r·min-1, 1 min), followed by formamide addition (0.22%) and then incubation with Dowex cation exchange resin (50 g per g of dry biomass), provided the highest amount of extracted EPS (mostly proteins), with low cell lysis. Furthermore, extracted EPS were characterized by size exclusion chromatography, and the obtained fingerprints revealed similar profiles for the three benthic microorganisms with a majority of low molecular weight polymers (400 to 11 300 Da). However, additional EPS of high (>600 000 Da) and intermediate (20 000 to 80 000 Da) molecular sizes were specifically detected in the diatom extracts.

Resilience of aggregated microbial communities subjected to drought--small-scale studies.

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.

Biofilms inactivate the free-living stage of Batrachochytrium dendrobatidis, the most destructive pathogen for vertebrate diversity.

Emerging infectious diseases threaten biodiversity and human health. Many emerging pathogens have aquatic life stages and all immersed substrates have biofilms on their surface, i.e., communities of microorganisms producing a gelatinous matrix. However, the outcome of the interactions between environmental biofilms and pathogens is poorly understood. Here we demonstrate that biofilms reduce the survival of the most impactful pathogen for vertebrate diversity, the invasive chytrid fungus Batrachochytrium dendrobatidis. Effects on its zoospores varied with biofilm composition in controlled settings and biofilm compositional variation also coincided with divergent impacts of chytridiomycosis on amphibian populations in nature. Our results suggest that biofilms form a biotic component of ecosystem resistance to Batrachochytrium dendrobatidis by reducing environmental transmission, and that they could be used to develop nature-based technologies to limit the impacts and spread of this invasive chytrid fungus. Our study warrants further research into the interactions between environmental biofilms and pathogenic and/or invasive micro-organisms.

The sensitivity of aquatic microbial communities to a complex agricultural contaminant depends on previous drought conditions.

In intermittent rivers, which represent a prominent part of worldwide rivers, aquatic organisms are exposed to sequential disturbances including flow cessation, potentially associated with water warming, desiccation process and flow resumption. At flow resumption, pollutants stored in soil and washed by rainfalls can reach fresh waters. The interaction between contamination and river intermittency is poorly understood. In this study, we aimed at understanding in what extent the intensity of dry period combined or not to water warming drives the sensitivity of aquatic communities to a complex agricultural run-off (ARO) during rewetting. Phototrophic biofilms, at the basis of freshwater food webs, were chosen as a model of community. Biofilms grown in laboratory were first exposed to a disturbance crossing two temperature conditions (not warmed, 22°C or warmed, 32°C) and three dry periods (no drying, short (3 days), or long (3 months)). Then they were exposed to a chemical mix of nitrates, copper and 3 pesticides at 6 gradual concentrations. Various descriptors associated with biofilm structure and function were assessed one week after ARO addition. When undisturbed biofilms were exposed to ARO, they shifted toward a more heterotrophic state as they lost algal richness and diversity, and gross primary production tended to decrease. Warming alone only slightly modified the sensitivity of biofilms to ARO, with lower effects on algal richness and a trend to increase the effect on gross primary production. In contrast, the association of warming and a dry period strongly modified the sensitivity to ARO, certainly due to the selection of generalist species and/or physiological acclimation inducted by the first disturbance. This study emphasizes the importance of considering water intermittency in the management of the ecological risk of chemicals in aquatic ecosystems.

Photosynthetic microorganisms effectively contribute to bryophyte CO2 fixation in boreal and tropical regions.

Photosynthetic microbes are omnipresent in land and water. While they critically influence primary productivity in aquatic systems, their importance in terrestrial ecosystems remains largely overlooked. In terrestrial systems, photoautotrophs occur in a variety of habitats, such as sub-surface soils, exposed rocks, and bryophytes. Here, we study photosynthetic microbial communities associated with bryophytes from a boreal peatland and a tropical rainforest. We interrogate their contribution to bryophyte C uptake and identify the main drivers of that contribution. We found that photosynthetic microbes take up twice more C in the boreal peatland (~4.4 mg CO2.h-1.m-2) than in the tropical rainforest (~2.4 mg CO2.h-1.m-2), which corresponded to an average contribution of 4% and 2% of the bryophyte C uptake, respectively. Our findings revealed that such patterns were driven by the proportion of photosynthetic protists in the moss microbiomes. Low moss water content and light conditions were not favourable to the development of photosynthetic protists in the tropical rainforest, which indirectly reduced the overall photosynthetic microbial C uptake. Our investigations clearly show that photosynthetic microbes associated with bryophyte effectively contribute to moss C uptake despite species turnover. Terrestrial photosynthetic microbes clearly have the capacity to take up atmospheric C in bryophytes living under various environmental conditions, and therefore potentially support rates of ecosystem-level net C exchanges with the atmosphere.

Warming lowers critical thresholds for multiple stressor-induced shifts between aquatic primary producers.

In aquatic ecosystems, excessive nutrient loading is a global problem that can induce regime shifts from macrophyte- to phytoplankton-dominated states with severe consequences for ecosystem functions. Most agricultural landscapes are sites of nutrient and pesticide loading, which can interact with other stressors (e.g., warming) in additive, antagonistic, synergistic or reversed forms. The effects of multiple stressors on the resilience of macrophyte-dominated states and on critical thresholds for regime shifts are, however, unknown. We test the effects of individual and combined stressors of warming, nitrate, and various pesticides typically found in agricultural run-off (ARO) on the growth of macrophytes, periphyton, and phytoplankton in microcosms. We applied a one-level replicated design to test whether ARO induces a regime shift and a multifactorial dose-response design to model stressor thresholds and disentangle stressor interactions along a gradient. The individual stressors did not induce a regime shift, but the full ARO did. Nitrate and pesticides acted synergistically, inducing a shift with increasing phytoplankton biomass and decreasing macrophyte biomass. Warming amplified this effect and lowered critical thresholds for regime shifts. Shallow aquatic ecosystems in agricultural landscapes affected by global warming thus increasingly risk shifting to a turbid, phytoplankton-dominated state, and negatively impacting ecosystem service provisioning. Multiple stressor interactions must be considered when defining safe operating spaces for aquatic systems.

Ecotoxicity of Heteroaggregates of Polystyrene Nanospheres in Chironomidae and Amphibian.

Due to their various properties as polymeric materials, plastics have been produced, used and ultimately discharged into the environment. Although some studies have shown their negative impacts on the marine environment, the effects of plastics on freshwater organisms are still poorly studied, while they could be widely in contact with this pollution. The current work aimed to better elucidate the impact and the toxicity mechanisms of two kinds of commercial functionalized nanoplastics, i.e., carboxylated polystyrene microspheres of, respectively, 350 and 50 nm (PS350 and PS50), and heteroaggregated PS50 with humic acid with an apparent size of 350 nm (PSHA), all used at environmental concentrations (0.1 to 100 µg L-1). For this purpose, two relevant biological and aquatic models-amphibian larvae, Xenopus laevis, and dipters, Chironomus riparius-were used under normalized exposure conditions. The acute, chronic, and genetic toxicity parameters were examined and discussed with regard to the fundamental characterization in media exposures and, especially, the aggregation state of the nanoplastics. The size of PS350 and PSHA remained similar in the Xenopus and Chironomus exposure media. Inversely, PS50 aggregated in both exposition media and finally appeared to be micrometric during the exposition tests. Interestingly, this work highlighted that PS350 has no significant effect on the tested species, while PS50 is the most prone to alter the growth of Xenopus but not of Chironomus. Finally, PSHA induced a significant genotoxicity in Xenopus.

Multiple-stressor exposure of aquatic food webs: Nitrate and warming modulate the effect of pesticides.

Shallow lakes provide essential ecological and environmental services but are exposed to multiple stressors, including agricultural runoff (ARO) and climate warming, which may act on different target receptors disrupting their normal functioning. We performed a microcosm experiment to determine the individual and combined effects of three stressors-pesticides, nitrate and climate warming-on two trophic levels representative of communities found in shallow lakes. We used three submerged macrophyte species (Myriophyllum spicatum, Potamogeton perfoliatus, Elodea nuttallii), eight benthic or pelagic microalgal species and three primary consumer species (Daphnia magna, Lymnaea stagnalis, Dreissena polymorpha) with different feeding preferences for benthic and pelagic primary producers. Eight different treatments consisted of a control, only nitrate, a pesticide cocktail, and a combination of nitrate and pesticides representing ARO, each replicated at ambient temperature and +3.5°C, mimicking climate warming. Pesticides negatively affected all functional groups except phytoplankton, which increased. Warming and nitrate modified these effects. Strong but opposite pesticide and warming effects on Myriophyllum drove the response of the total macrophyte biomass. Nitrate significantly suppressed Myriophyllum final biomass, but not overall macrophyte and microalgal biomass. Nitrate and pesticides in combination caused a macrophyte decline, and the system tipped towards phytoplankton dominance. Strong synergistic or even reversed stressor interaction effects were observed for macrophytes or periphyton. We emphasize the need for more complex community- and ecosystem-level studies incorporating multiple stressor scenarios to define safe operating spaces.

Asynchronous recovery of predators and prey conditions resilience to drought in a neotropical ecosystem.

The predicted increase in the intensity and frequency of drought events associated with global climate change will impose severe hydrological stress to freshwater ecosystems, potentially altering their structure and function. Unlike freshwater communities' direct response to drought, their post-drought recovery capacities remain understudied despite being an essential component driving ecosystem resilience. Here we used tank bromeliad as model ecosystem to emulate droughts of different duration and then assess the recovery capacities of ecosystem structure and function. We followed macroinvertebrate predator and prey biomass to characterize the recovery dynamics of trophic structure (i.e. predator-prey biomass ratio) during the post-drought rewetting phase. We showed that drought significantly affects the trophic structure of macroinvertebrates by reducing the predator-prey biomass ratio. The asynchronous recovery of predator and prey biomass appeared as a critical driver of the post-drought recovery trajectory of trophic structure. Litter decomposition rate, which is an essential ecosystem function, remained stable after drought events, indicating the presence of compensatory effects between detritivores biomass and detritivores feeding activity. We conclude that, in a context of global change, the asynchrony in post-drought recovery of different trophic levels may impact the overall drought resilience of small freshwater ecosystems in a more complex way than expected.

Effects of 2E,4E-decadienal on motility and aggregation of diatoms and on biofilm formation.

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.

The Response of Extracellular Polymeric Substances Production by Phototrophic Biofilms to a Sequential Disturbance Strongly Depends on Environmental Conditions.

Phototrophic biofilms are exposed to multiple stressors that can affect them both directly and indirectly. By modifying either the composition of the community or the physiology of the microorganisms, press stressors may indirectly impact the ability of the biofilms to cope with disturbances. Extracellular polymeric substances (EPS) produced by the biofilm are known to play an important role in its resilience to various stresses. The aim of this study was to decipher to what extent slight modifications of environmental conditions could alter the resilience of phototrophic biofilm EPS to a realistic sequential disturbance (4-day copper exposure followed by a 14-day dry period). By using very simplified biofilms with a single algal strain, we focused solely on physiological effects. The biofilms, composed by the non-axenic strains of a green alga (Uronema confervicolum) or a diatom (Nitzschia palea) were grown in artificial channels in six different conditions of light intensity, temperature and phosphorous concentration. EPS quantity (total organic carbon) and quality (ratio protein/polysaccharide, PN/PS) were measured before and at the end of the disturbance, and after a 14-day rewetting period. The diatom biofilm accumulated more biomass at the highest temperature, with lower EPS content and lower PN/PS ratio while green alga biofilm accumulated more biomass at the highest light condition with lower EPS content and lower PN/PS ratio. Temperature, light intensity, and P concentration significantly modified the resistance and/or recovery of EPS quality and quantity, differently for the two biofilms. An increase in light intensity, which had effect neither on the diatom biofilm growth nor on EPS production before disturbance, increased the resistance of EPS quantity and the resilience of EPS quality. These results emphasize the importance of considering the modulation of community resilience ability by environmental conditions, which remains scarce in the literature.

Heteroaggregates of Polystyrene Nanospheres and Organic Matter: Preparation, Characterization and Evaluation of Their Toxicity to Algae in Environmentally Relevant Conditions.

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.

Disentangling the direct and indirect effects of agricultural runoff on freshwater ecosystems subject to global warming: A microcosm study.

Aquatic ecosystems are exposed to multiple stressors such as agricultural run-off (ARO) and climate-change related increase of temperature. We aimed to determine how ARO and the frequency of its input can affect shallow lake ecosystems through direct and indirect effects on primary producers and primary consumers, and whether warming can mitigate or reinforce the impact of ARO. We performed a set of microcosm experiments simulating ARO using a cocktail of three organic pesticides (terbuthylazine, tebuconazole, pirimicarb), copper and nitrate. Two experiments were performed to determine the direct effect of ARO on primary producers (submerged macrophytes, periphyton and phytoplankton) and on the grazing snail Lymnaea stagnalis, respectively. Three different ARO concentrations added as single doses or as multiple pulses at two different temperatures (22°C and 26°C) were applied. In a third experiment, primary producers and consumers were exposed together to allow trophic interactions. When functional groups were exposed alone, ARO had a direct positive effect on phytoplankton and a strong negative effect on L. stagnalis. When exposed together, primary producer responses were contrasting, as the negative effect of ARO on grazers led to an indirect positive effect on periphyton. Periphyton in turn exerted a strong control on phytoplankton, leading to an indirect negative effect of ARO on phytoplankton. Macrophytes showed little response to the stressors. Multiple pulse exposure increased the effect of ARO on L. stagnalis and periphyton when compared with the same quantity of ARO added as a single dose. The increase in temperature had only limited effects. Our results highlight the importance of indirect effects of stressors, here mediated by grazers and periphyton, and the frequency of the ARO input in aquatic ecosystems.

Chemical interactions in diatoms: role of polyunsaturated aldehydes and precursors.

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.

Physiological responses of three mono-species phototrophic biofilms exposed to copper and zinc.

In freshwater ecosystem, phototrophic biofilms play a crucial role through adsorption and sequestration of organic and inorganic pollutants. However, extracellular polymeric substance (EPS) secretion by phototrophic biofilms exposed to metals is poorly documented. This work evaluated the physiological responses of phototrophic biofilms by exposing three microorganisms (cyanobacterium Phormidium autumnale, diatom Nitzschia palea and green alga Uronema confervicolum) to 20 and 200 µg L-1 of Cu or 60 and 600 µg L-1 of Zn, both individually and in combination. Analysis of metal effects on algal biomass and photosynthetic efficiency showed that metals were toxic at higher concentrations for these two parameters together and that all the strains were more sensitive to Cu than to Zn. U. confervicolum was the most impacted in terms of growth, while P. autumnale was the most impacted in terms of photosynthetic efficiency. In consequence to metal exposure at higher concentrations (Cu200, Zn600 and Cu200Zn600), a higher EPS production was measured in diatom and cyanobacterium biofilms, essentially caused by an overproduction of protein-like polymers. On the other hand, the amount of secreted polysaccharides decreased during metal exposure of the diatom and green alga biofilms. Size exclusion chromatography revealed specific EPS molecular fingerprints in P. autumnale and N. palea biofilms that have secreted different protein-like polymers during their development in the presence of Zn600. These proteins were not detected in the presence of Cu200 despite an increase of proteins in the EPS extracts compared to the control. These results highlight interesting divergent responses between the three mono-species biofilms and suggest that increasing protein production in EPS biofilms may be a fingerprint of natural biofilm against metal pollutants in freshwater rivers.

Interactive Impacts of Silver and Phosphorus on Autotrophic Biofilm Elemental and Biochemical Quality for a Macroinvertebrate Consumer.

Autotrophic biofilms are complex and fundamental biological compartments of many aquatic ecosystems. In particular, these biofilms represent a major resource for many invertebrate consumers and the first ecological barrier against toxic metals. To date, very few studies have investigated the indirect effects of stressors on upper trophic levels through alterations of the quality of biofilms for their consumers. In a laboratory study, we investigated the single and combined effects of phosphorus (P) availability and silver, a re-emerging contaminant, on the elemental [carbon (C):nitrogen (N):P ratios] and biochemical (fatty acid profiles) compositions of a diatom-dominated biofilm initially collected in a shallow lake. We hypothesized that (1) P and silver, through the replacement of diatoms by more tolerant primary producer species, reduce the biochemical quality of biofilms for their consumers while (2) P enhances biofilm elemental quality and (3) silver contamination of biofilm has negative effects on consumers life history traits. The quality of biofilms for consumers was assessed for a common crustacean species, Gammarus fossarum, by measuring organisms' survival and growth rates during a 42-days feeding experiment. Results mainly showed that species replacement induced by both stressors affected biofilm fatty acid compositions, and that P immobilization permitted to achieve low C:P biofilms, whatever the level of silver contamination. Gammarids growth and survival rates were not significantly impacted by the ingestion of silver-contaminated resource. On the contrary, we found a significant positive relationship between the biofilm P-content and gammarids growth. This study underlines the large indirect consequences stressors could play on the quality of microbial biomass for consumers, and, in turn, on the whole food web.

Colony induction and growth inhibition in Desmodesmus quadrispina (Chlorococcales) by allelochemicals released from the filamentous alga Uronema confervicolum (Ulotrichales).

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.

Nutrient effects on the genetic and functional diversity of aquatic bacterial communities.

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.