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National audience; L’évolution du niveau de tolérance des communautés microbiennes aux toxiques etii) des approches focalisant sur les capacités de biodégradation des pesticides organiques par les microorganismes hétérotrophes. De ce fait, nous avons mené un suivi trisannuel (2008 -2011) sur une rivière située dans la zone viticole du Beaujolais afin d’évaluer l’intérêt de ces deux types d’approches pour appréhender les conséquences écologiques de l’interdiction d’utilisation du diuron (effective en décembre 2008). Avant son interdiction, ce composé était le principal herbicide détecté dans le cours d’eau et ce, depuis de nombreuses années. Les résultats de cette étude montrent que l’interdiction d’utilisation du diuron a engendré une diminution du niveau de contamination du cours d’eau se traduisant par une diminution progressive i ) des capacités de tolérance des biofilms phototrophes à ce toxique et ii) des capacités des communautés sédimentaires microbiennes à le dégrader. Les conclusions de cette étude offrent donc des perspectives particulièrement intéressantes en vue de développer des bio-indicateurs d’exposition aux pesticides basées sur les mécanismes d’adaptation microbienne aux pesticides.

International audience; Microorganisms are vital in mediating the earth's biogeochemical cycles; yet, despite our rapidly increasing ability to explore complex environmental microbial communities, the relationship between microbial community structure and ecosystem processes remains poorly understood. Here, we address a fundamental and unanswered question in microbial ecology: 'When do we need to understand microbial community structure to accurately predict function?' We present a statistical analysis investigating the value of environmental data and microbial community structure independently and in combination for explaining rates of carbon and nitrogen cycling processes within 82 global datasets. Environmental variables were the strongest predictors of process rates but left 44% of variation unexplained on average, suggesting the potential for microbial data to increase model accuracy. Although only 29% of our datasets were significantly improved by adding information on microbial community structure, we observed improvement in models of processes mediated by narrow phylogenetic guilds via functional gene data, and conversely, improvement in models of facultative microbial processes via community diversity metrics. Our results also suggest that microbial diversity can strengthen predictions of respiration rates beyond microbial biomass parameters, as 53% of models were improved by incorporating both sets of predictors compared to 35% by microbial biomass alone. Our analysis represents the first comprehensive analysis of research examining links between microbial community structure and ecosystem function. Taken together, our results indicate that a greater understanding of microbial communities informed by ecological principles may enhance our ability to predict ecosystem process rates relative to assessments based on environmental variables and microbial physiology.

International audience; At the scale of geomorphological units (rif␣es, pools, and gravel bars), the contribution of the hyporheic zone to the functioning of streams and rivers depends on the hydrological exchanges between surface water and groundwater. These exchanges are largely controlled by sediment structure and texture, which are dif␣cult to assess with classical methods (shovelling and freeze coring). We aimed to evaluate the ability of a non-destructive method, ground-penetrating radar (GPR), to detect sediment structures associated with subsurface biologically active zones in a gravel-bed river. After GPR data acquisition and processing, a three-dimensional reconstruction of a gravel bar permitted the identi␣cation of two sediment facies: a cobble/gravel lithofacies denoted as ‘coarse’ and a sand/gravel lithofacies denoted as ‘␣ne’. We installed piezometers along two longitudinal pro␣les (each corresponding to a lithofacies identi␣ed by GPR) and monitored hydraulic head and temperature for 20days. Water and sediments were sampled along the two pro␣les to measure water physicochemistry, sediment characteristics, bacterial abundance and activity, and interstitial invertebrate assemblages. These measurements con␣rmed that the two pro␣les were characterized by distinct hydrological ␣ow rates and associated biological activities. Rapid water transport in the coarse pro␣le fuelled the hyporheic zone with organic matter, whereas water and organic matter supplies were lower in the ␣ne pro␣le. Consequently, the higher supply of organic matter in the coarse pro␣le was associated with higher microbial activities and invertebrate density and diversity. Therefore, GPR could be an ef␣cient tool to detect the sediment features playing a key role in hyporheic zone functioning.

International audience; Water resources and aquatic ecosystems are an essential concern of any policy for sustainable development. In the context of the European Water Framework Directive, which aims to achieve good chemical and ecological status of waters, the herbicide diuron was banned in France in December 2008. In lotic ecosystems, benthic microbial assemblages are considered as useful potential indicators of ecological status because they integrate the effects of multiple anthropogenic disturbances and have strong capacities to adapt to novel environmental conditions. Field studies have thus revealed that in situ diuron exposure can induce microbial adaptation leading to an increase in diuron biodegradation potential of sediment microbial communities. It suggests that microbial biodegradation capacities can represent a powerful ecological indicator for monitoring spatio-temporal variations in pesticide contamination and evaluating associated ecological effects. The aim of this study was thus to evaluate the use of freshwater sediment biodegradation potential as an ecological indicator for monitoring microbial recovery following a decrease in chronic pesticide exposure. For this purpose, a four-year case study (2008–2011) was conducted in a small stream (Morcille river) long exposed to high diuron concentrations, increasing from upstream to downstream. Our results show that the ban on diuron resulted in a progressive decrease in its concentrations in the Morcille river over the survey period. The decrease in the level of chronic diuron exposure in the river caused a fall in sediment diuron-mineralizing capacities, assessed by radiorespirometry using [ring-U-14C] diuron. It thus revealed a corresponding recovery of microbial communities, showing that the use of freshwater sediment biodegradation potential may be useful for assessing microbial recovery after a decrease in chronic exposure to pollutants, opening prospects for developing a new class of ecological indicator to monitor the recovery of biological quality of water resources.

National audience

International audience; 1. Aquifers are considered to be controlled bottom-up because of their dependence on organic matter supply from surface ecosystems. Microorganisms are generally assumed to form the base of the food web and to respond strongly to organic matter supply. Although the bottom-up control of microorganisms by carbon sources has been well documented, the potential top-down control of obligate groundwater invertebrates on microorganisms has never been addressed in alluvial aquifers. 2. The main aims of the present study were (i) to quantify the relative influences of the activity of a subterranean amphipod (Niphargus rhenorhodanensis) and the flux of dissolved organic carbon (DOC) on organic matter processing and microbial activity, biomass and abundance in slow filtration columns mimicking an alluvial aquifer, and (ii) to determine the feeding rate of N. rhenorhodanensis on sedimentary microbes by tracing the flux of a 13C-labelled source of DOC in batches (closed systems). 3. Slow filtration column experiments showed that microbial abundance, biomass and activity were primarily controlled by DOC flux, whereas the activity of N. rhenorhodanensis had only a slight effect on the microbial compartment. Modelling of carbon fluxes in the 13C-tracer experiments indicated that the feeding activity of the amphipod was too low to significantly modify microbial growth and activity. 4. Our experiments supported the hypothesis that groundwater ecosystems are controlled bottom-up. The small influence of N. rhenorhodanensis on the microbial compartment was probably linked to its slow metabolism. Our results highlight the need for further experiments to examine the relationship between metabolic rates of subterranean organisms and their role in ecosystem functioning.

Food webs play a crucial role in regulating the fluxes of matter and energy within ecosystems. Artificial recharge of aquifers relies heavily on the ability of groundwater biocenoses to degrade organic matter fluxes that is a condition to maintain the quality of groundwater. The objective of this work is to determine the impact of increased dissolved organic carbon supply on the trophic interactions between the microbial communities and invertebrate assemblages at the upper layers of groundwater artificially recharged with stormwater. Through a combined field and laboratory experimental approach, this work allows ranking the strength of relationship between dissolved organic carbon fluxes, environmental conditions, activity and diversity of microbial communities and abundance of invertebrate assemblages; Les réseaux trophiques jouent un rôle primordial dans la régulation des flux de matière et d’énergie au sein des écosystèmes. Dans le cadre des pratiques de recharge artificielle des aquifères, les biocénoses souterraines sont pleinement sollicitées et leur capacité à dégrader les flux de matière organique de surface conditionne le maintien de la qualité des eaux souterraines. L’objectif de ce travail est de déterminer l’influence d’une augmentation des flux de carbone organique dissous sur l’intensité des interactions trophiques entre les communautés microbiennes et les assemblages d’invertébrés au toit des nappes phréatiques rechargées artificiellement avec des eaux de ruissellement pluvial. A travers une approche expérimentale de terrain et de laboratoire, ce travail permet d’évaluer l’intensité des relations existant entre les flux de carbone organique dissous, les conditions environnementales, l’activité et la diversité de communautés microbiennes et l’abondance des communautés d’invertébrés.

International audience; L'urbanisation amène à imperméabiliser de façon massive la surface du sol, et donc à créer des bassins d'infiltration pour les eaux de pluie. Ces bassins provoquent des transports importants (hydriques, soluté, chaleur) qui peuvent perturber et polluer la nappe sous-jacente. Le milieu étant généralement très hétérogène, le calage entre expérimentation et modélisation est rarement satisfaisant à cause de la difficulté pour faire des mesures (accès au sous sol, faible densité et bruit). Nous proposons ici une étude numérique de l'influence de l'hétérogénéité du sol sur la répartition des champs de concentration en soluté et de température engendrés par un bassin d'infiltration. La modélisation confirme que l'évolution des concentrations en soluté est très bruitée par les hétérogénéités de perméabilité du sol. Le champ de température est beaucoup moins perturbé par les hétérogénéités, et donc plus facilement interprétable.

Food webs play a crucial role in regulating the fluxes of matter and energy within ecosystems. Artificial recharge of aquifers relies heavily on the ability of groundwater biocenoses to degrade organic matter fluxes that is a condition to maintain the quality of groundwater. The objective of this work is to determine the impact of increased dissolved organic carbon supply on the trophic interactions between the microbial communities and invertebrate assemblages at the upper layers of groundwater artificially recharged with stormwater. Through a combined field and laboratory experimental approach, this work allows ranking the strength of relationship between dissolved organic carbon fluxes, environmental conditions, activity and diversity of microbial communities and abundance of invertebrate assemblages, Les réseaux trophiques jouent un rôle primordial dans la régulation des flux de matière et d’énergie au sein des écosystèmes. Dans le cadre des pratiques de recharge artificielle des aquifères, les biocénoses souterraines sont pleinement sollicitées et leur capacité à dégrader les flux de matière organique de surface conditionne le maintien de la qualité des eaux souterraines. L’objectif de ce travail est de déterminer l’influence d’une augmentation des flux de carbone organique dissous sur l’intensité des interactions trophiques entre les communautés microbiennes et les assemblages d’invertébrés au toit des nappes phréatiques rechargées artificiellement avec des eaux de ruissellement pluvial. A travers une approche expérimentale de terrain et de laboratoire, ce travail permet d’évaluer l’intensité des relations existant entre les flux de carbone organique dissous, les conditions environnementales, l’activité et la diversité de communautés microbiennes et l’abondance des communautés d’invertébrés.