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The connectivity of environmental compartments through chemical and biological exchanges is often difficult to study. However, understanding the functioning of fluxes is essential in the context of climate change and the assessment of anthropogenic impacts. These exchanges can be realized through water cycle fluxes establishing interactions between the atmosphere, surface water and land.To examine the interactions between the atmosphere, surface water and land via water fluxes, we conducted a large scale field study at the watershed level, involving multiple disciplines from chemistry to meteorology and microbiology. The chemical and biological contents of water from the atmosphere (cloud and rain), to mid-mountain hydrological continuum (streams, wetlands and lake) and soil (agricultural plots) were assessed in the natural and agricultural area from Puy De Dôme (Central France) along an altitudinal gradient from puy de Dôme Mountain summit (1465 m asl) to the plain (~ 600 m asl). We set up experimental procedures for sampling, handling and analysing each environmental matrix, and the environmental context was characterized through meteorological and hydrological measurements and models. The biological and chemical variables included: isotopes of water (1H/2H and 16O/18O; laser spectroscopy), major ions (Na+, NH4+, K+, Mg2+, Ca2+, Cl-, NO3-, SO4-, IC), amino acids (LC-MS), bacterial diversity (16S metabarcoding and high-throughput sequencing) and microbial enzymatic activity associated with the nitrogen, carbon and phosphorus cycles (fluorimetric activity assays for whole Beta-Glucosidase, Leucine Aminopeptidase and Phosphatase activities).The multi-compartment analysis revealed significant differences between the compartments by the chemical variables, highlighting the compartment specificity. We estimated a chemical flux of major ions and amino acids from the atmosphere to the surface (soil and surface water). Bacterial diversity analysis showed a core community in these compartments, confirming their connectivity. Thereafter, we tried to explain bacterial diversity by the chemical variables from the studied compartments. Our analysis on microbial enzymatic activity showed an enzymatic activity associated with the nitrogen, carbon and phosphorus cycles in clouds and rain.Here, our study contributes to the understanding of atmosphere-surface interaction through field observations and atmospheric models and we attempted to better understand environmental fluxes. Our field study emphasized the importance of considering the interaction of environmental compartments in future investigations for future and gobal assessments of anthropogenic impacts, such as agrosystem effects to natural ecosystems. Key words:Field observations, Environmental interaction, Chemical flux, Microbial diversity, Atmosphere, Surface water, Soil.

International audience; The TIRAMISU collaboration gathers expertise from biologists, physicists, radiochemists and geologists within the Zone-Atelier Territoires Uranifères (ZATU) in France to study the response of microorganisms living in naturally radioactive mineral springs. Mineral springs are isolated ecosystems, extremely stable over geological time scales, which display different physicochemical and radiological parameters compared to their surroundings. Water and sediment samples collected in 27 mineral springs of the volcanic Auvergne region (Massif Central, France) have been analyzed for their microbial biodiversity and radionuclide content. Among the microorganisms present, diatoms (a microalgae species), widely used as environmental bioindicators of water quality, have shown to display an exceptional abundance of teratogenic forms in the most radioactive springs studied (radon activity up to 3700 Bq/L). The current work presents a first assessment of the dose received by the diatoms inhabiting naturally radioactive mineral springs. The radiological risk for microorganisms living in freshwater environments was estimated using the ERICA tool. Most of the sampled mineral springs were highly above the risk threshold of 10 mGy/h due to the large concentrations of radium in the sediments (up to 50 Bq/g). The complete radiological data on water and sediments are used as inputs to Monte Carlo simulations at micro- (GATE) and nano- (Geant4-DNA) scale in order to assess the direct and indirect damages on the diatom DNA.

International audience

Leaf litter decomposition is a key mechanism in headwater streams, allowing the transfer of nutrients and energy into the entire food web. However, chemical contamination resulting from human activity may exert a high pressure on the process, possibly threatening the structure of heterotrophic microbial communities and their decomposition abilities. In this study, the rates of microbial Alnus glutinosa (Alnus) leaf decay were assessed in six French watersheds displaying different land use (agricultural, urbanized, forested) and over four seasons (spring, summer, autumn, winter). In addition, for each watershed at each sampling time, both upstream (less-contaminated) and downstream (more-contaminated) sections were monitored. Toxicities (estimated as toxic units) predicted separately for pesticides and pharmaceuticals as well as environmental parameters (including nutrient levels) were related to microbial decay rates corrected for temperature and a range of fungal and bacterial community endpoints, including biomass, structure, and activity (extracellular ligninolytic and cellulolytic enzymatic activities). Results showed that agricultural and urbanized watersheds were more contaminated for nutrients and xenobiotics (higher pesticides and pharmaceuticals predicted toxicity) than forested watersheds. However, Alnus decay rates were higher in agricultural and urbanized watersheds, suggesting compensatory effects of nutrients over xenobiotics. Conversely, fungal biomass in leaves was 2-fold and 1.4-fold smaller in urbanized and agricultural watersheds than in the forested watersheds, respectively, which was mostly related to pesticide toxicity. However, no clear pattern was observed for extracellular enzymatic activities except that β-glucosidase activity positively correlated with Alnus decay rates. Together, these results highlight microbial communities being more efficient for leaf decomposition in polluted watersheds than in less contaminated ones, which is probably explained by changes in microbial community structure. Overall, our study showed that realistic chemical contamination in stream ecosystems may affect the biomass of Alnus-associated microbial communities but that these communities can adapt themselves to xenobiotics and maintain ecosystem functions.

9 páginas.- 7 figuras.- 4 tablas.- referencias.- Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2020.105503., A slow release formulation (SRF) of the herbicide mesotrione (MS) [2-(4-mesyl-2-nitrobenzoyl)cyclohexane-1,3-dione] was designed for extended weed control relative to that of conventional formulations. The SRF was designed by incorporating the herbicide in micelles formed by the fatty amine ethoxylated surfactant Ethomeen T/15 (ET15) followed by their further sorption on the clay mineral sepiolite. The content of active substance (a.s.) was very high, 16.8% w.w. The surfactant per se was adsorbed by developing surface aggregates on sepiolite's surface. FTIR spectroscopy and zeta potential measurements demonstrated that electrostatic interactions were responsible of the sorption of MS by using micelles loaded with the herbicide, modifying the conformation of ET15 on sepiolite. Toxicity assays by using Microtox® tests yielded a value of IC50 for the SRF (52.6 ± 3.6 mg L−1) that was one order of magnitude higher than that reported for the commercial formulation Callisto® (1.1 ± 0.1 mg L−1). In vitro water release experiments stated that the release of MS was a controlled-diffusion process. Application of SRF and Callisto® as post-emergence in field trials of maize showed that after 10 days of treatment (DAT), the amount in the soil of the herbicide from the SRF was about two-fold higher than that of Callisto®, with a larger accumulation (42% of the total) in the upper soil layer (0–10 cm depth). No significant differences in soil accumulation at longer times and in the crop yield were observed. Consequently, the use of the designed sepiolite-based formulation significantly reduced MS leaching over the conventional formulation while maintaining the desired bioactivity for weed control., This research was supported by the grant P09-RNM-4581 from the Junta de Andalucía (Spain). The authors are grateful to Campi y Jove S.A. (Barcelona, Spain) for the supply of the surfactant. The authors also acknowledge the Analytical Services of Seville University (CITIUS) for zeta potential measurements, elemental and thermogravimetric analysis, and microscopic characterization.

Triketones, derived chemically from a natural phytotoxin (leptospermone), are a good example of allelochemicals as lead molecules for the development of new herbicides. Targeting a new and key enzyme involved in carotenoid biosynthesis, these latest-generation herbicides (sulcotrione, mesotrione and tembotrione) were designed to be eco-friendly and commercialized fifteen-twenty years ago. The mechanisms controlling their fate in different ecological niches as well as their toxicity and impact on different organisms or ecosystems are still under investigation. This review combines an overview of the results published in the literature on beta-triketones and more specifically, on the commercially-available herbicides and includes new results obtained in our interdisciplinary study aiming to understand all the processes involved (i) in their transfer from the soil to the connected aquatic compartments, (ii) in their transformation by photochemical and biological mechanisms but also to evaluate (iii) the impacts of the parent molecules and their transformation products on various target and non-target organisms (aquatic microorganisms, plants, soil microbial communities). Analysis of all the data on the fate and impact of these molecules, used pure, as formulation or in cocktails, give an overall guide for the assessment of their environmental risks. (C) 2016 Elsevier B.V. All rights reserved.

International audience

The present study investigates the sensitivity of laccase activity to the fungicide tebuconazole (TBZ) in order to seek for new functional toxicity descriptors in aquatic microbial communities associated to decomposing litter. With this aim, we analyzed the sensitivity of laccase from the different microbial components (fungi and bacteria growing separately and in co-existence), as well as that of their corresponding enzyme fractions (cell bound and diffusible), forming microbial communities in Alnus glutinosa leaves. Results show that fungi are pivotal for laccase activity in leaves and that their activity is repressed when they co-exist with bacteria. The sensitivity of laccase activity to the TBZ was only detectable in leaves colonized by fungi separately (Alatospora acuminata populations), but absent in those colonized by bacteria separately and/or mixed fungi plus bacteria. Specifically, the increase of TBZ concentration enhances laccase activity in Alatospora acuminata populations but decreases ergosterol concentration as well as the amount of 18S RNA gene copies. This activity response suggests a detoxification mechanism employed by the fungus in order to reduce TBZ toxicity. Besides, enzyme fractioning showed that laccase activity in the cell bound fraction (76% of the total activity) was sensitive to the fungicide, but not that in the diffusible fraction (24% of total activity). Hence, TBZ would influence laccase activity in the presence of fungal cells but not in enzymes already synthesized in the extracellular space. The present study highlights the importance of the biological complexity level (i. e. population, community, ecosystem) when seeking for appropriate functional ecotoxicity descriptors in aquatic microbial communities.

International audience; Intertidal mudflats are ecosystems submitted to natural hydrodynamical forcings during each tide. When the off-shore water flows at high tide, a proportion of the biofilm produced at low tide can be resuspended in the water column and interact with the pelagic food web. As a consequence, the resuspension creates a link between the benthos and the pelagos, modifying their properties and the stability of the meta-ecosystem they form together. The aim of this study is to describe the consequences of the microbial biofilm resuspension on the pelagic food web, and to investigate the question of the stability of the benthos–pelagos coupling resulting from the biofilm resuspension. Two food webs were considered, corresponding to different hydrodynamical conditions in sum-mer condition: one allowing the biofilm massive resuspension, and one without resuspension, but with particle sedimentation. The Monte-Carlo Markov Chain Linear Modelling was used to estimate the unknown flows of the food web. The comparison of the Ecological Network Analysis indices for the two food webs allowed defining their respective differences of structure and functioning. The results showed that the massive resuspension of the microbial biofilm stimulates pelagic primary production and microbial food web via a higher bacterivory. The higher activity of the whole system coupled with both a drop in the specialisation of the trophic pathways and a low cycling activity demonstrated that when massive resuspension occurs, the system is disturbed. In con-trast, when sedimentation occurs, the food webs show functioning features pointing out to a higher stability of the whole system.

The Microtox(®) test, using the prokaryote Vibrio fischeri, was employed to assess the toxicity of the maize herbicides S-metolachlor, benoxacor, mesotrione and nicosulfuron, and their formulated compounds: Dual Gold Safeneur(®), Callisto(®) and Milagro(®); alone and in mixtures. For each compound we obtained original IC50 values, with consistent higher toxicities for formulated compounds compared to active ingredients alone. Mixtures of the four herbicides, prepared according to application doses encountered in agriculture, were found to be toxic at a lower concentration than single molecules. Mesotrione and nicosulfuron mixture appeared to be highly toxic to V. fischeri, however, this recommended post-emergence combination for maize crops got its toxicity decreased in formulated compound mixtures, suggesting that chemical interactions could potentially reduce the toxicity. Data comparisons to theoretical models showed a good prediction of mixture toxicity by Concentration Addition concept. Results seemed to exclude any synergistic effects on V. fischeri for the tested herbicide mixtures. Additional work coupling these bioassay data to ecosystemic level studies (aquatic and soil compartments) and data on additives and degradation products toxicity, will help to fill the gap in our knowledge of the environmental impact of these xenobiotics and in the choice of a more sustainable use of pesticides.

International audience; This study aimed to investigate the dose-response effects of an herbicide on soil photosynthetic microbial communities, particularly cyanobacteria, using a microcosm approach. Pure mesotrione (active ingredient), and Callisto (a commercial formulation of this triketone herbicide), were spread at different rates on soil microcosm surfaces. Soil Chlorophyll concentrations were quantified to assess the photosynthetic biomass, and the genetic structure and diversity of the cyanobacterial community were investigated by a group-specific polymerase chain reaction followed by denaturing gradient gel electrophoresis. Dose-dependent responses were evidenced for both functional and structural parameters. No effect was detected in soils treated with 1 × AR (1-fold recommended application rate) irrespective of the herbicide formulation. At 10 × AR (10-fold recommended application rate), only Callisto treatment induced significant decreases of photosynthetic biomass, whereas structural parameters were less affected. At the 100 × AR (100-fold recommended application rate), both pure mesotrione and Callisto had strong negative impacts on soil chlorophyll concentrations and cyanobacterial genetic structure and diversity. At both the 10 × AR and 100 × AR treatments, Callisto induced significant stronger effects than pure mesotrione. In addition, indicators of photosynthetic biomass, compared with structural parameters of cyanobacterial communities (within 14 days), responded (within 7 days) more quickly to herbicide stress. The results of this study underscore the relevance of soil photosynthetic microbial communities to develop indicators for herbicide risk assessment.

The trophic fate of benthic bacteria in an intertidal mudflat (Brouage, Marennes-Oleron, France) was evaluated in situ, and environmental parameters that potentially influence the consumption of bacteria by the most abundant organisms of the meio- and macrofauna were identified. Variations in grazing rates were estimated using 15N pre-enriched bacteria at different temporal and spatial scales on a cross-shore transect over the mudflat. Grazing incubations were performed in microcosms with freshly collected grazers. Environmental factors varied more by season than by day or sampling station. Bacterial uptake by grazers did not appear to be strongly influenced by abiotic factors and was not linked to bacterial abundance. Algal abundance was negatively correlated with bacterivory in both the nematode community and the foraminifer Ammonia tepida, suggesting that bacteria constitute an alternative resource that is consumed when algae are less abundant. Bacteria were mainly ingested by the mudsnail Hydrobia ulvae and secondarily by nematodes with grazing rates of copepods and A. tepida being considerably lower. The estimated grazing in the upper and middle part of the mudflat represented 7% and 28% of bacterial production, respectively. In the lower mudflat, daily grazing never represented >3% of bacterial production throughout the year. Consequently, grazing appears to be a minor factor in the regulation of bacterial production. Bacterivory did not vary clearly according to season; consequently, the fate of bacteria in this benthic food web is poorly structured by season.

International audience; Microbial communities driving the nitrogen cycle contribute to ecosystem services such as crop production and air, soil, and water quality. The responses to herbicide stress of ammonia-oxidizing and ammonia-denitrifying microbial communities were investigated by an analysis of changes in structure-function relationships. Their potential activities, abundances (quantitative PCR), and genetic structure (denaturing gradient gel electrophoresis) were assessed in a microcosm experiment. The application rate (1 × FR, 0.45 μg g(-1) soil) of the mesotrione herbicide did not strongly affect soil N-nutrient dynamics or microbial community structure and abundances. Doses of the commercial product Callisto® (10 × FR and 100 × FR) or pure mesotrione (100 × FR) exceeding field rates induced short-term inhibition of nitrification and a lasting stimulation of denitrification. These effects could play a part in the increase in soil ammonium content and decrease in nitrate contents observed in treated soils. These functional impacts were mainly correlated with abundance shifts of ammonia-oxidizing Bacteria (AOB) and Archaea (AOA) or denitrifying bacteria. The sustained restoration of nitrification activity, from day 42 in the 100 × FR-treated soils, was likely promoted by changes in the community size and composition of AOB, which suggests a leading role, rather than AOA, for soil nitrification restoration after herbicide stress. This ecotoxicological community approach provides a nonesuch multiparameter assessment of responses of N-cycling microbial guilds to pesticide stress.

International audience; Soil is a primary resource used by mankind to ensure its needs mainly through agriculture. Its sustainability is regulated by the indigenous organisms it contains such as microorganisms. Current agricultural practices employ mixtures of pesticides to ensure the crops yield and can potentially impair these non-target organisms. However despite this environmental reality, studies dealing the susceptibility of microorganisms to pesticide mixtures are scarce. In this context, we designed a 3-month microcosm study to assess the ecotoxicity of realistic herbicide mixtures of formulated S-metolachlor (Dual Gold Safeneur®), mesotrione (Callisto®), and nicosulfuron (Milagro®) on the abundance, the diversity, and the activities of microorganisms from a “clay/organic matter-rich” soil, with a particular attention given to N-cycle communities. These communities appeared to be quite resistant to realistic mixtures even if transient effects occurred on the N-cycle-related communities with an increase of ammonification and an inhibition of nitrification as a short-term effect, followed by an increase of denitrification and an accumulation of nitrates. As nitrates are known to be highly leachable with a strong pollution potential, intensive studies should be carried out at field level to conclude on this potential accumulation and its consequences. Moreover, these data now need to be compared with other agricultural soils receiving these herbicide mixtures in order to bring general conclusion on such practices.

Soil phototrophic microorganisms, contributors to soil health and food webs, share their particular metabolism with plants. Current agricultural practices employ mixtures of pesticides to ensure the crops yields and can potentially impair these non-target organisms. However despite this environmental reality, studies dealing the susceptibility of phototrophic microorganisms to pesticide mixtures are scarce. We designed a 3 months microcosm study to assess the ecotoxicity of realistic herbicide mixtures of formulated S-metolachlor (Dual Gold Safeneur(®)), mesotrione (Callisto(®)) and nicosulfuron (Milagro(®)) on phototrophic communities of two soils (Limagne vertisol and Versailles luvisol). The soils presented different colonizing communities, with diatoms and chlorophyceae dominating communities in Limagne soil and cyanobacteria and bryophyta communities in Versailles soil. The results highlighted the strong impairment of Dual Gold Safeneur(®) treated microcosms on the biomass and the composition of both soil phototrophic communities, with no resilience after a delay of 3 months. This study also excluded any significant mixture effect on these organisms for Callisto(®) and Milagro(®) herbicides. We strongly recommend carrying on extensive soil studies on S-metolachlor and its commercial formulations, in order to reconsider its use from an ecotoxicological point of view.

International audience; Resuspension thresholds in terms of friction velocity were experimentally quantified for the prokaryotes, protists and for the first time, viruses of intertidal mudflat biofilms. Differences in resuspension thresholds could be related to the type, behaviour and size of microorganisms and their association with particles. Free microorganisms (viruses, bacteria and some nanoflagellates) were resuspended by weak flow at friction velocities lower than 2 cm s− 1. Chlorophyll a, some nanoflagellates and attached bacteria were resuspended together with the bed's muddy sediment, which required friction velocities larger than 3 cm s− 1. Diatoms smaller than 60 μm were resuspended at velocities between 3 and 5 cm s− 1, while those larger than 60 μm were resuspended at higher friction velocities (5.5 to 6.5 cm s− 1).The thresholds of resuspension also depended on the micro-scale position of microorganisms in the sediment (horizontal and vertical distributions). In the field, the vertical distribution of chlorophyll a (a proxy of microphytobenthos) was skewed, with a maximum in the first 2 mm of sediment. Along the neap–spring tidal cycle, chlorophyll a revealed an increase in MPB biomass in the first 2 mm of the sediment, in relation to light increases with exposure durations. The horizontal distribution of chlorophyll a could be inferred from erosion experiments. During the initial phase of biofilm growth, the distribution of chlorophyll a seemed horizontally homogeneous, and was uniformly eroded at the beginning of the increase in chlorophyll a. From these results, we can make a hypothesis: in the subsequent phase of biofilm growth until the maximum of emersion duration, the eroded quantity of chlorophyll a was larger than expected based from chlorophyll a vertical distribution, suggesting that biofilm horizontal distribution became patchy and enriched chlorophyll a was preferentially eroded. When emersion duration and biofilm growth decreased, the trend was reversed, and eroded quantity of chlorophyll a was lower than expected from chlorophyll a vertical distribution, suggesting that areas with low chlorophyll a were preferentially eroded. Such erosion patterns when biofilm growth decreased probably resulted from the bulldozing activity of a surficial sediment bioturbator, the gastropod Peringia ulvae. Our study did not directly prove this horizontal distribution but it should be further discussed. This distribution needs to be studied to acquire real evidence of patchy distributions.

International audience; In intertidal mudflats, during low tide exposure, microphytobenthos (MPB) migrate vertically through the surface sediment and form, with the heterotrophic bacteria, a transient biofilm. Inside this biofilm, multiple interactions exist between MPB and bacteria. These micro-organisms secrete a wide range of extracellular polymeric substances (EPS), which are major components of the biofilm matrix. In this study, we used a tidal mesocosm experiment in order to decipher the interactions of the MPB-EPS-bacteria complex within the biofilm. We tried to determine if the EPS could control bacterial activities and/or production and/or richness according to the age of the biofilm and to the immersion/emersion period. The dynamics of biomasses of MPB and prokaryotes, the bacterial production, the hydrolysis of predominating organic constituents in the dissolved organic carbon (DOC) pool (i.e., carbohydrates and polypeptides), and the bacterial structure were studied in relation to the different EPS fractions (carbohydrates and proteins: colloidal and bound) dynamics during 8 days. Our experiment had emphasized the influence of the environmental conditions (light, immersion/emersion) on the interactions within the biofilm and also on the effects on biofilm ageing. Bacterial production was always inhibited by the bound EPS-carbohydrate, especially during low tide. Our results suggest that the concentration and composition of EPS had a major role in the bacterial/MPB interactions: these interactions can be either positive or negative in order to regulate the productive phases of MPB and bacteria.

Relationships between bacteria, microphytobenthos and extracellular polymeric substances (EPS) that make up microbial biofilms over bare mudflats were investigated at an hourly frequency during two 14-day spring– neap cycles in winter and summer 2008. Bacterial abundance and total chl a concentration were lower in summer (0.78 × 108 ± SD 0.39 × 108 cell.m−2 and 59.0 ± SD 10.42 mgchla.m−2) than in winter (3.7 × 108 ± SD 1.9 × 108 cell.m−2 and 106.64 ± SD 11.29 mgchla.m−2), coinciding with a high abundance of the gastropod Peringia ulvae in summer, which subsequently impacted 1st-cm chl a concentration by intense grazing. Bound and colloidal EPS carbohydrate temporal patterns were similar in winter (5.71 ± SD 3.95 and 4.67 ± SD 3.45 μg.g−1, respectively) butwere different in summer (14.9±SD 4.05 and 5.60±SD 4.50 μg.g−1, respectively). Carbohydrate colloidal EPS appeared to be related to light and salinity, while 1st-mm chl a concentration was negatively affected by strong salinities and predation pressure by P. ulvae. The fluctuations of colloidal carbohydrates were remarkably similar in the two seasons with peaks just after spring tideswhen the highest irradiance was received by microphytobenthic cells. Apparently, colloidal EPS carbohydrates can protect cells against the high salinity values ranging from 32.3 to 50.4 PSU. The presence of bound EPS carbohydrates may be linked to sediment colonization and resistance of biofilm activity. Proteins in EPS were absent in winter and represented a small proportion in summer (10%), but they appeared to be a good indicator of potential synergistic effects between MPB and bacteria in summer. Conversely, bound EPS carbohydrates reached high levels in winter, while the number of bacteria decreased simultaneously, suggesting a negative effect on bacterial growth in the absence of proteins in EPS. There was a lower proportion (31%) of low molecular weight EPS in summer than in winter (83%), possibly in relation to desiccation.

International audience; Thaumarchaeota have been recognized as the main drivers of aerobic ammonia oxidation in many ecosystems. However, little is known about the role of ammonia-oxidizing Archaea (AOA) and Bacteria (AOB) in lacustrine ecosystems. In this study, the photic zone of three contrasted freshwater ecosystems located in France was sampled during two periods: winter homothermy (H) and summer thermal stratification (TS), to investigate the distribution of planktonic AOA and AOB. We showed that AOB were predominant in nutrient-rich ecosystems, whereas AOA dominated when ammonia concentrations were the lowest and during winter, which could provide a favorable environment for their growth. Moreover, analyses of archaeal libraries revealed the ubiquity of the thaumarchaeal I.1a clade associated with higher diversity of AOA in the most nutrient-poor lake. More generally, this work assesses the presence of AOA in lakes, but also highlights the existence of clades typically associated with lacustrine and hot spring ecosystems and specific ecological niches occupied by these microorganisms.

International audience; In order to reduce the amounts of pesticides used, and thereby their associated risks, new generations of less environmentally dangerous molecules with lower weight are currently being used in the mixtures sprayed on crops. Few studies have been made, however, to analyse their impact on the soil, and more particularly on the microorganisms living in the soil which maintain the essential functions of this ecosystem. By taking a microcosmic approach, we were able to assess the impact of the maize herbicides "cocktail" Mesotrione and S-metolachlor on global soil microbial activity, biomass, and structures, by using the formulated compounds, respectively, Callisto and Dual Gold (both registered brands of Syngenta). Our results highlighted a synergetic effect in "cocktail" microcosms resulting in an increase in the Mesotrione herbicide dissipation time and in an impact on the microbial community at onefold field rate equally to more than a single herbicide used at tenfold field rate.

International audience; Dissipation kinetics of mesotrione, a new triketone herbicide, sprayed on soil from Limagne (Puy-de-Dôme, France) showed that the soil microflora were able to biotransform it. Bacteria from this soil were cultured in mineral salt solution supplemented with mesotrione as sole source of carbon for the isolation of mesotrione-degrading bacteria. The bacterial community structure of the enrichment cultures was analyzed by temporal temperature gradient gel electrophoresis (TTGE). The TTGE fingerprints revealed that mesotrione had an impact on bacterial community structure only at its highest concentrations and showed mesotrione-sensitive and mesotrione-adapted strains. Two adapted strains, identified as Bacillus sp. and Arthrobacter sp., were isolated by colony hybridization methods. Biodegradation assays showed that only the Bacillus sp. strain was able to completely and rapidly biotransform mesotrione. Among several metabolites formed, 2-amino-4-methylsulfonylbenzoic acid (AMBA) accumulated in the medium. Although sulcotrione has a chemical structure closely resembling that of mesotrione, the isolates were unable to degrade it

The fate of benthic bacterial biomass in benthic food webs is a topic of major importance but poorly described. This paper describes an alternative method for evaluation of bacterial grazing rate by meiofauna and macrofauna using bacteria pre-enriched with stable isotopes. Natural bacteria from the sediment of an intertidal mudflat were cultured in a liquid medium enriched with 15NH4Cl. Cultured bacteria contained 2.9% of 15N and were enriched sufficiently to be used as tracers during grazing experiments. Cultured bacteria presented a biovolume (0.21 μm3) and a percentage of actively respiring bacteria (10%) similar to those found in natural communities. The number of Operational Taxon Units found in cultures fluctuated between 56 and 75% of that found in natural sediment. Despite this change in community composition, the bacterial consortium used for grazing experiments exhibited characteristics of size, activity and diversity more representative of the natural community than usually noticed in many other grazing studies. The bacterial ingestion rates of three different grazers were in the range of literature values resulting from other methods: 1149 ngC ind−1h−1 for the mud snail Hydrobia ulvae, 0.027 ngC ind−1 h−1 for the nematode community, and 0.067 ngC ind−1 h−1 for the foraminifera Ammonia tepida. The alternative method described in this paper overcomes some past limitations and it presents interesting advantages such as short time incubation and in situ potential utilisation.

The aminopeptidase response of bacterial communities from anoxic sediments to inputs of organic matter was investigated by substrate enrichment experiments with particulate organic matter (phytoplankton) and polymer (proteins, cellobiose and starch) and monomer (glucose and amino acids) organic compounds. Changes in specific aminopeptidase activity were observed over an 11 days incubation period under in situ temperatures. The pattern of this activity was related to polymer compounds and more precisely to enrichments with proteins. The results showed that monomer compounds did not inhibit activity. Thus, mechanisms of aminopeptidase regulation seem to be different in freshwater sediment from other aquatic environments.

Human activity through industry, urbanization and agriculture, has led to the production and release of a large amounts of chemical compounds (including pesticides and pharmaceuticals) into the biosphere. One of the problematic related to the xenobiotic compounds fate is their transfer to aquatic ecosystems and the alteration of diversity and activity of microbial communities. Microbial communities associated with immersed leaf-litter can be impacted by these compounds, and in turn, alter global processes such as the carbon and nutrient cycling in the stream ecosystem. Accordingly, this thesis work aims to assess the effects of realistic chemical contamination on microbial leaf-litter decomposition process in streams.The first chapter of this thesis was focused on the comparison of microbial decomposition activity in alder leaves in six watersheds presenting different land uses (agricultural, urbanized, forested) over four seasons (spring, summer, autumn, winter). The effect of the gradient of contamination on microbial organic matter processing from upstream to downstream sections in each watershed was also assessed. Monitoring revealed that microbial decomposition of leaves was slightly higher in contaminated watersheds (agricultural and urbanized) in comparison with control ones (forested), probably because of the compensation effect by nutrients over xenobiotics. However, this compensation mechanism was partial since fungal biomass accumulated in leaves was greatly reduced in contaminated watersheds. Overall, this highlights microbial communities being more efficient for leaf decomposition in polluted watersheds than in the less contaminated ones, which is probably explained by changes in microbial community structure.The second chapter of this thesis aimed to evaluate in vitro the specific interactions between nutrients (nitrogen and phosphorus) and pesticides (herbicide and fungicide, alone or in mixture) exposure on microbial communities during leaf-litter decomposition. High nutrient concentrations (eutrophic conditions) tended to exacerbate the effects of pesticides on leaf decomposition rates suggesting that the compensatory mechanism of nutrients over pesticides observed in the previous part is probably concentration dependent and does not always apply to aquatic microbial communities. Moreover, a stimulation in laccase activity was observed when microbial communities were exposed to the fungicide, suggesting a role of this enzyme in detoxification mechanisms. However, the fact that such stimulation was not observed when exposed to the mixture of both pesticides (herbicide and fungicide) suggest that the interaction between these two molecules impaired the ability of microbial communities to display properstress response. These results constitute the first evidence of the potential interaction between an herbicide and a fungicide on leaf-associated microbial communities functioning. (...); L’activité humaine, au travers de l’industrie, l’urbanisation ou encore l’agriculture, a conduit à la production et au relargage d’une grande quantité de produits chimiques (dont les xénobiotiques) dans la biosphère. Une des problématiques liée aux composés xénobiotiques est leur transfert vers les écosystèmes aquatiques et l’altération de la diversité et des activités microbiennes. Ces communautés peuvent être impactées par ces composés, potentiellement affectant en retour les grands processus écosystémiques tels que le cycle du carbone. Ce travail de thèse vise donc à évaluer les effets d'une contamination chimique réaliste sur les processus de décomposition microbiens de la matière organique dans les écosystèmes lotiques via l’utilisation de la litière de feuilles comme descripteur.Une première partie de ces travaux de thèse a été consacrée à décrire et à comparer la décomposition microbienne (bactéries et champignons) de la litière de feuilles au travers de la structure des communautés microbiennes (structure génétique et biomasse) et les activités associées (activités enzymatiques extracellulaires) dans six bassins versants présentant des occupations des sols différentes (urbanisés, agricoles et forestier) et au cours de quatre saisons (printemps, été, automne et hiver). L'effet du gradient de contamination depuis l'amont vers aval sur ce processus a également été évalué. La décomposition microbienne s'est avérée légèrement plus élevée pour les bassins versants agricoles et urbanisés par rapport aux bassins versants forestier (contrôle), probablement en raison de l'effet de la compensation des nutriments sur les xénobiotiques. Cependant, cette compensation ne s’est avérée que partielle, la biomasse fongique ayant été considérablement réduite dans les bassins versants contaminés. Ces résultats mettent en évidence une meilleure efficacité des communautés microbiennes pour la décomposition des feuilles dans les bassins versants présentant une contamination chimique, probablement par l’intermédiaire de changements au niveau de la structure des communautés microbiennes.La deuxième partie de ces travaux de thèse consistait à évaluer in vitro les interactions entre l’eutrophisation des cours d’eau et des concentrations réalistes en pesticides (herbicides et fongicides, seuls ou en mélange) sur l’activité microbienne de décomposition de la litière de feuilles. Les fortes concentrations en nutriments (condition eutrophe) ont eu tendance à exacerber les effets des pesticides sur les taux de décomposition microbiens, suggérant que l'effet compensatoire des nutriments sur les pesticides observé dans la partie précédente dépend de la concentration et ne s'applique pas toujours aux communautés microbiennes aquatiques.De plus, une stimulation de l’activité laccase a été observée chez les communautés microbiennes exposées au fongicide, ce qui suggèrent l’utilisation de cette enzyme en tant que mécanisme de détoxification. Cependant, le fait que cette stimulation n’ait pas été observée en présence du mélange de pesticides (herbicide et fongicide) suggère que l’interaction entre ces deux composés peut altérer la capacité des communautés microbiennes à se défende contre le stress. Ces résultats constituent la première description de l'interaction potentielle entre un herbicide et un fongicide sur les communautés microbiennes associées à la litière de feuilles. (...)

These experiments are part of the actual problematic of understanding the poorknown effects of pesticides mixtures in soil ecosystem. The herbicidal commercial formulations of S-metolachlor (Dual Gold Safeneur®), mesotrione (Callisto®) and nicosulfuron (Milagro®) were applied in mixtures at the recommended field rate. Formulated compounds are more toxic than active ingredients alone (Microtox®) and no synergistic or antagonistic effects were recorded. However, these mixtures impaired rapidly and transitory the Limagne soil microbial communities, without modifying the global parameters of structure and abundance. Effects were also recorded on sensitive communities, such as phototrophic, nitrifying and denitrifying communities. A comparison of mixtures effects on phototrophic communities from two soils underlined the high toxicity of Dual Gold Safeneur® and challenged its use from an ecotoxicological point of view.., Ces travaux s’inscrivent dans une problématique actuelle de compréhension des effets, encore peu connus, des mélanges de pesticides dans le sol. Les formulations d’herbicides S-métolachlore (Dual Gold Safeneur®), mésotrione (Callisto®) et nicosulfuron (Milagro®) ont été appliquées en mélanges et à la dose agronomique recommandée. Les molécules formulées sont plus toxiques que les molécules actives seules (Microtox®) et aucun effet synergique ou antagoniste des mélanges n’a été observé. Cependant, ces mélanges impactent transitoirement et à court terme les communautés microbiennes du sol de Limagne, sans modifier les paramètres généraux de structure et d’abondance. Toutefois, des effets ont été mis en évidence sur des communautés sensibles telles que les phototrophes ou celles impliquées dans les processus de nitrification et de dénitrification. De plus, une comparaison des effets des mélanges sur les communautés phototrophes de deux sols souligne l’importante toxicité du Dual Gold Safeneur® et remet en cause son utilisation d’un point de vue écotoxicologique.