Activités microbiennes anaérobies et écodynamique des contaminants métalliques et organométalliques (cas de l étain et du mercure)

Le tributylétain (TBT) et le mercure (Hg) ainsi que leurs dérivés organiques sont des micropolluants métalliques, ubiquistes et persistants à l état de traces dans les écosystèmes aquatiques. Les processus microbiologiques d alkylation et de déalkylation des formes organiques plus toxiques de l étain et du mercure (méthyl-mercure, tributyl-étain ou méthylétains), en conditions oxiques ou anoxiques, sont peu connus. Ce travail porte dans une première partie sur l étude de la biodégradation du TBT par des communautés bactériennes simplifiées en culture continue. Les potentiels de dégradation du TBT et de méthylation de l étain inorganique, e.g. Sn(IV), des souches isolées de ce réacteur, tout comme ceux de différents microorganismes aérobies et/ou anaérobies, ont été caractérisés grâce à l utilisation de traceurs isotopiques stables et ont permis de définir certaines voies d alkylation et de déalkylation. Le second volet de ce document est consacré à l étude des capacités de microorganismes sulfato-réducteurs (MSR) à méthyler et à déméthyler le Hg et le méthylmercure. L utilisation de traceurs isotopiques stables du Hg a permis d étudier l aspect systématique du phénomène pour des microorganismes isolés de l environnement et sous différentes conditions physiologiques (fermentation, respiration nitrate, sulfato-réduction, limitation en substrat carboné). Finalement, l étude du fractionnement isotopique du Hg lors de sa méthylation par un MSR (Desulfobulbus propionicus, DSM6523) souligne l importance qu il y a à mieux comprendre le fonctionnement de ces deux mécanismes co-occurrents, dans la régulation de la production nette de méthylmercure dans les écosystèmes aquatiques.Tributyltin (TBT) and mercury (Hg) as well as their organic derivatives are ubiquitous and persistent metallic micro-pollutants present at trace level in aquatic ecosystems. The microbiological alkylation and dealkylation processes of the more toxic organic forms of tin and mercury (methylmercury, tributyltin or methyltin) under oxic and anoxic conditions are poorly understood. The first part of this study reports on TBT biotransformations in simplified bacterial communities under continuous culture. The potential for TBT biodegradation and inorganic tin methylation, e.g. Sn(IV), of strains isolated from this reactor as well as various aerobic and anaerobic microorganisms were simultaneously characterized by means of stable isotopic tracers, allowing to define some alkylation and dealkylation pathways. The second section of this document is dedicated to the investigation of the capacities of sulphate-reducing prokaryotes (SRP) for mercury methylation and demethylation. The use of Hg stable isotopic tracers allowed the systematic study of this phenomenon in environmental organisms under different physiological conditions (fermentation, nitrate respiration, sulphate respiration and substrate starvation). In conclusion, the study of isotopic fractionation of Hg during its methylation by a SRM (Desulfobulbus propionicus, DSM6523) highlighted the need to better understand the functioning of the co-occurring mechanisms implied in the regulation of net methylmercury production in aquatic ecosystems.PAU-BU Sciences (644452103) / SudocSudocFranceF

In vitro simulation of oscillatory redox conditions in intertidal sediments: N, Mn, Fe, and P coupling

International audienceIn coastal environments, oscillating redox conditions represent a functional state affecting organic matter mineralization. Such transient diagenetic processes remain difficult to study in situ, and we therefore designed a specific reactor to provide experimental results that are environmentally relevant in this context. Here, we present the results of two independent experiments carried out with sediment having contrasting Fe, Mn contents, collected from a coastal tidal lagoon (the Arcachon bay) and a mesotidal estuary (Adour river). Sediment and overlying water were mixed to form slurries that were submitted to redox oscillations to assess the diagenetic mechanisms that affect N, P, Fe, Mn, and S. Changing from anoxic to oxic conditions, we observed a rapid oxidation of dissolved Fe(II) and dissolved inorganic phosphorus (DIP) was apparently trapped by the newly formed Fe-oxyhydroxides (Fe-ox). DIP was totally titrated in the coastal lagoon sediment, but not in estuarine sediment, where the initial amount of Fe available was lower. In both experiments, Mn(II) was only slowly oxidized during the oxidation events and a major part of Mn(II) was adsorbed on new Fe-ox. In coastal lagoon sediment, ammonium remained constant in oxic conditions while nitrate was produced from organic-N mineralization. On the contrary, in estuarine sediment, ammonium was quantitatively oxidized to nitrate. When the conditions became anoxic again, direct reduction of nitrate to ammonium occurred in coastal lagoon sediment. Anaerobic production of nitrate occurred in estuarine sediment, probably because Mn-oxides (Mn-ox), which had a high concentration, acted as an oxidant for ammonium. Consequently, nitrate production prevented Fe(II) accumulation. The Mn-N-Fe coupling outlined here is an apparent indirect oxidation of Fe(II) by Mn-ox through anaerobic nitrification (with Mn-ox) and denitrification (with Fe-ox). This coupling also implied P availability because of the strong control of P by Fe. These experimental results show that nutrient dynamics in oscillatory redox environments such as the estuarine turbidity zone, bioturbated sediment, or tidal permeable sediments highly depends on Mn-and Fe-ox availability

Measurements of gaseous mercury exchanges at the sediment-water, water-atmosphere and sediment-atmosphere interfaces of a tidal environment (Arcachon Bay, France)

International audienceThe elemental mercury evasion from non-impacted natural areas is of significant importance in the global Hg cycle due to their large spatial coverage. Intertidal areas represent a dynamic environment promoting the transformations of Hg species and their subsequent redistribution. A major challenge remains in providing reliable data on Hg species variability and fluxes under typical transient tidal conditions found in such environment. Field experiments were thus carried out to allow the assessment and comparison of the magnitude of the gaseous Hg fluxes at the three interfaces, sediment-water, sediment-atmosphere and water-atmosphere of a mesotidal temperate lagoon (Arcachon Bay, Aquitaine, France) over three distinct seasonal conditions. The fluxes between the sediment-water and the sediment-atmosphere interfaces were directly evaluated with field flux chambers, respectively static or dynamic. Water-atmosphere fluxes were evaluated from ambient concentrations using a gas exchange model. The fluxes at the sediment-water interface ranged from −5.0 to 5.1 ng m−2 h−1 and appeared mainly controlled by diffusion. The occurrence of macrophytic covers (i.e.Zostera noltii sp.) enhanced the fluxes under light radiations. The first direct measurements of sediment-atmosphere fluxes are reported here. The exchanges were more intense and variable than the two other interfaces, ranging between −78 and 40 ng m−2 h−1 and were mostly driven by the overlying atmospheric Hg concentrations and superficial sediment temperature. The exchanges between the water column and the atmosphere, computed as a function of wind speed and gaseous mercury saturation ranged from 0.4 to 14.5 ng m−2 h−1. The flux intensities recorded over the intertidal sediments periodically exposed to the atmosphere were roughly 2 to 3 times higher than the fluxes of the other interfaces. The evasion of elemental mercury from emerged intertidal sediments is probably a significant pathway for Hg evasion in such tidal environments exhibiting background contamination level

Methylation and dealkykation of tin compounds by sulfate- and nitrate-reducing bacteria

International audienceIn this study, axenic cultures of sulfate-reducing (SRB) and nitrate-reducing (NRB) bacteria were examined for their ability to methylate inorganic tin and to methylate or dealkylate butyltin compounds. Environmentally relevant concentrations of natural abundance tributyltin (TBT) and 116Sn-enriched inorganic tin were added to bacterial cultures to identify bacterial-mediated methylation and dealkylation reactions. The results show that none of the Desulfovibrio strains tested was able to induce any transformation process. In contrast, Desulfobulbus propionicus strain DSM-6523 degraded TBT either under sulfidogenic or non-sulfidogenic conditions. In addition, it was able to alkykate 116Sn-enriched inorganic tin leading to the formation of more toxic dimethyltin and trimethyltin. A similar capacity was observed for incubations of Pseudomonas but with a much greater dealkykation of TBT. As such, Pseudomonas sp. ADR42 degraded 61% of the initial TBT under aerobic conditions and 35% under nitrate-reducing conditions. This is the first work reporting a simultaneous TBT degradation and a methylation of both inorganic tin species and TBT dealkykation products by SRB and NRB under anoxic conditions. These reactions are environmentally relevant as they can control the mobility of these compounds in aquatic ecosystems; as well as their toxicity toward resident organisms

Identical Hg Isotope Mass Dependent Fractionation Signature during Methylation by Sulfate-Reducing Bacteria in Sulfate and Sulfate-Free Environment

Inorganic mercury (iHg) methylation in aquatic environments is the first step leading to monomethylmercury (MMHg) bioaccumulation in food webs and might play a role in the Hg isotopic composition measured in sediments and organisms. Methylation by sulfate reducing bacteria (SRB) under sulfate-reducing conditions is probably one of the most important sources of MMHg in natural aquatic environments, but its influence on natural Hg isotopic composition remains to be ascertained. In this context, the methylating SRB <i>Desulfovibrio dechloracetivorans</i> (strain BerOc1) was incubated under sulfate reducing and fumarate respiration conditions (SR and FR, respectively) to determine Hg species specific (MMHg and IHg) isotopic composition associated with methylation and demethylation kinetics. Our results clearly establish Hg isotope mass-dependent fractionation (MDF) during biotic methylation (−1.20 to +0.58‰ for δ²⁰²Hg), but insignificant mass-independent fractionation (MIF) (−0.12 to +0.15‰ for Δ²⁰¹Hg). During the 24h of the time-course experiments Hg isotopic composition in the produced MMHg becomes significantly lighter than the residual IHg after 1.5h and shows similar δ²⁰²Hg values under both FR and SR conditions at the end of the experiments. This suggests a unique pathway responsible for the MDF of Hg isotopes during methylation by this strain regardless the metabolism of the cells. After 9 h of experiment, significant simultaneous demethylation is occurring in the culture and demethylates preferentially the lighter Hg isotopes of MMHg. Therefore, depending on their methylation/demethylation capacities, SRB communities in natural sulfate reducing conditions likely have a significant and specific influence on the Hg isotope composition of MMHg (MDF) in sediments and aquatic organisms

Transformation, Localization, and Biomolecular Binding of Hg Species at Subcellular Level in Methylating and Nonmethylating Sulfate-Reducing Bacteria

Microbial activity is recognized to play an important role on Hg methylation in aquatic ecosystems. However, the mechanism at the cellular level is still poorly understood. In this work subcellular partitioning and transformation of Hg species in two strains: <i>Desulfovibrio</i> sp. BerOc1 and <i>Desulfovibrio desulfuricans</i> G200 (which exhibit different Hg methylation potential) are studied as an approach to the elucidation of Hg methylation/demethylation processes. The incubation with isotopically labeled Hg species (¹⁹⁹Hgi and Me²⁰¹Hg) not only allows the determination of methylation and demethylation rates simultaneously, but also the comparison of the localization of the originally added and resulting species of such metabolic processes. A dissimilar Hg species distribution was observed. In general terms, monomethylmercury (MeHg) is preferentially localized in the extracellular fraction; meanwhile inorganic mercury (Hgi) is associated to the cells. The investigation of Hg binding biomolecules on the cytoplasmatic and extracellular fractions (size exclusion chromatography coupled to ICP-MS) revealed noticeable differences in the pattern corresponding to the Hg methylating and nonmethylating strains