Insights into bio-oxidation and bio-reduction of arsenic in contaminated marine sediments
Résumé
Arsenic (As) occurs naturally and anthropogenically in the environment and chronic exposure to this element is directly linked to human health problems. Recently, a study has revealed high As concentrations along the South Marseille littoral (France) and an excess of carcinogenic effect level through mussel consumption. If the marine cycle of As is well described in the water column, only scarce data exist on its transport from the sediment to the water column. The role of bacteria on arsenic speciation (arsenite (AsIII) oxidation, arsenate (AsV) reduction) and thus on its mobility, is now widely acknowledged. However, the bacterial cycle of As in marine sediments remains largely unknown. Within the ASEDMAR project, supported by the French National Research Agency (reference 2008 CESA-003), the final aim being the elaboration of a model integrating biogeochemistry, bioavailability and toxicity of As in marine sediments, our task is to study the impact of the bacterial compartment on the sequestration and mobilization of arsenic, taking its chemical forms into account that will directly influence its mobility and bioavailability. Sediments were sampled from two seaports, one being polluted with As by several industrial activities, and the other one as a reference for low arsenic content. Sediment microcosms in defined media were used for the determination of kinetics of potential AsIII oxidation and AsV reduction. Molecular methods based on the functional genes aoxB (AsIII oxidation) and arrA (AsV respiration) were applied in order to evaluate their diversity. As we recently developed the DGGE fingerprinting technique on aoxB genes, the aoxB-carrying community was studied in more details. Altogether, our results showed a very active bacterial community against As in the marine sediments, where active AsIII oxidizers and AsV reducers co-exist at both locations. Our results give insights into a likely transfer of As from the sediment toward the water column.