PHYTAMBA

Understanding of mercury (Hg) transformation pathways in the aquatic systems is an active research area. However opposite to the extensive work on bacteria, the role of the phototrophic microorganisms in both abiotic and biotic transformations is not well understood. The primary goal of the present proposal is therefore to get new mechanistic insights into the role of phytoplankton in the Hg cycling and transformations in surface waters. Phytoplankton may affect Hg cycling and transformations directly (e.g. through biotic transformation of the accumulated metal, via methylation/demethylation, reduction/oxidation processes) and/or indirectly through the excretion of ligands with strong Hg affinity, such as thiols, that likely trigger or influence different abiotic transformation pathways. The specific aims of the project are thus twofold: (i) to determine the kinetics and prevailing mechanisms of Hg biotic transformation pathways mediated by phytoplankton; (ii) to examine to what extent and under which conditions the biogenic ligands produced by the phytoplankton will further affect additional abiotic processes. Althought the high affinity of mercury to SH- groups (both inorganic Hg(II) and methylHg) is well accepted, we plan to investigate a wide spectrum low and high molecular mass complexes originating from phytoplankton. This multidisciplinary hypothesis and result’s driven project encompasses a large range of expertise at the cross-road between analytical and environmental chemistry, enriched and natural stable isotopes, biochemistry and ecotoxicology. Hyphenated techniques based on gas or liquid chromatography coupling with ICPMS or molecular mass spectrometry will be used throughout the project to perform speciation advances between Hg compounds and their binding ligands. Isotopically labelled Hg compounds will also be used to investigate simultaneous pathways at trace levels along with the different biotic and abiotic experiments to be performed. Finally, natural isotopic fractionation of Hg will also be investigated to develop new proxy of parallel biotic and abiotic pathways that could be further applied to decipher Hg biogeochemical pathways in the natural aquatic environments. To date, such integrative experimental approach has never been applied to the Hg – microorganisms interactions at the molecular and isotopic scale. Since the importance of phytoplankton in Hg biogeochemistry was always questioned but never systematically addressed, numerous new findings are expected. They will be of upmost importance for the development of Hg biogeochemical modelling, mitigation strategies and rational environmental risk assessment in aquatic system. The present collaborative initiative is expected to serve as a trampoline for further development of the mutually beneficial research cooperation and exchange between the research partners, both sharing interests and strongly committed in the Hg biogeochemistry research. It will take advantage of the high complementarity, earlier collaborative experience and synergy of the expertise between the two teams, which warranty the successful realization of the research objectives tackling one of the important current challenges in the toxic trace element biogeochemistry.