Cruising the marine halomethane cycle: Microbiology, biochemistry and geochemistry of reference bacteria, new microbial players and underlying processes

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Volatile organohalides are toxic to living organisms. Furthermore, they contribute to the destruction of stratospheric ozone. Seas and oceans are major sources of these compounds that prominently comprise halomethanes (chloro-, bromo-, and iodomethane). They are produced by microorganisms and algae but also represent nutrients for bacterial life. The sources and sinks of halomethanes, as well as their resulting fluxes in the marine environment and the atmosphere, remain poorly characterized. In particular, only one pathway for microbial degradation of chloromethane has been elucidated so far, although many studies in the literature clearly suggest that others exist.The 36-month French-German research project MAHABIO ambitions to gain a comprehensive view on global biogeochemical cycles of volatile halogenated compounds. It proposes a resolutely interdisciplinary, innovative, and tightly focussed approach to explore and characterize bacterial processes associated with production and consumption of chloromethane in the marine environment. MAHABIO associates two French and two German partners, each bringing complementary areas of expertise. It is organized in three work packages : i) study of chloromethane consumption by marine samples and characterization of dehalogenases involved from reference strains, including under hitherto neglected anoxic conditions; ii) quantification of bacterial production of chloromethane in marine samples and by reference bacterial strains; and iii) integrative study of concomitant production and consumption of chloromethane by synthetic bacterial communities. The synergistic expertise of MAHABIO partners is a key asset of the project. In addition, many prerequisites for this project have been validated in preliminary experiments. These include cultivation of marine chloromethane-degrading bacteria, stable isotope analysis with the three elements of chloromethane, and preliminary genetic and proteomic characterization of chloromethane utilization by a marine chloromethane-degrading bacterium with yet uncharacterized pathway. In-depth analysis of each aspect of the project has also made it possible to explicitly identify associated risks, and to propose fallback solutions. Major scientific and technological advances are envisaged from MAHABIO in the production and consumption of chloromethane such as the biochemical, enzymatic and genetic characterization of novel bacterial enzymes involved, and the establishment of characteristic biomarkers and isotopic signatures associated with chloromethane production and consumption. Taken together, the expected results will contribute in developing renewed, state-of-the-art technologies that contribute to a better understanding of global biogeochemical cycles.