Concentrations of sedimentary molybdenum (Mo) have been used as a proxy for paleooceanographic redox conditions based on the distinctive behavior of Mo under oxic versus euxinic (anoxic and sulfidic) conditions. However, the mechanisms that govern Mo sequestration in various euxinic settings are not fully resolved. It has been proposed that sulfate-reducing bacteria (SRB), the main driver and regulator of euxinic conditions, reductively immobilize molybdate (MoO42-), a structural analog to sulfate (SO42-). Controversially, only circumstantial evidence was provided, which contradicts the more widely accepted paradigm that Mo inhibits SRB growth. The major goal of the current study is to understand the interactions among Mo, ferrous iron (Fe2+), and SRB through systematic experimentation with a focus on combinations of conditions that may lead to reductive precipitation of Mo mediated by Fe2+ and/or SRB. Specifically, we will characterize the behavior of Mo (i.e., aqueous speciation, degree of association with SRB cells, and the composition, structure, and Mo valence state of its precipitates) in sulfidic solutions generated and maintained by SRB. Solutions and precipitates will be analyzed using UV-Vis spectrophotometry, transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS). Preliminary results show that Desulfovibrio vulgaris immobilized Mo more effectively than Desulfotigum balticum, which could challenge the assumption of former studies that little distinction exists in the biological control of Mo by SRB. Moreover, upon Fe2+ addition, the D. vulgaris solutions remained unaffected, indicating the formed Mo-OM complexes were stable enough to prevent remobilization of the Mo and sulfide, suggesting a high preservation potential of thiomolybdate-OM complexes. Based on these preliminary results, we hypothesize that (1) the Mo precipitates in the biological experiments will exhibit strong association with the periplasm of SRB cell walls, (2) the initial presence of Fe2+ will induce Mo precipitation but decrease the cellular association of the precipitates, and (3) Mo(VI) will be reduced to Mo(IV) in both SRB- and Fe-mediated precipitates. The results of these biological Mo sequestration experiments will further our understanding of Mo behavior in natural euxinic environments and refine redox reconstructions.

Biological sequestration of molybdenum under sulfidic conditions: The role of sulfate reducing bacteria

ACS Fall 2021

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