Sulfide as a Chemoautotrophic Electron Donor for Dissimilatory Arsenate Reduction in Mono Lake, California

In aqueous systems, arsenic occurs as arsenate [As(V)] or as arsenite [As(III)], with the latter form being more toxic and mobile. Mono Lake, California is a meromictic soda lake (pH = 9.8; salinity = 70-90 g/L) with exceptionally high arsenic content ( ∼200 μ M), a consequence of hydrothermal inputs combined with evaporative concentration. Previous work has shown that arsenic speciation changes from As(V) to the more reduced As(III) with vertical transition from the lake's surface oxic waters to its unmixed, anoxic bottom waters and that dissimilatory reduction is responsible for the observed change in arsenic speciation. Rates of in situ dissimilatory As(V) reduction measured by radiotracer ( ∼1- 6 μ mol/L/d) were estimated to be significant enough to mineralize up to 14% of annual primary productivity. Subsequent lab-based investigations with As(V)-amended ( ∼1-2 mM) bottom water displayed significantly higher rates (150-260 μ mol/L/d) of As(V) reduction and were not limited by the availability of organic electron donors such as acetate, lactate, malate and glucose. The focus of this study was to identify a natural source of electrons for As(V) reduction in Mono Lake. While Mono Lake contains plentiful dissolved organic carbon ( ∼7 mM) this material is usually refractory and resistant to bacterial oxidation. Alternatively, the anoxic bottom waters contain high concentrations of sulfide ions ( ∼1-2 mM) that could potentially serve as an electron donor for dissimilatory As(V) reduction. In a time course experiment with As(V)-amended Mono Lake bottom water, we observed oxidation of sulfide linked to the reduction of As(V) to As(III). This reaction did not occur in filter sterilized controls and sulfide loss did not occur in samples lacking As(V). In bottom water amended with additional sulfide (total = 6 mM) and As(V), we observed a linear relationship between rates of dissimilatory As(V) reduction and As(V) concentration. The highest rate observed under these conditions was ∼3 mmol/L/d, over 1000-fold higher than Mono Lake in situ rates. We isolated an anaerobic bacterium from Mono Lake bottom water, strain MLMS-1, that grows in mineral salts media by oxidizing sulfide to sulfate and reducing As(V) to As(III). MLMS-1 grew with a 4:1 stoichiometry of As(V) reduced to sulfide oxidized, indicating an 8 electron transfer. MLMS-1 aligned by 16S rDNA amplification and sequencing in the δ -Proteobacteria, being closely related to the sulfate-reducing bacteria of the genus Desulfobulbus. However, strain MLMS-1 does not grow with sulfate as an electron acceptor.