DISSIMILATORY ARSENATE REDUCTION AND CHEMOAUTOTROPHIC ARSENITE OXIDATION ASSOCIATED WITH SEDIMENTS FROM A SALT-SATURATED SODA LAKE

Evaporative and hypersaline soda lakes from arid, closed-basin settings may represent modern terrestrial analogs for the oceans of ancient Mars and present day Europa (Kempe and Kazmierczak, 2002). In volcanic regions, hydrothermal input and evapo-concentration can enrich these lakes in As and other chalcophile elements. Despite the presence of abundant sulfate, the salinity of these lakes inhibits dissimilatory sulfate reduction because this process does not provide sufficient energy for bacteria to maintain their internal osmotic salt balance (Oren, 1999). Anaerobic respiratory pathways that utilize arsenate [As(V)] as a terminal electron acceptor are bio-energetically more favorable than sulfate reduction. Recent studies (Blum et al., 1998; Oremland et al., 2002; Hoeft et al., 2004) have demonstrated dissimilatory arsenate reduction (DAsR) coupled to the oxidation of both organic carbon and sulfide, as well as chemotrophic arsenite [As(III)] oxidation, in the anoxic water column and sediments of Mono Lake, CA (pH=9.8, salinity=90 g/L, [As]=200 mM). DAsR represents a significant pathway for microbial respiration and the mineralization of primary productivity in soda lakes, and As(V) may represent the most important electron acceptor present where conditions approach salt saturation.

Searles Lake is an alkaline (pH 9.8), salt-saturated (340 g/L), As-rich (3 mM), partially dry lake in the Mojave Desert, CA. We conducted experiments with As-amended slurries of Searles Lake sediment in an artificial media designed to mimic the lake water chemistry. Live, anaerobic incubations of Searles Lake sediment completely reduced 1 mM As(V) to As(III) using both organic and inorganic electron donors. The most rapid rate of As(V) reduction occurred when hydrogen was supplied as an electron donor (164 mM/L/d), followed by sulfide-amended slurries (94 mM/L/d) and lactate amended slurries (60 mM/L/d). Biological As(III) oxidation was also observed in aerobic incubations of lake sediment, however anaerobic incubations amended with nitrate did not oxidize As(III). We have isolated a novel As(V) reducing bacterium from these samples, strain SLAS-1, that demonstrates optimum arsenic-dependent anaerobic growth at salinity concentrations > 250 g/L. No growth was observed for this organism at salinities < 200 g/L.