REDOX TRANSFORMATIONS OF ARSENIC(V) AND ARSENIC(III) BY BACTERIA IN TWO HYPERSALINE SODA LAKES AND THEIR RELATIONSHIP TO THE BIOGEOCHEMICAL CYCLES OF SULFATE, NITRATE, AND METHANE

Mono Lake and Searles Lake are hypersaline and alkaline (pH = 9.8) soda lakes that occupy closed basins along the arid eastern escarpment of the Sierra Nevada in California. These lakes are naturally As rich due to hydrothermal activity and evaporative concentration. Mono Lake is characterized by a salinity of 90 g/L and contains 200 mM dissolved As. Searles Lake, a partially-dry residual playa, exhibits saturated salt concentrations (>300 g/L) and As concentrations of 3.9 mM. We conducted experiments with As(V)-, As(III)-, nitrate-, or sulfate-amended slurries of Mono or Searles Lake sediments in artificial media over a wide range of salinities (25 – 346 g/L). Methane production was also monitored in the headspace of the slurries. Rates of anaerobic As(V)-reduction, denitrification, and methanogenesis, in addition to the rate of aerobic As(III) oxidation, all demonstrated an inverse relationship to total salinity in sediments from both lakes. These processes all persisted at low but detectable rates in salt-saturated (346 g/L) sediment slurries. Sulfate reduction, however, was inhibited in salt-saturated slurries of Mono Lake sediment and was completely undetectable in Searles Lake sediments at any salinity. Anaerobic respiratory pathways that utilize As(V) or nitrate as electron acceptors are bioenergetically favorable to sulfate reduction. Our results suggest that sulfate reduction may not provide sufficient energy for bacterial osmoadaptation at salt saturated conditions, and is therefore excluded from salt saturated settings such as Searles Lake. We also extracted, PCR-amplified, and DGGE separated sedimentary DNA from As(V)-reduction slurries incubated at low (25 g/L), medium (100 g/L) and high (325 g/L) salinity for each lake to obtain 16S rDNA phylogenetic markers and the arrA partial functional gene for dissimilatory As(V) reduction. As(V) reduction is accomplished by notably distinct microbial populations at low versus high salinity in both lakes. Insights gained by studying biological redox processes that affect As speciation in these As rich, extreme settings may be applicable to freshwater settings characterized by comparably lower concentrations of As, such as contaminated reservoirs and aquifers.