Effects of imposed salinity gradients on dissimilatory arsenate-reduction, sulfate-reduction, and other microbial processes in sediments from two California soda lakes

U.S. Geological Survey, Menlo Park, CA 94025; Department of Environmental Sciences, University of California at Riverside, Riverside, CA 92521; Department of Environmental Toxicology, University of California at Santa Cruz, Santa Cruz, CA 95064

	Abstract

Salinity effects on microbial community structure and on potential rates of arsenate reduction, arsenite oxidation, sulfate reduction, denitrification, and methanogenesis were examined in sediment slurries from two California soda lakes. We conducted experiments with Mono and Searles Lake sediments over a wide range of salt concentrations (25 - 346 g L^-1). With the exception of sulfate reduction, rates of all processes demonstrated an inverse relationship to total salinity. However, each of these processes persisted at low but detectable rates at salt saturation. Denaturing gradient gel electrophoresis (DGGE) analysis of partial 16S rRNA genes amplified from As(V)-reduction slurries revealed that distinct microbial populations grew at low (25 - 50 g L^-1), intermediate (100 - 200 g L^-1), and high (> 300 g L^-1) salinity. At intermediate and high salinities, a close relative of a cultivated As-respiring halophile was present. These results suggest that organisms adapted to more dilute conditions can remain viable at high salinity and rapidly repopulate the lake during periods of rising lake level. In contrast to As reduction, sulfate-reduction in Mono Lake slurries was undetectable at salt-saturation. Furthermore, sulfate reduction was excluded from Searles Lake sediments at any salinity despite the presence of abundant sulfate. Sulfate reduction occurred in Searles Lake sediment slurries only following inoculation with Mono Lake sediment, indicating the absence of sulfate-reducing flora. Experiments with borate-amended Mono Lake slurries suggest that the notably high (0.46 molal; "m") concentration of borate in the Searles Lake brine was responsible for the exclusion of sulfate-reducers from that ecosystem.