We characterized the arsenate-reducing, sulfide-oxidizing population of Mono Lake, California, by analyzing the distribution and diversity of rrnA, cbbL, and dissimilatory arsenate reductase (arrA) genes in environmental DNA, arsenate-plus sulfide-amended lake water, mixed cultures, and isolates. The arsenate-reducing community was diverse. An organism represented by an rrnA sequence previously retrieved from Mono Lake and affiliated with the Desulfobulbaceae (Deltaproteobacteria) appears to be an important member of the arsenate-reducing, sulfide-oxidizing community. Sulfide oxidation coupled with arsenate reduction appears to proceed via a two-electron transfer, resulting in the production of arsenite and an intermediate S compound that is subsequently disproportionated. A realgar-like As/S mineral was formed in some experiments.Soda lakes and similar alkaline, hypersaline environments are widespread today and have been important features of the hydrosphere since the earliest stages of the formation of the Earth's oceans (15). Because of their constrained watersheds, local geology strongly influences the composition of dissolved salts in these endorheic (closed-basin) lakes. Hydrothermal processes associated with volcanism in the watershed of Mono Lake, an endorheic lake in eastern California, have led to the accumulation of arsenic compounds (12,17). These compounds play an active role in the lake's biogeochemical cycles (26), serving both as electron acceptors for the oxidation of organic matter (23) and as electron donors (24). Arsenate reduction in Mono Lake appeared to be relatively insensitive to organic-carbon additions (11,26), suggesting that another electron donor, possibly sulfide, might be significant in this environment, a conclusion that was supported by geochemical evidence (12).Arsenate reduction coupled with sulfide oxidation was first demonstrated with an archived sample of Mono Lake bottom water (10) and was subsequently repeated with slurries of sediments from an arsenic-enriched playa basin south of Mono Lake (25). An organism (deltaproteobacterium strain MLMS-1) capable of chemoautotrophic growth by oxidizing sulfide with arsenate was isolated from the Mono Lake sample (10). While Hoeft et al. (10) noted the relatively close alignment (ϳ97% rrnA sequence similarity) of strain MLMS-1 with ribotypes previously detected in Mono Lake (13), extensive characterizations of Mono Lake microbial assemblages using cultureindependent techniques (13; J. T. Hollibaugh, unpublished data) failed to retrieve ribotypes more closely related to strain MLMS-1. This prompted us to examine arsenate reduction coupled with sulfide oxidation in enrichments of freshly collected water samples and to compare arsenate-reducing, sulfide-oxidizing populations with nitrate-reducing, sulfideoxidizing populations in similar enrichments. We also compared the populations we obtained in this way with diagnostic sequences retrieved directly from DNA extracted from lake water samples.
MATERIALS AND METHODSSample collection. Wat...