Characterization of novel arsenite oxidation pathways in Mono Lake, California, USA
Fisher, Jenny Carole
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Two novel arsenite oxidation processes in Mono Lake, CA, and the organisms involved in these processes were identified. Arsenate was produced when anoxic live Mono Lake water samples were amended with arsenite and selenate, but it was not produced in filtered or killed control samples. A pure culture capable of selenate-dependent anaerobic arsenite oxidation was isolated from Mono Lake. Washed cells experiments with this culture demonstrated that the oxidation of arsenite is tightly coupled to the reduction of selenate. Strain ML-SRAO is not autotrophic and grows optimally on lactate with selenate as the electron acceptor. TStrain ML-SRAO is a Gram-positive, non-motile, spore-forming rod that can respire oxygen, nitrate, and arsenate in addition to selenate. The arsenate reductase gene (arrA) from strain ML-SRAO was highly similar to arrA genes from other Mono Lake arsenate reducers. TComparison of 16S rDNA sequences of closely related organisms showed that ML-SRAO and six other classically defined Bacillus species formed a distinct phyletic group The nearest Trelative, Bacillus agaradhaerens DSM 8721, was 96.1% similar based on comparison of nearly full length (>1500 bp) 16S rRNA gene sequences. We proposed that Bacillus agaradhaerans TDSM 8721 and five other closely related species be reclassified to the genus Natrobacillus gen. Tnov. The distinct phylogenetic identity and metabolic capabilities of ML-SRAO suggest that it is a novel species, for which the name Natrobacillus oremlandii sp. nov. is proposed. Tthe effects of sulfide on aerobic arsenite oxidation in alkaline lake water samples and in laboratory enrichment cultures were also examined. Significant arsenite oxidation occurred only in treatments with bacteria present, and production of arsenate was greatly enhanced by the addition of sulfide or thiosulfate. IC-ICP-MS analysis of samples showed that mono- and di-thioarsenate formed in arsenite + sulfide amended lake water. Enrichment culture experiments suggest that sulfur-oxidizing bacteria use free or arsenic-bound sulfur as a growth substrate and directly or indirectly transform arsenite and thioarsenates to arsenate during growth. Sulfur-driven arsenite oxidation and microbial thioarsenate transformation may be important biogeochemical processes in the arsenic cycle of our study site (Mono Lake, CA, USA) and other alkaline environments as well.