Microbially mediated transformation of arsenic and sulfur compounds in Mono Lake, California, USA
Edwardson, Christian Frederick
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The transformation of arsenic, sulfur, and thioarsenic compounds was examined in alkaline, hypersaline Mono Lake, California, USA. The microbial communities were surveyed at five depths along a redox gradient at the deepest station in the lake using both 16S rRNA tag pyrosequencing and metatranscriptomics approaches. Most of the abundant OTUs observed in the 16S amplicon survey had been found previously in Mono Lake, thus indicating the microbial community remained relatively stable through periods of meromixis and mixing. Similar taxa were found through metatranscriptomics, but abundances varied, especially with Bacteroidetes (relatively less abundant) and Proteobacteria (relatively more abundant). The metatranscriptome samples were examined for arsenic- and sulfur-transforming bacteria by identifying transcript hits to enzymes catalyzing arsenic and sulfur redox transformations. Arsenite oxidase transcripts were dominated by the alternative (arxA) rather than the canonical (aioA) arsenite oxidase and were affiliated with organisms not previously identified as being involved in arsenite oxidation (Thioalkalivibrio and Halomonas). Arsenate reductase (arrA) transcripts were dominated by Deltaproteobacteria and Firmicutes. Transcripts of important sulfur cycle genes (soxB, aprA, dsrA) were dominated by sulfur-oxidizing Gammaproteobacteria and Deltaproteobacteria at 15 and 18 m, with a transition to sulfate reducing Deltaproteobacteria at 18-31 m. These results highlight the shift from arsenite and sulfide oxidation in the oxycline (15-18 m) to arsenate and sulfate reduction in the anoxic bottom waters (18-31 m), as has been detected by previous rate measurements. A phototrophic enrichment culture that was dominated by the purple sulfur bacteria Ectothiorhodospira sp. was obtained by amending Mono Lake water with thioarsenic compounds. The culture was able to convert thioarsenic compounds, arsenite, thiosulfate and monothioarsenate to obtain energy for photosynthetic growth. Monothioarsenate was used as the sole electron donor for anoxygenic photosynthesis, and the transformation was light-dependent. Additional pure cultures of members of the Ectothiorhodospiraceae were tested and most were able to convert monothioarsenate to arsenate. These findings highlight the important interactions between sulfur and arsenic in hypersaline, alkaline environments where arsenic is prevalent. The links between the bacteria that oxidize sulfur and arsenite and bacteria that reduce sulfate and arsenate are clearer.