GEOCHEMISTRY AND MICROBIOLOGY OF ARSENIC IN EL TATIO GEYSER FIELD, CHILE

El Tatio geyser field, northern Chile, impacts regional water quality, policy, and agricultural commerce due to high, naturally-occurring arsenic (As) concentrations in the geothermal waters. Microbial mats and sediments were analyzed using XANES and EXAFS spectroscopy at both the As and Fe K-edges, combined with ATR-FTIR spectroscopy and molecular genetics, to understand As cycling in the circumneutral pH, non-sulfidic thermal waters. Arsenite [As(III)] dominates geyser discharge water upstream, but arsenate [As(V)] dominates downstream. As(III) oxidation in the water occurs both day and night, suggesting limited photic oxidation. As(V) reduction may be linked to photosynthesis due to higher reduction rates during the day than at night. XANES and EXAFS scans show a predominance of As(V) in the sediment with the mats having more organo-arsenic than inorganic arsenic. Downstream samples show As sorbed as an inner-sphere complex to organic matter and ferric oxides. Some downstream mats contain appreciable As(III) due to sequestration occurring for both As species. Differences in As speciation are likely due to microbial activity, therefore microbial diversity of the mats was evaluated within the 16S rRNA gene framework. Near the geyser pool (at ~76-80^oC), biofilms covering opal and loose sediment consisted of different microbial groups than from within the thick microbial mats downstream. Several groups belonged to the candidate divisions OP10, OP11, and WS-6, forming novel clades, with the photosynthetic Chloroflexi group being abundant throughout the microbial mats. Sequences from the arsenite oxidase (aroA) gene belonged predominantly to Chloroflexi and Proteobacteria, which is interesting because Chloroflexi have not been previously linked to As(III) oxidation in geothermal habitats, and could be a possible contributor to the As(V) flux. Arsenic contamination in these waters is a major issue for downstream users and this research could lead to a greater understanding of aqueous As remediation and geothermal development.