Biofilm Development, Microbial Diversity and Dissolved Organic Matter from El Tatio, Chile: Implications for Metalloid Biogeochemical Cycling

El Tatio is one of the largest geyser zones known, located at >4400 m elevation in Region II, Chile. Microbial diversity, fluid composition, and the biofilm geochemistry are being investigated to better understand toxic metalloid mobility and biogeochemical cycling. Hydrothermal fluids are near neutral pH, Na-Cl type, with ~450 μmol/L total arsenic and 21 μmol/L antimony. Biofilm development occurs in silica-precipitating fluids containing 2.6-5.8 mmol/L dissolved Si. As(V), speciated by x-ray absorption spectroscopy, partitions to the hydrous ferric oxides (HFO) in the mats. Mats contain only 0.8 - 3.0% (dry w/w) total organic carbon, with the balance principally as Si and HFO, with trace amounts of other inorganic constituents. As temperature decreases, silica and iron precipitate and the mats resemble a silica-rich, ferrihydrite-gel rather than organic biofilms. 16S rRNA gene sequences from biofilms indicates communities proximal to geyser pools (at ~76-80 ^oC) are dominated by Chloroflexi-like organisms, with representation from the OP10, OP11, and WS-6 candidate divisions and Bacteriodetes. Downstream communities are dominated by Bacteriodetes, with lesser abundances of Chloroflexi, Gammaproteobacteria, Chlorobi, and Planctomycetes. Based on fluorescence spectroscopy, spectral characteristics of dissolved organic matter (DOM) were closely related to microbial protein signatures and not to terrestrially-influenced or soil-derived humic substances. These results imply that microbial production and cycling of DOM within the discharging hydrothermal fluids may be occurring, but also that accumulation of organic carbon is minimal because intense photooxidation degrades the microbial DOM. The presence of As may be influencing the fluorescence of humic-like components but not protein-like components, suggesting that the interactions between microbial DOM and As may also be a significant, possibly due to a photosensitive ligand-metal complex. The biofilms at El Tatio provide unique opportunities to investigate biofilm-mediated metalloid cycling that may be relevant to water resources and to geothermal power production in the region.