Collaborative Research: Biogeochemical Controls on Antimony and Arsenic Mobility in a Siliceous Hydrothermal Systems

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EAR-0545336/EAR-0544960Philip C. BENNETT & Annette S. ENGELEl Tatio is one of the largest geyser zones known, with over 100 mapped hydrothermal features, and it is a cultural treasure and regional tourism resource for Chile Region II. Geyser water supports isolated microbial mat communities in the otherwise barren high altitude landscape and discharge water forms the headwaters of the Rio Salado, an essential water resource for downstream rural villages. El Tatio is also however a natural source of the toxic elements arsenic (As) and antimony (Sb) that impact regional water quality and water policy. The biogeochemistry, microbial cycling, and ultimate mobility of the As and Sb in this system remain as critical unknown parameters to this ecosystem and water resource. This project will address four broad research questions: What are the biogeochemical controls on As and Sb mobility? How do the spatial and temporal variations in geyser fluid composition influence lotic microbial communities? Are As and Sb used as sources of chemical energy for chemolithotrophs, or electron acceptors for metal-respiring bacteria? What are the influences of Sb and As on the endolithic communities in the geyserite? El Tatio is a uniquely extreme environment distinct from the more intensely studied geyser regions; yet, compared to the classic hydrothermal systems El Tatio is almost unknown. This natural laboratory offers the opportunity to study As and Sb biogeochemistry in situ at very high concentration and to directly observe fundamental geochemical relationships. We will characterize the biogeochemical controls on As and Sb redox speciation in the geyser waters and geyserite (silica) using interdisciplinary field and laboratory geochemical and microbiological methods. This will include an examination of the relationships between habitat hydro-geochemistry and microbial community composition of both the planktonic and endolithic communities using molecular- and culture-based techniques. Field precipitation experiments will be supplemented by laboratory sequential extraction methods and hydrothermal silica precipitation experiments to examine the partitioning behavior of As and Sb. Sb isotope ratios will be characterized in the water and silica solids to test for potential microbial fractionation. Temporal studies of discharge volume and chemical composition will be used to estimate the mass flux of toxic metals to the down stream communities.El Tatio can provide insight into the parameters that govern and define the microbial ecology of extreme environments in Earth's geologic systems. The relationship between UV stress and toxic metal biogeochemistry in this unique siliceous habitat will also provide insight into early-earth habitats and the origins of life. Further, the influence of El Tatio on the regional villages involves balancing preservation of an irreplaceable natural resource with understanding of the impacts of toxic metals on water quality.