BIOLOGICAL AND GEOCHEMICAL INFLUENCES ON ARSENIC REDOX PROCESSES IN A GEOTHERMAL ENVIRONMENT

Natural surface waters receiving geothermal input frequently have elevated arsenic concentrations. Numerous geothermal springs within the Alvord Basin, OR, contain arsenic at 1-6 mg/L, and geochemical studies of springs associated with Borax Lake, a group of springs within the Alvord Basin, showed that As(III) was predominant at springs with source temperatures above 75°C. We are studying the effect of microorganisms on arsenic redox processes and chose five springs with source temperatures ranging from 76 to 91°C for further study. For each spring, the pH, temperature, and oxygen concentrations were measured at 30 to 50cm intervals downstream from the source, and water was collected for measurement of As(III) and total arsenic. Two springs showed a decrease in As(III) but not total arsenic as water flowed from the source, suggesting As(III) was being oxidized to As(V). A geochemical assessment was done to determine abiotic effects on As(III) oxidation, and molecular and microbiological studies were done to determine the biotic effects on As(III) oxidation. Enrichments for As(III) oxidizers were initiated to isolate and characterize microorganisms involved in As(III) oxidation. Using molecular tools, we compared the overall microbial diversity of springs showing As(III) oxidation with springs showing no As(III) oxidation. In addition, within each of the five springs we compared the diversity of genes coding for the arsenite oxidase enzyme to assess the potential for using these genes as indicators of As(III) oxidizing activity. We show that a two-pronged biological and geochemical approach enables a complete picture of arsenic redox processes occurring in a complex natural system.