| Abstract
| - Geothermal springs within Yellowstone National Park(YNP) often contain arsenic (As) at concentrations of 10−40 μM, levels that are considered toxic to many organisms.Arsenite (As(III)) is often the predominant valence state atthe point of discharge but is rapidly oxidized to arsenate(As(V)) during transport in shallow surface water. The currentstudy was designed to establish rates and possiblemechanisms of As(III) oxidation and to characterize thegeochemical environment associated with predominantmicrobial mats in a representative acid−sulfate−chloride(pH 3.1) thermal (58−62 °C) spring in Norris Basin, YNP.At the spring origin, total soluble As was predominantlyAs(III) at concentrations of 33 μM. No oxidation ofAs(III) was detected over the first 2.7 m downstream fromthe spring source, corresponding to an area dominatedby a yellow filamentous S0-rich microbial mat. However,rapid oxidation of As(III) to As(V) was observed between 2.7and 5.6 m, corresponding to termination of the S0-richmats, decreases in dissolved sulfide, and commencementof a brown Fe/As-rich mat. Rates of As(III) oxidationwere estimated, yielding an apparent first-order rateconstant of 1.2 min-1 (half-life = 0.58 min). The oxidationof As(III) was shown to require live organisms present justprior to and within the Fe/As-rich mat. Complementaryanalytical tools used to characterize the brown mat revealedan As:Fe molar ratio of 0.7 and suggested that thisfilamentous microbial mat contains iron(III) oxyhydroxidecoprecipitated with As(V). Results from the current work arethe first to provide a comprehensive characterization ofmicrobially mediated As(III) oxidation and the geochemicalenvironments associated with microbial mats in acid−sulfate−chloride springs of YNP.
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