IDENTIFICATION AND DISTRIBUTION OF ARSENIC SPECIES IN AN IRON-OXIDIZING MICROBIAL MAT COMMUNITY

Andrea L. Foster*, Roger P. Ashley, and James J. Rytuba, U.S. Geological Survey, 345 Middlefield Rd. MS 901, Menlo Park, CA 94025-3591, USA

The recent failure of a mine tailings dam at the Lava Cap mine, Nevada City district, California, released As-rich (500-1000 ppm) particulate material to creeks and a small freshwater lake (Lost Lake) in the area. Fe-hydroxide rich Microbial mats collected from a small creek near Lost Lake are enriched in arsenic approximately 1000-fold relative to the waters from which they were collected. The mat community is dominated by a sheathed bacterium (tentatively identified as Leptothrix ochracea on the basis of its distinct "empty drinking straw" morphology), but epifluoresence microscopy using nucleic acid probes several additional unidentified community members. Scanning electron micrographs indicate that the sheaths commonly exceed 10 microns in length and have an average diameter of 1 micron. Leptothrix spp. are known to oxidize Fe(II) to Fe(III), but not all members of this genus have been shown to derive energy from the process. X-ray absorption fine structure (XAFS) spectroscopic analysis (conducted in bulk without sample alteration) indicates that As occurs predominantly in the pentavalent state, that it is associated with Fe oxides, and that adsorption or coprecipitation with Fe oxides is the primary mode of association. We will present data from epifluoresence microscopy, transmission electron microscopy and molecular phylogenetic analysis to characterize the microbial community present in the mat, identify and characterize the Fe oxides present, and determine the distribution of As in mat-associated Fe oxide. In addition, we will discuss the effects of seasonal mat formation, As accumulation, and As speciation on the seasonal As fluxes from this and similar mine drainage environments.