Molecular analysis of microbial community structure in an arsenite-oxidizing acidic thermal spring

Electron microscopy (EM), denaturing gradient gel electrophoresis (DGGE) an d 16S rDNA sequencing were used to examine the structure and diversity of m icrobial mats present in an acid-sulphate-chloride (pH 3.1) thermal (58-62 degreesC) spring in Norris Basin, Yellowstone National Park, WY, USA, exhib iting rapid rates of arsenite oxidation. Initial visual assessments, scanni ng EM and geochemical measurements revealed the presence of three distinct mat types. Analysis of 16S rDNA fragments with DGGE confirmed the presence of different bacterial and archaeal communities within these zones. Changes in the microbial community appeared to coincide with arsenite oxidation ac tivity. Phylogenetic analysis of 1400 bp 16S rDNA sequences revealed that c lone libraries prepared from both arsenic redox active and inactive bacteri al communities were dominated by sequences phylogenetically related to Hydr ogenobacter acidophilus and Desulphurella sp. The appearance of archaeal 16 S rDNA sequences coincided with the start of arsenite oxidation, and sequen ces were obtained showing affiliation with both Crenarchaeota and Euryarcha eota. The majority of archaeal sequences were most similar to sequences obt ained from marine hydrothermal vents and other acidic hot springs, although the level of similarity was typically just 90%. Arsenite oxidation in this system may result from the activities of these unknown archaeal taxa and/o r the previously unreported arsenic redox activity of H. acidophilus- or De sulphurella-like organisms. If the latter, arsenite oxidation must be inhib ited in the initial high-sulphide zone of the spring, where no change in th e distribution of arsenite versus arsenate was observed.