Microbial oxidation of pyrite: Experiments using microorganisms from an extreme acidic environment

Surface colonization and microbial dissolution of pyrite were studied in th e laboratory and by in situ surface colonization experiments conducted at I ron Mountain, California. Laboratory experiments involved organisms obtaine d from Iron Mountain and cultured in pH <1.0, 42 degrees C solutions design ed to enrich for chemolithotrophic species present at acid-generating sites . Planktonic and sessile microorganisms grew in enrichment cultures contain ing small amounts of yeast extract. The maximum density of attached cells w as approximately 8 x 10(6) cells/cm(2). Attachment was specific for pyrite and occurred nonrandomly; rod-shaped bacteria tended to orient parallel to {100} and {110} pyrite. Attachment resulted in formation of euhedral dissol ution pits. Cultures grown without yeast extract contained only planktonic cells and euhedral dissolution pits were not developed on the pyrite surfac e. All cultured organisms were identified as bacteria by fluorescence in si tu hybridization and domain-specific probes. Leptospirillum ferrooxidans co mprised 10-40% of planktonic organisms in both enrichments. Thiobacillus fe rrooxidans was not identified in either enrichment. Oxidation rates were ap proximately equivalent in both enrichments (4 x 10(-7) pM Fe/cell.day) over a 28 day period. Pyrite cubes were exposed to natural solutions at Iron Mountain for two mon ths. A subset of samples was exposed only to solutions that had passed thro ugh 0.22 mu m Teflon filters. Denser colonization (by distinctive elongate bacteria not observed in laboratory cultures) occurred on pyrite in filter- covered vessels. Attachment specificity, orientation, and resulting degrada tion morphology were similar to that observed in laboratory cultures. Resul ts show that interaction between attached cells and pyrite surface is highl y specific and the impact on surface morphology evolution is different from that associated with planktonic microorganisms, despite the similarity in effect (per cell) on total dissolution rates.