Phylogeny of microorganisms populating a thick, subaerial, predominantly lithotrophic biofilm at an extreme acid mine drainage site

An unusually thick (similar to 1 cm) slime developed on a slump of finely d isseminated pyrite ore within an extreme acid mine drainage site at Iron Mo untain, near Redding, Calif, The slime was studied over the period of 1 yea r. The subaerial form of the slime distinguished it from more typical subme rged streamers. Phylogenetic analysis of 16S rRNA genes revealed a diversit y of sequences that were mostly novel. Nearest relatives to the majority of sequences came from iron-oxidizing acidophiles, and it appears that iron o xidation is the predominant metabolic characteristic of the organisms in th e slime, The most abundant of the 16S rRNA genes detected were from organis ms related to Leptospirillum species. The dominant sequence (71% of clones) may represent a new genus. Sequences within the Archaea of the Thermoplasm ales lineage were detected. Most of these mere only distantly related to kn own microorganisms, Also, sequences affiliating with Acidimicrobium were de tected. Some of these mere closely related to "Ferromicrobium acidophibcs," and others were affiliated with a lineage only represented by environmenta l clones. Unexpectedly, sequences that affiliated within the delta subdivis ion of the Proteobacteria were detected. The predominant metabolic feature of bacteria of this subdivision is anaerobic sulfate or metal reduction. Th us, microenvironments of low redox potential possibly exist in the predomin antly oxidizing environments of the slime. These results expand our knowled ge of the biodiversity of acid mine drainage environments and extend our un derstanding of the ecology of extremely acidic systems.