Microbial communities and interactions in extremely acidic environments were studied in this project. Microbial biomass growing as biofilms ("acid streamers") in acidic metal-rich water was collected from a chalybeate spa and an abandoned pyrite mine in North Wales, U.K.. The microbial communities found in these materials and those in extremely acidic, metal-rich waters were investigated using a combination of cultivation-dependent and cultivation-independent techniques. The majority (>90%) of microbes in the samples were unculturable using the methods used. Culture-independent approaches revealed that the acid-streamer microbial community was predominantly bacterial and that the dominant members were previously uncharacterized β-proteobacteria. Significant variations in microbial communities were found between streamers from different locations; microbes detected included members of the classes α- and γ-Proteobacteria, Acidobacteria Actinobacteria, Bacilli and Nitrospira.
Sulfidogenesis at low pH (3.8-4.2) was demonstrated by acidophilic/acid-tolerant sulfate reducing bacteria (aSRB), Desulfosporosinus-isolate M1, and
by a defined mixed-culture of M1 grown in mixed culture with an apparently
obligately aerobic heterotrophic acidophile (Acidocella-isolate PFBC). The
aSRB-isolate was found to be an incomplete oxidizer of glycerol (producing
acetic acid) and was also able to use molecular hydrogen as electron donor.
A sulfidogenic bioreactor, containing the defined mixed culture, was studied
in detail. No acetic acid accumulation was recorded when glycerol was
supplied as electron donor and the amount of sulfide produced indicated that
glycerol was being completely oxidized to CO₂. A hypothesis was developed
to explain the observed phenomena, involving interspecies transfer of hydrogen. Subsequent experimental data supported this hypothesis, and suggested a mechanism whereby sulfidogenesis may occur in extremely acidic environments.