A study was carried out to elucidate microbial interactions in extremely acidic, metal-rich environments and to evaluate the potential of pure cultures and consortia of acidophilic micro-organisms to remediate mine waters. Two processes, dissimilatory iron oxidation and sulfate reduction, both of which have central roles in the biogeochemical cycling of iron and sulfur in the lithosphere, formed the major focus of the study. Novel isolates of acidophilic bacteria were obtained from study sites and used to develop new prototype systems for bioremediating acidic, metal-rich mine waters. Two systems were examined, one of which accelerated ferrous iron oxidation, while the other used sulfate-reducing bacteria to generate alkalinity and precipitate metals as sulfides. The microbiology of the novel sulfidogenic system that operated at low pH was examined in detail, and confirmed to comprise a syntrophic consortium of acidophilic/acid-tolerant sulfate-reducing and acetate-degrading bacteria. Novel species of acidophilic sulfate-reducing bacteria were also obtained from an abandoned copper mine in south-east Spain; preliminary characterisation of these bacteria suggested that they could be highly effective in low pH
sulfidogenic bioreactors.
Interactions between the geochemistry and micro-organisms in acidic, metal-rich waters at two abandoned copper mines (Mynydd Parys, north Wales, U.K. and Cantareras, Huelva province, Spain) were examined. In both locations, iron-oxidising bacteria were present in large numbers both in the flowing water (Acidithiobacilus ferrooxidans) and immobilised in
massive "acid streamer" growths (notably the novel β-proteobacterium "Ferriovalis acidosiris"). Despite the presence of these bacteria, no significant changes in concentrations of soluble iron were detected from the point of discharge to ~100 metres downstream, at both mine sites. At Mynydd Parys, the relatively high flow rate of the AMO stream was thought to be the reason why ferrous iron oxidation was not detected, whereas in the Cantareras stream, ferric iron reduction by heterotrophic acidophiles present in the streamer growths and fuelled by organic carbon originating from photosynthetic eukaryotes that colonised the streamer surface, was considered to be the major reason why net iron oxidation was not apparent.
Packed-bed bioreactors containing immobilized iron-oxidising bacteria were tested for their potential to oxidise ferrous iron in low temperature (~15 °C) and low pH (~2) waters, with the objective of selectively removing iron from acid mine drainage as an oxidised mineral (e.g. schwertmannite). A novel acidophilic iron-oxidising isolate ("Ferriovalis acidosiris") was superior to others when the bioreactors were operated in continuous flow mode, while others acidophiles (a Ferrimicrobium-like isolate, and Leptospirillum ferrooxidans) were superior in reducing concentrations if ferrous iron to very low levels. A prototype iron oxidation biosystem was installed at the Mynydd Parys site, and data obtained over 12 months showed this increased both aeration of the mine water stream and net ferrous iron oxidation.