BIOGEOCHEMICAL CYCLING OF IRON AND SULFUR IN LEACHING ENVIRONMENTS

Bacterial dissimilatory reduction of iron and sulphur in extremely aci dic environments is described. Evidence for reduction at two disused m ine sites is presented, within stratified 'acid streamers' growths and in sediments from an acid mine drainage stream. A high proportion (ap prox. 40%) of mesophilic heterotrophic acidophiles were found to be ca pable of reducing ferric iron (soluble and insoluble forms) under micr oaerophilic and anoxic conditions. Mixed cultures of Thiobacillus ferr ooxidans and Acidiphilium-like isolate SJH displayed cycling of iron i n shake flask and fermenter cultures. Oxido-reduction of iron in mixed cultures was determined by oxygen concentration and availability of o rganic substrates. Some moderately thermophilic iron-oxidising bacteri a were also shown to be capable of reducing ferric iron under conditio ns of limiting oxygen when grown in glycerol/yeast extract or elementa l sulphur media. Cycling of iron was observed in pure cultures of thes e acidophiles. Sulphate-reducing bacteria isolated from acid streamers could be grown in acidified glycerol/yeast extract media (as low as p H 2.9), but not in media used conventionally for their laboratory cult ure. An endospore-forming, non-motile rod resembling Desulfotomaculum has been isolated. This bacterium has a wide pH spectrum, and appears to be acid-tolerant rather than acidophilic.