Moderately thermophilic iron oxidising bacteria isolated from a pyritic coal deposit showing spontaneous combustion

Seven Fe²⁺-oxidising acidophilic bacterial strains were isolated at 50 °C from a pyritic coal from Collie, Australia and from a Fe²⁺-oxidising fluidised bed reactor running at 60 °C which was originally inoculated with the Collie coal. The 16S rRNA gene of five of the strains was partially sequenced. The strains isolated from the reactor were closely related (99% similarity in gene sequence) to Sulfobacillus thermosulfidooxidans and the strains isolated directly from the coal had a 97% gene sequence similarity to "Sulfobacillus yellowstonensis". The sequenced strains each had an optimum temperature for growth using an organic carbon source of approximately 50 °C and a maximum temperature for growth of approximately 60 °C. All strains required a mixotrophic medium for growth on Fe²⁺ or on an organic carbon source. Of the five strains tested, all grew on pyrite whereas only three of the strains grew on chalcopyrite. The "S. yellowstonensis"-like strains grew better on S⁰ than the other strains. The S. thermosulfidooxidans-like strains were more tolerant then the other strains to Cu²⁺ and Cl^-. The "S. yellowstonensis"-like strains contained ω-cycloheptyl-18:0 and ω-cycloheptyl-18:0 2OH phospholipid fatty acids, whereas the S. thermosulfidooxidans-like strains contained no ω-cyclofatty acids in the phospholipid fraction. These strains have potential to be used in the bioleaching of heaps, especially the S. thermosulfidooxidans-like strains. Work is continuing to further characterise their bioleaching abilities and their usefulness in a commercial heap leach environment.