The study of acidophilic, moderately thermophilic iron-oxidizing bacteria

Abstract

This study has divided the most frequently isolated types of moderate thermophiles into three groups: isolates of Sulfobacillus thermosulfidooxidans (mol% G + C 47-50), an isolate referred to as strain NAL and other closely related species (mol% G+C 54-57), and the type previously referred to as strain TH3 (mol% G+C 68). An enrichment culture was obtained that could efficiently solubilise a range of mineral sulphides at 48oC under air. Characterisation of this culture indicated the presence of two organisms essential for efficient growth under air: a typical S. thermosulfidooxidans group organism (isolate ICH), and a strain TH3 group organism (isolate ICP). Strain ICP appeared to possess an inducible, high affinity transport system for carbon dioxide during growth under air (unlike any previously studied moderate thermophiles), but extensive oxidation of ferrous iron was not achieved even at enhanced carbon dioxide levels. This lack of oxidation appeared to be the result of autotrophically-growth strain ICP having an apparent higher affinity for the end-product of iron oxidation, ferric iron (Ki 0.4 mM), than the substrate, ferrous iron (Km 0.5 mM). Only when a mixed culture of strain ICP and strain ICH was grown did extensive oxidation occur. A comparative mineral leaching study, with a mesophilic, a moderately thermophilic, and an extremely thermophilic culture indicated that the moderately thermophilic culture was the most robust during the dissolution of a range of minerals. This culture gave consistently better mineral dissolution rates than the mesophilic culture, clearly indicating their immediate commercial potential. In comparison the extremely thermophilic culture often produced faster rates of mineral dissolution than the moderately thermophilic culture, but appeared sensitive to agitation at high mineral pulp densities (10% (w/v)), limiting any present commercial applications of these organisms