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Bacteria from the genus Acidithiobacillus are often associated with biominingand acid mine drainage. Biomining utilises acidophilic, sulphur and ironoxidising microorganisms for recovery of metals from sulphidic low grade oresand concentrates. Acid mine drainage results in acidification and contaminationwith metals of soil and water emanating from the dissolution of metal sulphidesfrom deposits and mine waste storage. Acidophilic microorganisms play acentral role in these processes by catalysing aerobic oxidation of sulphides.Acceleration of mineral solubilisation is a positive aspect in biomining whereas,in acid mine drainage it is undesirable and accordingly, microbial iron andsulphur oxidation is promoted in the first case and measures are taken to inhibitit in the second case. In this thesis, several approaches were taken in order tounderstand and increase oxidation efficiency in biomining and to gain an insightinto the biochemical reactions taking place in these environments. A laboratoryscale bioreactor was designed and tested allowing simulation of bioleaching inheaps of mine tailings at different aeration, irrigation and particle size conditions(Paper I). A new psychrotolerant strain of Acidithiobacillus ferrooxidans wascharacterised that has an application in boreal heap bioleaching. Iron, reducedinorganic sulphur compound oxidation and bioleaching of various ores by thisstrain was studied as well as gene expression during oxidation of tetrathionateand/or ferrous iron (Papers III & IV). Expression and regulation of atetrathionate hydrolase from Acidithiobacillus caldus, a key enzyme in reducedinorganic sulphur compound metabolism of this bacterium was investigated andthe presence of this enzyme in a bioleaching mixed culture was shown. The genecluster that harbours the gene coding for tetrathionate hydrolase (tetH) wasdescribed for the first time (Paper II).