The Role of Enhanced Heterotrophic Bacterial Growth on Iron Oxidation by Acidithiobacillus ferrooxidans

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Addition of organic carbon substrate (glucose) profoundly affected the growth of cultures of acidophilic bacteria typical of acid mine drainage (AMD) sites: the iron-oxidizing autotrophic bacteria, Acidithiobacillus ferrooxidans , and a common heterotrophic strain, Acidiphilium acidophilum . Growth of A. ferrooxidans on soluble ferrous iron media was significantly inhibited in the presence of 1,000 mg/L glucose, regardless of the initial cell density, and in spite of favorable pH and aeration conditions. Interestingly, inhibition of A. ferrooxidans was reduced with addition of the heterotroph, apparently due to the consumption of glucose because the onset of iron oxidation coincided with reduction in glucose concentration in the medium. Another mechanism, local production of CO 2 by A. acidophilum provided inorganic carbon required by A. ferrooxidans cells, was investigated. Although no direct proof of interspecies CO 2 exchange was identified, iron oxidation was enhanced and glucose inhibition reduced with incubation of A. ferrooxidans cultures with 5% CO 2 in air. When oxygen was limited and glucose was added to the acidic coculture, the ultimate amount of iron oxidized was significantly lower and the ferric iron produced was subsequently reduced by the heterotrophs as conditions became anoxic. Attribution of ferric iron reduction to A. acidophilum was confirmed in pure culture experiments where a zero-order iron-reduction rate of 458 - 26.9 w mol Fe/L-day (25.6 - 1.5 mg Fe/L-day) was observed. Bacterial iron reduction also led to an increase in pH from 2.5 to 4.0. Thus, the addition of glucose or some other organic electron donor could provide an in situ or ex situ bioremediation strategy to raise pH at AMD sites resulting in a lower amount of metal leaching into drainage water by promoting reducing conditions favorable to iron reduction.