The effects of nitrate on substrate utilisation by some iron(II)- and sulfur-oxidising Bacteria and Archaea

• Bacteria showed signs of adapting to nitrate in growth media.
• Archaea did not adapt to nitrate in growth media.
• Nitrate offers a potential redox-control strategy for high-temperature bioleaching.

The influence of nitrate on batch cultures of selected Acidithiobacillus, Sulfobacillus, Acidianus, Sulfolobus and Metallosphaera species capable of utilising iron(II) and reduced sulfur substrates was examined. Nitrate was added to cultures in media containing both ferrous ions and tetrathionate. The presence of nitrate resulted in decreased planktonic cell numbers, increased microbial lag times and lower ferrous ion and tetrathionate utilisation rates. These results varied with nitrate concentration, species and adaptive history. Based on the results of batch culture tests, nitrate was added to chalcopyrite concentrate bioleaching experiments to control the redox potential. Bacteria in bioleaching tests at 30 and 45 °C exhibited adaptation to nitrate resulting in high redox potentials. However, when archaea were used in bioleaching tests at 60 °C, ferrous ion oxidation was suppressed in the presence of nitrate. At a nitrate concentration of 20–30 mM, a redox potential of 430–460 mV (vs Ag/AgCl) was maintained during the 10 week experiment. At this redox potential copper extraction was increased by 20% compared to a culture without nitrate, potentially offering a method of redox control for high-temperature stirred-tank bioleaching with archaea.