(Per)chlorate reduction at high temperature: Physiological study of Archaeoglobus fulgidus and potential implications for novel souring mitigation strategies

The recent finding that Archaeoglobus fulgidus is able to couple (per)chlorate reduction to growth expanded this trait to the hyperthermophilic range of life. This sulfate-reducing archaeon is considered to be one of the major contributors to souring in hot oil reservoirs. Therefore, it is important to study its physiology in depth, particularly in view of novel souring mitigation strategies. A. fulgidus does not possess the classical (per)chlorate reduction pathway, as it lacks the key enzyme chlorite dismutase. Rather, the microorganism seems to couple (per)chlorate reduction to sulfur metabolism. Growth experiments show the strict necessity of sulfur compounds to sustain perchlorate reduction. Furthermore, the chemical formation of elemental sulfur was observed during perchlorate reduction, a compound that is biologically reduced again. Additional experiments showed that tetrathionate, but not elemental sulfur and polysulfide, serves as an electron acceptor for growth by A. fulgidus. Taken together these results provide further evidence for the importance of chemical and biological redox reactions involving sulfur compounds during (per)chlorate reduction. In non-reduced media also, nitrate could be reduced by A. fulgidus, though not coupled to growth. This observation and the fact that A. fulgidus had prolonged adaptation phases on sulfate after long-lasting growth on perchlorate are of interest in the development of new souring mitigation strategies using nitrate and/or (per)chlorate.