7. CO-Oxidizing Anaerobic Thermophilic Prokaryotes

Being a potent electron donor (E^0' _CO/CO2 = −520 mV), CO may serve as an energy source for anaerobic prokaryotes. The main sources of CO in hot environments inhabited by anaerobic thermophiles are volcanic exhalations and thermal degradation of organic matter. A number of phylogenetically diverse anaerobic prokaryotes, both Bacteria and Archaea, are known to metabolize CO. CO transformation may be coupled to methanogenesis, acetogenesis, hydrogenogenesis, and sulfate or ferric iron reduction. This review will mainly focus on the diversity, ecology, physiology, and certain genomic features of the hydrogenogenic species, which are most numerous among the currently recognized thermophilic anaerobic carboxydotrophs and many of which were isolated and described in recent years. Among them are diverse Firmicutes, Dictyoglomi, and Eury- and Crenarchaeota. Despite their phylogenetic diversity, they employ similar enzymatic mechanisms of the СО + Н₂О → СО₂ + Н₂ process. The key enzyme of anaerobic CO utilization, the Ni-containing CO dehydrogenase, forms in hydrogenogens an enzymatic complex with the energy-converting hydrogenase, and genomic analysis shows this enzymatic complex to be encoded by a single-gene cluster.