A Cytochrome c-Containing Periplasmic Nitrate Reductase in the Acetogen Sporomusa ovata

Sporomusa ovata is one of the few acetogenic bacteria that have cytochromes and quinones, but their role is unknown. In addition to reducing CO₂ to acetate, S. ovata can also use nitrate as an alternative electron acceptor, but the enzymes involved, a possible function of cytochromes and quinones and the bioenergetics of nitrate reduction remain elusive. Under heterotrophic and autotrophic growth conditions, the presence of nitrate led to higher optical densities and decreased acetate production. Under heterotrophic conditions with fructose as an electron donor, nitrate was preferred over CO₂ as an electron acceptor. Under autotrophic conditions with H₂ as an electron donor, nitrate and CO₂ were used simultaneously. Genome analyses revealed a nitrate reduction island with genes encoding a periplasmic cytochrome c-containing nitrate reductase, nitrite reductase, cytochrome c and heme biosynthesis. The expression of this nitrate reduction island was strongly induced by the presence of nitrate. Enzyme assays demonstrated membrane-bound nitrate and nitrite reductase activities exclusively in nitrate-grown cells. Heme peroxidase staining confirmed the presence of cytochromes in nitrate-grown cells. In sum, nitrate reduction to ammonium is catalysed by a membrane-bound electron transport chain involving cytochromes and potentially coupled to energy conservation in S. ovata.