Introduction: Any acetogenic bacterium known to date generates ATP via two ferredoxin-dependent respiratory chains, containing either the Rnf or the Ech complex^[1]. These two membrane-bound respiratory enzymes do not have cytochromes. However, some species such as Moorella thermoacetica or Sporomusa ovata contain b- and c-type cytochromes respectively^[2,3], whose functions remain to be identified.

Goals: To unravel the role of cytochromes in the physiology of S. ovata.

Materials & Methods: Bioinformatics, growth experiments, metabolite analyses, determination of transcript levels, biochemistry, cytochrome c-levels.

Results: Addition of nitrate led to higher optical densities and to reduced acetate concentrations in S. ovata when grown on fructose, methanol or H₂ + CO₂. Nitrate was reduced and nitrate reduction was CO2/bicarbonate independent. Inspection of the genome sequence revealed nap genes coding for a periplasmic nitrate reductase as well as genes encoding cytochrome c and heme biosynthesis and the genes coding for a periplasmic nitrite reductase. Transcriptome analyses revealed induction of expression of these genes by nitrate. Biochemical analyses confirmed the presence of cytochrome c in nitrate-grown cells.

Summary: Cytochrome c is apparently involved in the utilization of an alternative electron acceptor in S. ovata, nitrate, but not in the primary energetics. Rnf is still active in S. ovata under these conditions. Whether nitrate reduction produces additional ATP remains to be identified.

[1] Schuchmann, K., Müller, V. (2014) Nat Rev Microbiol 12: 809-21

[2] Rosenbaum, F. P., Müller, V. (2021) Extremophiles 25: 413-24

[3] Kremp, F., Roth, J., Müller, V. (2022) Microbiol. Spectr 10: e0138522