Genomic Analysis of Calderihabitans maritimus KKC1, a Thermophilic, Hydrogenogenic, Carboxydotrophic Bacterium Isolated from Marine Sediment.

KKC1 is a thermophilic, hydrogenogenic carboxydotroph isolated from a submerged marine caldera. Here, we describe the sequencing and feature analysis of the KKC1 genome. Genome-based phylogenetic analysis confirmed that KKC1 was most closely related to the genus , which includes well-studied acetogenic members. Comparative genomic analysis revealed that, like , KKC1 retained both the CO-reducing Wood-Ljungdahl pathway and energy-converting hydrogenase-based module activated by reduced ferredoxin, but it lacked the HydABC and NfnAB electron-bifurcating enzymes and pyruvate:ferredoxin oxidoreductase required for ferredoxin reduction for acetogenic growth. Furthermore, KKC1 harbored six genes encoding CooS, a catalytic subunit of the anaerobic CO dehydrogenase that can reduce ferredoxin via CO oxidation, whereas possessed only two CooS genes. Our analysis revealed that three genes formed known gene clusters in other microorganisms, i.e., -acetyl coenzyme A (acetyl-CoA) synthase (which contained a frameshift mutation), -energy-converting hydrogenase, and -FAD-NAD oxidoreductase, while the other three had novel genomic contexts. Sequence composition analysis indicated that these genes likely evolved from a common ancestor. Collectively, these data suggest that KKC1 may be highly dependent on CO as a low-potential electron donor to directly reduce ferredoxin and may be more suited to carboxydotrophic growth compared to the acetogenic growth observed in , which show adaptation at a thermodynamic limit. KKC1 and members of the genus are phylogenetically related but physiologically distinct. The former is a hydrogenogenic carboxydotroph that can grow on carbon monoxide (CO) with H production, whereas the latter include acetogenic bacteria that grow on H plus CO with acetate production. Both species may require reduced ferredoxin as an actual "energy equivalent," but ferredoxin is a low-potential electron carrier and requires a high-energy substrate as an electron donor for reduction. Comparative genomic analysis revealed that KKC1 lacked specific electron-bifurcating enzymes and possessed six CO dehydrogenases, unlike species. This suggests that KKC1 may be more dependent on CO, a strong electron donor that can directly reduce ferredoxin via CO dehydrogenase, and may exhibit a survival strategy different from that of acetogenic , which solves the energetic barrier associated with endergonic reduction of ferredoxin with hydrogen.