Comparative Physiology and Genomics of Thermincola and Carboxydocella Strains and Description of Two Novel Isolates.

Thermophilic microorganisms, such as those inhabiting hydrothermal environments, play key roles in carbon monoxide (CO) metabolism, thereby influencing global carbon cycling. Members of the genera Thermincola and Carboxydocella are capable of CO oxidation via the water-gas shift reaction, generating H2, but comprise only two and three validly described species, respectively. In this study, we report the isolation of two novel bacterial strains: strain AZ34E, affiliated with Thermincola, and strain AZ29I, affiliated with Carboxydocella. Comparative genomic and phylogenetic analyses revealed that all currently described isolates within Thermincola and Carboxydocella each represent a single species within their respective genera. Thermincola genomes contain four gene copies encoding [Ni-Fe] CO dehydrogenases, while Carboxydocella genomes harbour six copies with diverse predicted functional roles, suggesting high metabolic flexibility for CO oxidation in these thermophiles. The isolation of the two novel strains, both members of the Bacillota_B phylum, motivated a broader genomic survey across this lineage. This search uncovered numerous candidate organisms with genomic potential for CO oxidation, substantially expanding the diversity of putative carboxydotrophs. These findings highlight the value of targeted genomic mining approaches, which may reveal a much wider array of CO-oxidising microorganisms than currently recognised.