Isolation and characterization of novel carboxydotrophic Moorella strains for employment as potential thermophilic biocatalysts

Carboxydotrophic hydrogenogenic and acetogenic bacteria utilize carbon monoxide (CO) as their primary carbon and energy source. Hydrogenogenic bacteria employ the process of hydrogenogenesis to convert CO into H₂ gas, thereby generating energy and H₂ as a renewable energy carrier. In contrast, acetogenic bacteria employ the acetyl-CoA pathway to convert CO into acetic acid or other organic acids, which can be utilized in the synthesis of bulk chemicals and biofuels. The use of these bacteria in biotechnological processes allows the conversion of waste gases or syngas containing CO into valuable end products such as H₂ and biofuels, thereby supporting sustainable energy solutions and a circular economy. Thermophilic carboxydotrophs, such as members of the genus Moorella, are of particular interest due to their capacity to grow at elevated temperatures. This trait reduces the costs associated with cooling of bioreactors, minimizes the risks of contamination, and facilitates the recovery of volatile products. The objective of this project is to isolate novel carboxydotrophic strains from the thermophilic Moorella genus, followed by a comprehensive genomic and physiological characterization of their CO-oxidizing metabolism.

Thermophilic enrichments resulted in the identification of a novel species, Moorella carbonis sp. nov., and six additional strains of the species Moorella thermoacetica and Moorella humiferrea. Growth experiments demonstrated that M. carbonis and one M. humiferrea strain exhibited carboxydotrophic hydrogenogenesis, resulting in the production of H₂ and CO₂ as end products. The remaining five strains performed acetogenesis, converting CO primarily to acetate. Genome analysis revealed that acetogenic Moorella strains encode a formate dehydrogenase upstream of the energy-conserving hydrogenase complex (Ech1), whereas hydrogenogenic strains encode a monofunctional carbon monoxide dehydrogenase (CooS) instead. Moreover, the CooS/Ech1 complex of hydrogenogenic Moorella strains differed from the highly conserved CooS/Ech-complex observed in all other described carboxydotrophic hydrogenogenic bacteria.

In conclusion, novel carboxydotrophic hydrogenogenic and acetogenic Moorella strains have been successfully isolated from environmental samples and primarily differed in the genes encoded in synteny to the Ech1 complex involved in energy conservation.