Formate-Dependent Heterodisulfide Reduction in a Methanomicrobiales Archaeon

Hydrogenotrophic methanogens produce CH₄using H₂as an electron donor to reduce CO₂. In the absence of H₂, many are able to use formate or alcohols as alternate electron donors. Methanogens from the order Methanomicrobiales are capable of growth with H₂, but many lack genes encoding hydrogenases that are typically found in other hydrogenotrophic methanogens. In an effort to better understand electron flow in methanogens from the Methanomicrobiales, we undertook a genetic and biochemical study of heterodisulfide reductase (Hdr) in Methanoculleus thermophilus. Hdr catalyzes an essential reaction by coupling the first and last steps of methanogenesis through flavin-based electron bifurcation. Hdr from M. thermophilus copurified with formate dehydrogenase (Fdh) and only displayed activity when formate was supplied as an electron donor. We found no evidence of an Hdr-associated hydrogenase, and H₂could not function as an electron donor, even with Hdr purified from cells grown on H₂. We found that cells catalyze a formate hydrogenlyase activity that is likely essential for generating the formate needed for the Hdr reaction. Together, these results highlight the importance of formate as an electron donor for methanogenesis and suggest the ability to use formate is closely integrated into the methanogenic pathway in organisms from the order Methanomicrobiales. Importance Methanogens from the order Methanomicrobiales are thought to prefer H₂as an electron donor for growth. They are ubiquitous in anaerobic environments, such as in wastewater treatment facilities, anaerobic digesters, and the rumen, where they catalyze the terminal steps in the breakdown of organic matter. However, despite their importance, the metabolism of these organisms remains understudied. Using a genetic and biochemical approach, we show that formate metabolism is closely integrated into methanogenesis in Methanoculleus thermophilus. This is due to a requirement for formate as the electron donor to heterodisulfide reductase (Hdr), an enzyme responsible for catalyzing essential reactions in methanogenesis by linking the initial CO₂fixing step to the exergonic terminal reaction of the pathway. These results suggest that hydrogen is not necessarily the preferred electron donor for all hydrogenotrophic methanogens and provide insight into the metabolism of methanogens from the order Methanomicrobiales.