Reversible Hydrogenase Activity Confers Flexibility to Balance Intracellular Redox in Moorella thermoacetica

Methanol and Carbon Monoxide Metabolism of the Thermophile Moorella caeni

Moorella species are thermophilic acetogens that primarily produce acetate from one-carbon (C1) compounds including CO, CO2 (+H2), methanol and formate. Notably, Moorella caeni DSM 21394T displays a hydrogenogenic metabolism on CO and an acetogenic metabolism on methanol. Furthermore, M. caeni is unable to use CO2 (+H2) and grows only on formate in the presence of a methanogen or when thiosulfate is added as an electron acceptor. Presently, all theoretical frameworks for C1 metabolism in Moorella species are derived from experimental and genomic analyses of Moorella thermoacetica, which exhibits an acetogenic metabolism with all C1 substrates. In this study, we applied a transcriptomics approach to elucidate the mechanisms underlying the C1 metabolism of Moorella caeni during growth on methanol and CO. Our results indicate that respiratory Complex 1, a proton-translocating (ubi)quinone oxidoreductase, is the primary respiratory enzyme in methanol-grown cells of M. caeni. Conversely, in CO-grown cells, an energy-conserving hydrogenase complex (Ech) appears to be the primary respiratory complex, alongside respiratory Complex 1. This study provides insight into the C1 metabolism of M. caeni and reveals variations in gene syntenies related to C1 metabolism among the Moorella genus.

Harnessing acetogenic bacteria for one-carbon valorization toward sustainable chemical production

The pressing climate change issues have intensified the need for a rapid transition towards a bio-based circular carbon economy. Harnessing acetogenic bacteria as biocatalysts to convert C1 compounds such as CO2, CO, formate, or methanol into value-added multicarbon chemicals is a promising solution for both carbon capture and utilization, enabling sustainable and green chemical production. Recent advances in the metabolic engineering of acetogens have expanded the range of commodity chemicals and biofuels produced from C1 compounds. However, producing energy-demanding high-value chemicals on an industrial scale from C1 substrates remains challenging because of the inherent energetic limitations of acetogenic bacteria. Therefore, overcoming this hurdle is necessary to scale up the acetogenic C1 conversion process and realize a circular carbon economy. This review overviews the acetogenic bacteria and their potential as sustainable and green chemical production platforms. Recent efforts to address these challenges have focused on enhancing the ATP and redox availability of acetogens to improve their energetics and conversion performances. Furthermore, promising technologies that leverage low-cost, sustainable energy sources such as electricity and light are discussed to improve the sustainability of the overall process. Finally, we review emerging technologies that accelerate the development of high-performance acetogenic bacteria suitable for industrial-scale production and address the economic sustainability of acetogenic C1 conversion. Overall, harnessing acetogenic bacteria for C1 valorization offers a promising route toward sustainable and green chemical production, aligning with the circular economy concept.

Thermophilic Moorella thermoacetica as a platform microorganism for C1 gas utilization: physiology, engineering, and applications

In the context of the rapid development of low-carbon economy, there has been increasing interest in utilizing naturally abundant and cost-effective one-carbon (C1) substrates for sustainable production of chemicals and fuels. Moorella thermoacetica, a model acetogenic bacterium, has attracted significant attention due to its ability to utilize carbon dioxide (CO2) and carbon monoxide (CO) via the Wood-Ljungdahl (WL) pathway, thereby showing great potential for the utilization of C1 gases. However, natural strains of M. thermoacetica are not yet fully suitable for industrial applications due to their limitations in carbon assimilation and conversion efficiency as well as limited product range. Over the past decade, progresses have been made in the development of genetic tools for M. thermoacetica, accelerating the understanding and modification of this acetogen. Here, we summarize the physiological and metabolic characteristics of M. thermoacetica and review the recent advances in engineering this bacterium. Finally, we propose the future directions for exploring the real potential of M. thermoacetica in industrial applications.

Enhancing acetone production from H2 and CO2 using supplemental electron acceptors in an engineered Moorella thermoacetica

Acetogens grow autotrophically and use hydrogen (H₂) as the energy source to fix carbon dioxide (CO₂). This feature can be applied to gas fermentation, contributing to a circular economy. A challenge is the gain of cellular energy from H₂ oxidation, which is substantially low, especially when acetate formation coupled with ATP production is diverted to other chemicals in engineered strains. Indeed, an engineered strain of the thermophilic acetogen Moorella thermoacetica that produces acetone lost autotrophic growth on H₂ and CO₂. We aimed to recover autotrophic growth and enhance acetone production, in which ATP production was assumed to be a limiting factor, by supplementing with electron acceptors. Among the four selected electron acceptors, thiosulfate and dimethyl sulfoxide (DMSO) enhanced both bacterial growth and acetone titers. DMSO was the most effective and was further analyzed. We showed that DMSO supplementation enhanced intracellular ATP levels, leading to increased acetone production. Although DMSO is an organic compound, it functions as an electron acceptor, not a carbon source. Thus, supplying electron acceptors is a potential strategy to complement the low ATP production caused by metabolic engineering and to improve chemical production from H₂ and CO₂.