Carbon monoxide (CO) is a gas that is toxic to various organisms including humans and even microbes; however, it has low redox potential, which can fuel certain microbes, namely, CO oxidizers. Hydrogenogenic CO oxidizers utilize an energy conservation system via a CO dehydrogenase/energy-converting hydrogenase complex to produce hydrogen gas, a zero emission fuel, by CO oxidation coupled with proton reduction. Biochemical and molecular biological studies using a few model organisms have revealed their enzymatic reactions and transcriptional response mechanisms using CO. Biotechnological studies for CO-dependent hydrogen production have also been carried out with these model organisms. In this chapter, we review recent advances in the studies of these microbes, which reveal their unique and versatile metabolic profiles and provides future perspectives on ecological roles and biotechnological applications. Over the past decade, the number of isolates has doubled (37 isolates in 5 phyla, 20 genera, and 32 species). Some of the recently isolated ones show broad specificity to electron acceptors. Moreover, accumulating genomic information predicts their unique physiologies and reveals their phylogenomic relationships with novel potential hydrogenogenic CO oxidizers. Combined with genomic database surveys, a molecular ecological study has unveiled the wide distribution and low abundance of these microbes. Finally, recent biotechnological applications of hydrogenogenic CO oxidizers have been achieved via diverse approaches (e.g., metabolic engineering and co-cultivation), and the identification of thermophilic facultative anaerobic CO oxidizers will promote industrial applications as oxygen-tolerant biocatalysts for efficient hydrogen production by genomic engineering.

一氧化碳（CO）是一种对多种生物（包括人类甚至微生物）有毒的气体；但是，它具有较低的氧化还原电势，可以为某些微生物提供燃料，即CO氧化剂。产氢的CO氧化剂利用CO脱氢酶/能量转换加氢酶复合物利用节能系统，通过CO氧化与质子还原相结合，产生氢气（零排放燃料）。使用一些模型生物的生化和分子生物学研究已经揭示了使用CO的酶促反应和转录反应机制。也已经对这些模型生物进行了依赖CO的氢气生产的生物技术研究。在本章中，我们回顾了这些微生物研究的最新进展，揭示了它们独特而通用的代谢谱，并提供了生态作用和生物技术应用的未来展望。在过去的十年中，分离株的数量增加了一倍（5个门，20个属和32个种的37个分离株）。最近分离出的一些化合物对电子受体显示出广泛的特异性。此外，不断积累的基因组信息可预测它们的独特生理结构，并揭示它们与新型潜在的产氢CO氧化剂的亲缘关系。结合基因组数据库调查，一项分子生态学研究揭示了这些微生物的广泛分布和低丰度。最后，通过多种方法（例如，代谢工程和共培养），已实现了生氢CO氧化剂的最新生物技术应用，