Abstract
Methanothermobacter Met2 is a metagenome-assembled genome (MAG) that encodes a putative mixotrophic methanogen constituting the major populations in thermophilic fixed-bed anaerobic digesters. In order to characterize its physiology, the present work isolated an archaeon (strain Met2-1) that represents Met2-type methanogens by using a combination of enrichments under a nitrogen atmosphere, colony formation on solid media and limiting dilution under high partial pressures of hydrogen. Strain Met2-1 utilizes hydrogen and carbon dioxide for methanogenesis, while the growth is observed only when culture media are additionally supplemented with acetate. It does not grow on acetate in the absence of hydrogen. The results demonstrate that Methanothermobacter sp. strain Met2-1 is a novel methanogen that exhibits obligate mixotrophy.
Highlights
Methanogens are archaea that produce methane as the catabolic end product [1]
Thermophilic fixed-bed anaerobic digesters (TFDs) facilitate high-rate conversion of organic wastes into methane [3], and commercial TFDs are operated in Japan and other countries for the treatment of food and brewery wastes [4]
It has been found that metagenome-assembled genomes (MAGs) Mes1 and Met2 represent two major populations that account for over 30% and 20%, respectively, of the total biofilm populations in TFD [5]
Summary
Methanogens are archaea that produce methane as the catabolic end product [1]. In nature, methanogens play important roles in organic-matter decomposition under anaerobic conditions [1]. Reconstructed catabolic pathways indicate that the Met MAG does not code for a carbon-monoxide dehydrogenase/acetyl-CoA synthase complex (CODH/ACS), one of the essential enzymes in the methanogenic pathways [8]. This finding suggests the possibility that Met is incapable of utilizing the acetyl-CoA pathway for CO2 fixation and of producing methane from acetate. It is considered that Met utilizes CO2/H2 only for conserving energy by methanogenesis, while acetate is used as a carbon source Based on these assumptions, Met is likely a novel obligate mixotrophic methanogen that is entirely different from autotrophic hydrogenotrophic methanogens belonging to the genus Methanothermobacter. We deduce that Met2-type methanogens have ecological advantages over other hydrogenotrophic methanogens in TFD biofilms, and phenotypes associated with the lack of CODH/ACS would be relevant
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