Microbial Compositions in Carbon Monoxide Anaerobic Enrichment Cultures Using Sediment from Fresh Water Lake, Lake Biwa.

Both thermophilic and mesophilic hydrogenogenic carbon monoxide (CO) utilizers have been reported. Most thermophilic hydrogenogenic CO utilizers have been isolated from thermophilic environments, such as hot springs, deep-sea hydrothermal fields, and anaerobic bioreactors. However, some thermophilic hydrogenogenic CO utilizers have been found in mesophilic environments, such as marine sediments, lake sediments, and soils. Thermophilic hydrogenogenic CO utilizers may exist in mesophilic environments as dormant cells. Although the distribution of hydrogenogenic CO utilizers in thermophilic environments has been investigated, their distribution in low-temperature environments has not been fully addressed and remains unclear. Therefore, in this study, the distribution of hydrogenogenic CO utilizers at the bottom of Lake Biwa was investigated. The sediment samples from Lake Biwa were incubated under 20% CO + 80% N2. DNA was extracted from the CO-enriched samples and 16S rRNA amplicon sequencing analysis was performed. After incubation under the addition of CO for 48 h, a decrease in CO and an increase in hydrogen (H2) were observed. This suggests that potential hydrogenogenic CO utilizers exist at the bottom of Lake Biwa. To identify potential hydrogenogenic CO utilizers, we analyzed the microbial composition at the amplicon sequence variant (ASV) level and conducted metagenomic analysis to survey for the key gene to utilize CO, nickel-containing CO dehydrogenase (Ni-CODH) gene. We detected the known CO utilizer ASVs and short fragments of Ni-CODH catalytic subunit-like sequence. Thus, some of these prokaryotes might contribute to CO consumption in the CO enrichment cultures and are present at the bottom of Lake Biwa.