Microbial Diversity 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. It has been inferred that thermophilic hydrogenogenic CO utilizers exist in mesophilic environments as dormant cells. The distribution of hydrogenogenic CO utilizers in thermophilic environments has been investigated. However, 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 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 hydrogenigenic CO utilizers exist at the bottom of Lake Biwa. To identify potential hydrogenogenic CO utilizers, microbial diversity was analyzed at the amplicon sequence variant (ASV) level. The two dominant ASVs, whose relative abundance was over 0.1%, were both assigned to Caloramator australicus, although CO utilization by C. australicus has not been reported. Based on the results of this study, it was speculated that novel microorganisms containing candidates for potential hydrogenogenic CO utilizers may exist at the bottom of Lake Biwa.