Enrichment of Hydrogen-Oxidizing Bacteria Using a Hybrid Biological–Inorganic System Under a High CO2 Concentration or Atmospheric Air

Hybrid biological–inorganic (HBI) systems that integrate hydrogen-oxidizing bacteria (HOBs) with inorganic water-splitting catalysts have been developed as bioelectrochemical platforms for CO2 conversion. This study used an HBI system to enrich HOBs under atmospheric air or a high CO2 concentration. The HBI system operated with two different bioreactor configurations: a high-CO2 condition, in which the bioreactor was sealed with an air-tight stopper and the headspace gas was replaced with CO2 (100%), and an atmospheric air condition, in which the bioreactor was sealed with an air-permeable cap, gas exchange was allowed, and contamination was prevented. These bioreactors were inoculated with six rhizosphere soil samples, which were additionally enriched with a conventional gas-fermentation mixture. 16S rRNA gene amplicon sequencing revealed enrichment of the bacterial lineages mostly related to three main families under the high-CO2 condition. On the contrary, the open-air system facilitated enrichment of aerobic HOBs, potentially occurring through the enrichment of more diverse bacterial lineages. The different enrichment conditions imposed distinct selective pressures on HOBs. The obtained results indicated that HBI systems provide an alternative enrichment platform for expanding the diversity of HOBs applicable to CO2-based biomanufacturing.