Enrichment of salt-tolerant CO2-fixing communities in microbial electrosynthesis systems using porous ceramic hollow tube wrapped with carbon cloth as cathode and for CO2 supply

Abstract

Microbial inocula from marine origins are less explored for CO₂ reduction in microbial electrosynthesis (MES) system, although effective CO₂-fixing communities in marine environments are well-documented. We explored natural saline habitats, mainly salt marsh (SM) and mangrove (M) sediments, as potential inoculum sources for enriching salt-tolerant CO₂ reducing community using two enrichment strategies: H₂:CO₂ (80:20) enrichment in serum vials and enrichment in cathode chamber of MES reactors operated at -1.0 V vs. Ag/AgCl. Porous ceramic hollow tube wrapped with carbon cloth was used as cathode and for direct CO₂ delivery to CO₂ reducing communities growing on the cathode surface. Methanogenesis was dominant in both the M- and SM-seeded MES and the methanogenic Archaea Methanococcus was the most dominant genus. Methane production was slightly higher in the SM-seeded MES (4.9 ± 1.7 mmol) compared to the M-seeded MES (3.8 ± 1.1 mmol). In contrast, acetate production was almost two times higher in the M-seeded MES (3.1 ± 0.9 mmol) than SM-seeded MES (1.5 ± 1.3 mmol). The high relative abundance of the genus Acetobacterium in the M-seeded serum vials correlates with the high acetate production obtained. The different enrichment strategies affected the community composition, though the communities in MES reactors and serum vials were performing similar functions (methanogenesis and acetogenesis). Despite similar operating conditions, the microbial community composition of M-seeded serum vials and MES reactors differed from the SM-seeded serum vials and MES reactors, supporting the importance of inoculum source in the evolution of CO₂-reducing microbial communities.

Keywords: CO(2) reduction; CO(2) valorization; Carbon capture and utilization; Dual-function cathode; Salt-tolerant chemolithoautotrophs.