Conductive carbon nanoparticles inhibit methanogens and stabilize hydrogen production in microbial electrolysis cells

Nanosized conductive carbon materials have been reported to stimulate methanogenesis by anaerobic microbiomes, while other studies have shown their antimicrobial activities. The present study examined effects of conductive carbon nanoparticles (carbon black Vulcan, CB) on methanogenesis from glucose by anaerobic sludge. We found that a relatively high concentration (e.g., 2% w/v) of CB entirely inhibited the methanogenesis, where a substantial amount of acetate was accumulated after degradation of glucose. Quantitative real-time PCR assays and metabarcoding of 16S rRNA amplicons revealed that, while bacteria were stably present irrespective of the presence and absence of CB, archaea, in particular methanogens, were largely decreased in the presence of CB. Pure-culture experiments showed that methanogenic archaea were more seriously damaged by CB than fermentative bacteria. These results demonstrate that CB specifically inhibits methanogens in anaerobic sludge. We attempted to supplement cathode chambers of microbial electrolysis cells with CB for inhibiting methanogenesis from hydrogen, demonstrating that hydrogen is stably produced in the presence of CB.