Deciphering the role of biochar pseudocapacitance strengthening in promoting syntrophic methanogenesis: Microbial capacitance-tropism in propionate metabolism

Abstract

The redox functional groups of biochar can act as pseudocapacitance, alleviating acid accumulation and facilitating electron transfer in anaerobic digestion. Fe modification further enhances its pseudocapacitance properties. However, the specific role of pseudocapacitance strengthening in the electron transfer of volatile fatty acid syntrophic methanogenesis, as well as the factors inducing the enrichment behavior and metabolic mechanisms of microbes on its surface, remain unclear. In this study, experiments using acetate and propionate as substrates were conducted with the addition of biochar and electro-enhanced biochar, while 16S rRNA gene amplicon sequencing and metagenomic analysis were employed to investigate the underlying mechanisms. The results demonstrate that strengthening biochar’s pseudocapacitance significantly facilitates the electron shunt by accepting electrons, accelerating NADH/NAD+ conversion and ATP synthesis. This further promotes the oxidative degradation of propionate and acetate through the methylmalonyl-CoA (MMC) and syntrophic acetate oxidation (SAO) pathways. During methanogenesis, biochar directly transfers electrons to methanogens, with the strengthened pseudocapacitance further accelerating electron transfer through the CO2 reduction pathway and specifically promoting the methanation of propionate rather than acetate. Moreover, during propionate metabolism, Coprothermobacter and Methanosarcina exhibit capacitance-tropism in their enrichment on biochar, serving as key functional contributors to the biochar-microbial aggregates. These findings provide valuable insights for selecting exogenous materials in engineering applications to alleviate acid accumulation in anaerobic digestion systems.