Deciphering the Role of Conductive Materials in the Interaction between Syntrophic Bacteria and Hydrogenotrophic Methanogens: From Kinetics to Microbial Pathways

Conductive materials have been reported to enhance anaerobic digestion (AD) processes, but their role in the synergistic relationship between syntrophic bacteria and hydrogenotrophic methanogens (HMs) remains unclear yet. This study evaluated the holistic impact of conductive materials in an HMs-dominant anaerobic system (HMs over 95% in the archaeal community). Biochar (BC) and iron powder (IP) were selected as conductive materials with food waste and waste-activated sludge as co-substrates. The addition of IP and BC to HMs-dominant reactors significantly increased the methane yield (11.5–26%) and shortened the digestion period (32–45%) compared with the control. In addition, a novel developed integrated kinetic model revealed the diauxic peak for methanogenesis which could be significantly alleviated by the mediation of conductive materials. Furthermore, the degradation of volatile fatty acids followed a sequential order of acetate–butyrate–propionate. A smoother degradation was observed with the addition of conductive materials, while the control groups exhibited a 10 day lag phase between the degradation of butyrate and propionate. The microbial community analysis showed that BC and IP stimulated diverse syntrophic bacteria, that is, BC-enriched genus Longilinea and Aminicenantales and IP-accumulated Syntrophomonadaceae. Moreover, IP significantly facilitated HMs enrichment. Overall, this study offers new insights into the regulation of conductive materials on microbial pathways of syntrophic and hydrogenotrophic methanogenesis, with potential implications for the development of sustainable and efficient organic waste treatment strategies.