Double-edged sword effects of platinum-coated cathodes on methane production pathways in microbial electrolysis cell-coupled anaerobic digestion systems

Although platinum-coated cathodes (Pt-C) are commonly applied in microbial electrolysis cell-anaerobic digestion (MEC-AD) systems for methanogenesis, they exhibit inherent biotoxicity. At 0.8 V, Pt-C achieved the highest methane yield (445.71 mL/gCOD), but overall enhancement remained limited. This treatment enhanced inter-biofilm electron transfer processes and electrolyte transport, particularly at cathodes, thereby stimulating quorum sensing, proteolysis, amino acid/peptide transport, and acidification of substrates. It promoted the proliferation of cathodic unclassified_f_Methanobacteriaceae and Geobacteraceae, upregulated hmd, acs and coo, which drove hydrogenotrophic methanogenesis, acetoclastic methanogenesis, and syntrophic acetate oxidation via direct interspecies electron transfer. Conversely, it inhibited formate and methylotrophic methanogenesis pathways and associated genes. Notably, methylotrophic methanogens, despite low abundance, had a disproportionately significant role in methane production. These findings underscore Pt's "double-edged sword" effect, revealing its complex, selective influence on microbial metabolism and methanogenic pathways in MEC-AD systems, providing novel mechanistic insights into optimizing CH₄ production.