Pseudocapacitive conductive materials drive selective shifts in direct versus mediated electron transfer pathways...

Despite widespread application of conductive materials to enhance anaerobic digestion (AD), the dosage-dependent regulation of interspecies electron transfer pathways and its consequences for methane production remain poorly understood. Here, tannic acid-modified iron biochar (Fe-TA-C) was used to elucidate how dosage regulates electron transfer pathways in AD. Methane yield was maximized at 100 mg/g VS of Fe-TA-C, achieving a 24.1% increase over the control. At low to moderate dosages (50-100 mg/g VS), Fe-TA-C promoted direct interspecies electron transfer (DIET) through synergistic Fe(II)/Fe(III) redox cycling and intrinsic material conductivity. This was supported by upregulated pilA and ccdA, increased cytochrome c and conductive pili, and enrichment of the DIET-associated methanogen Methanothrix. In contrast, higher Fe-TA-C dosages induced a shift from DIET to mediated interspecies electron transfer (MIET). This transition was driven by the pronounced pseudocapacitive behavior of Fe-TA-C (324 mF/g), whereby quinone moieties and iron redox sites stored and released electrons with delay, weakening direct electron transfer. Concurrently, accumulation of quinone-type groups expanded endogenous quinone pools, reinforcing MIET pathways. This shift was evidenced by distinct redox signals (-0.38 V), upregulation of quinone biosynthesis genes (menA, ubiE), enrichment of MIET-related microbes, and significant inhibition by dicumarol. These findings reveal that Fe-TA-C dosage governs a pseudocapacitance-driven transition between DIET and MIET, providing a mechanistic basis for optimizing AD via tailored electron transfer strategies.