Biogas production enhancement from anaerobic digestion with magnetic biochar: Insights into the functional microbes and DIET

The application of magnetic biochar in anaerobic digestion (AD) has gained increasing attention. However, the underlying mechanisms remain insufficiently understood. This study systematically investigated the effects of magnetic biochar on functional microbial communities involved in methanogenesis and elucidated its role in promoting direct interspecies electron transfer (DIET) within AD systems. The addition of 40 mg g^-1 TS_added of magnetic biochar significantly enhanced methane production by 42.21 %, reaching 223.08 mL g^-1 TS with highest organic matter degradation efficiency. Microbial community analysis showed that magnetic biochar significantly enriched microorganisms associated with hydrolysis, acidogenesis, and methanogenesis, as well as electroactive microorganisms' abundance such as Geobacter spp., Syntrophus spp., P. aestuarii, and M. harundinacea, providing direct evidence for the DIET process of AD with magnetic biochar. Furthermore, the abundance of key genes involved in the DIET, including pilA, Fpo, and the genes encoded outer-membrane c-type cytochromes, was respectively upregulated by 44.49 %, 22.04 %, and 37.6 % in the presence of magnetic biochar. These findings suggest that magnetic biochar enhances the production of conductive pili and cytochrome c, facilitating extracellular electron transfer between syntrophic microorganisms. This accelerated electron transfer promotes CO₂ reduction to CH₄, ultimately improving methane production efficiency in the AD system. Moreover, the enhancement of hydrogenotrophic methanogenesis was particularly pronounced with magnetic biochar, further contributing to the improved AD performance. This study provides novel mechanistic insights into biochar-mediated DIET, offering a theoretical basis for optimizing biochar applications in AD.