Internal enhancement mechanism of biochar with graphene structure in anaerobic digestion: The bioavailability of trace elements and potential direct interspecies electron transfer

[Display omitted] •The bioavailability of trace elements was significantly multiplied with biochar.•The gene abundance and activity of enzymes relate to trace elements’ bioavailability.•Biochar having a graphene structure was studied in the promotion of DIET.•Pseudomonas could participate in DIET with Methanosaeta through biochar.•Biochar enhanced carbon dioxide reduction of the methane metabolism pathway. The main aim of the work is to understand the internal mechanism of biochar improving anaerobic digestion (AD) performance. This study evaluated the impact of biochar produced at 400 °C and 900 °C pyrolysis on the AD performance. The results indicated that the methane content increased from an inhibited value of around 60% in the control group to the theoretical value of 75% in the reactor with biochar produced at 900 °C pyrolysis, and the specific methane production increased to 725 mL/g VS/d. The bioavailability of trace elements (TEs) including Fe, Co and Ni was enhanced with the addition of biochar. Correspondingly, enzymatic activity test and metagenomic analysis signified the abundance of the enzyme gene as well as enzyme activity increased with the enhancement of the bioavailability of TEs by biochar. According to the metagenomic analysis of the microbial community and the metabolism pathway, Pseudomonas could participate in direct interspecies electron transfer (DIET) with Methanosaeta via biochar with graphene structure as an electron contact through carbon dioxide reduction so as to heighten methane metabolism. The obtained findings may provide new insight into the internal enhancement mechanism between enzymes as well as the bioavailability of TEs by biochar.