Distinct Mechanisms on Accelerating Electron Transfer to Facilitate Two-Stage Anaerobic Digestion Modulated by Various Microalgae Biochar

Microalgae-derived biochar are promising candidates to accelerate electron transfer during anaerobic digestion (AD) due to inherent advantages, but the mechanisms are unclear since they are highly related to microalgae species. In this work, distinct electron transfer mechanisms modulated by biochar derived from Scenedesmus sp. (SBC) and Chlorella sp. (CBC) were investigated during two-stage AD. Overall, adding biochar enhanced direct interspecies electron transfer (DIET) by increasing the relative abundance of related microorganisms like Firmicutes and Methanosaeta. Furthermore, SBC showed a foamy honeycomb structure with abundant functional groups, a rough surface, and irregular holes, which provided habitats for microorganism colonization and acted as an electron conductor for facilitating conductive material-mediated DIET (i.e., cDIET). Meanwhile, CBC showed a closed spherical granule structure having a smooth surface and low porosity, leading to stack of microorganisms on the biochar surface and causing bioelectrically triggered DIET (i.e., bDIET) via upregulated secretion of Flavins and C-type cytochromes. Results indicate that the electron transfer rate via bDIET was one order of magnitude higher than that via cDIET, resulting in a 53.9% increase on H-2 yield and a 9.1% increase on CH4 yield in the CBC group compared to SBC group. These findings can enrich knowledge gaps of electron transfer mechanisms modulated by microalgae biochar and may inspire more efficient AD processes.