Enhanced short-chain fatty acids production from food waste with magnetic biochar via anaerobic fermentation: Linking interfacial, extracellular, and intracellular electron transfer

Electron transfer is vital for microbial communities in anaerobic fermentation systems for food waste treatment. Considerable endeavors highlight the importance of interspecific electron transfer between acid-producing bacteria and methanogenic archaea. However, the contribution of electron transfer network from intracellular to extracellular in these microbes with conducive materials is unclear, and deserves more attention. This work demonstrated that magnetic biochar (as a typical conducive material) could enhance interfacial, extracellular and intracellular electron transfer networks during food waste fermentation, resulting in the improved production of short-chain fatty acids, accompanied by synchronous enhancements in solubilization, hydrolysis, and acidification steps. Further study revealed that magnetic biochar exhibited large surface area (28.5 m2/g) and high conductivity (97.6 μS/cm) with low charge transfer resistance and high catalytic activity, resulting in enriching acid-forming bacteria associated with electroactivity (e.g., Clostridium, Parabacteroides, and Fonticella). Also, magnetic biochar up-regulated the gene expressions involved in extracellular electron transfer, including membrane proteins (e.g., MtrA and MtrB) and conductive flagella (especially type IV pilus, e.g., PilA, PilB, and PilC), and intracellular electron transfer, particularly in the central pathway TCA cycle (e.g., acnA and sucA). In response to the evident stimulation of electron transfer networks (electron transport system activity increased by 204.1 % compared with the control), the critical gene expressions involved in fatty acid biosynthesis, requiring sufficient electrons and energy, were corresponding up-regulated (e.g., accA, accC, and accD), resulting in the promotion of short-chain fatty acids generation. This work would improve the in-depth understanding of electron transfer impacts on anaerobic systems and provide useful guidance for FW disposal.