Dynamic responses of the inter-microbial synergism and thermodynamic conditions attribute to the inhibition-and-relief effects of chitosan towards anaerobic digestion

Wide commercial applications of chitosan in food preservation and green packaging fields inevitably lead to the universal existence in food, as well as the food waste (FW) processing system. However, whether and how the chitosan, a class of biomacromolecule substances, lead to dysfunction of anaerobic digestion (AD) process of FW remains less understood. Herein, chitosan exhibited an inhibition-and-relief effect with the AD process proceeding, and 80 mg/g-FW of chitosan decreased the net methane yield of FW by 24.7 %. The dynamic effect was ascribed to the varied fates of chitosan and the coupling biotic/abiotic influencing on multi-steps. Chitosan enhanced substrate flocs agglomeration, restraining the release of organics to liquid phase and reducing the binding affinity to enzymes. Among the various microorganisms involved in different steps, chitosan severely inhibited aceticlastic and hydrogenotrophic methanogen at the levels of microbial abundance, activity and function. Genome-centric metagenomics analyses revealed that transient chitosan decreased the coenzyme-based synergism of various microbial taxa involved in acetic acid generation/consumption metabolisms, including syntrophic propionate-oxidizing bacteria, syntrophic butyrate-oxidizing bacteria and methanogen. With the elimination of chitosan, these inhibitions were relieved, and the accumulated acetic acid and the more favorable thermodynamic conditions finally attributed to the recovery of AD performance.