Promoting lignin exploitability in compost: A cooperative microbial depolymerization mechanism

Lignin, as the second largest biomass resource in the biomass plant, has the potential to be converted into other organic matter. Microbial mediated biocatalytic process in compost can depolymerize and utilize lignin environmentally. However, the source of lignin is complex and difficult to depolymerize, which limits the composting efficiency and reduces the humus quality. The purpose of the study was to find a breakthrough point by analyzing the difficulties of lignin depolymerization and reveal the potential contribution of lignin depolymerization to humus formation during composting. In this paper, the composition characteristics of lignin and the microbial action of lignin depolymerization were reviewed, including the microorganisms involved in lignin depolymerization, lignin depolymerases and microbial pathways, the potential pathway of lignin to form humus, and the environmental factors affecting lignin depolymerization during composting. Although the depolymerization ability of fungi on lignin has been studied for decades, the strict growth conditions of fungi limit their function during composting. Bacteria not only have good tolerance to environmental changes but also play an important role in the depolymerization of lignin. The results showed that the lignin depolymerization during composting was completed under the synergistic action of fungi and bacteria. Fungi deoxidized lignin β-O-4 and other bonding bonds by secreting depolymerases at low temperature, so as to depolymerize large molecules of lignin into oligomers and monomers, while bacteria degraded small molecules of lignin into nutrients that could be used by microorganisms at low temperature. At high temperature, bacteria can promote lignin depolymerization by secreting high temperature resistant depolymerases. Therefore, the study provides a theoretical basis for microbial cooperative depolymerization of lignin and formation of humus during composting. At the same time, it is necessary to add microbial agents and control the environment in different stages of compost to efficiently depolymerize lignin.