Material Conversion, Microbial Community Composition, and Metabolic Functional Succession During Algal Sludge Composting

Although bacterial and fungal communities play essential roles in organic matter degradation and humification during composting, their composition, interactions, abiotic compost properties, and succession patterns remain unclear. In this study, the succession of bacterial and fungal communities during algal sludge composting was explored using 16S and ITS rRNA amplicon sequencing. The compost rapidly entered the thermophilic phase (>50 °C) within the first phase. During the composting process, the diversity of bacterial and fungal communities did not show a significant response to the different composting phases. The physicochemical parameters and microbial community structures changed significantly during the thermophilic and cooling phases, particularly in the former, and gradually stabilized as the compost matured. Integrated random forest and network analyses suggested that the bacteria genera Geobacillus and Parapedobacter, along with the fungus genus Gilmaniella, could serve as potential biomarkers for different composting phases. The functional activity of the bacterial communities was obviously higher during the thermophilic phase than during the other phases, while fungal activity remained relatively high during both the thermophilic and cooling phases. Structural Equation Modeling (SEM) further indicated that bacterial communities primarily mediated nitrogen transformation and humification processes, while fungal communities mainly contributed to humification. These results cast a new light on understanding about microbial function during aerobic algal sludge composting.