Biodegradable microplastics influence on organic component microbial transformation during sludge composting

Sludge-based composting offers a promising pathway for sustainable resource recovery and pollution mitigation; however, the biodegradation mechanisms of biodegradable microplastics (MPs) during this process and their interactions with organic matter transformation have not yet been investigated. This study investigated the biodegradation response of polylactic acid (PLA) and the coupled dynamics of microbial communities and organic matter transformation during a 33-day sludge composting process at 55 °C and 70 °C. Scanning electron microscopy (SEM) revealed that thermophilic composting (70 °C) induced severe structural degradation of PLA-MPs, characterized by extensive void formation, surface wrinkling, and fragmentation. Compared to the mesophilic control, thermophilic composting significantly enhanced the transformation and turnover of key organic components, including amino acids (AAs), reducing sugars (RSs), polysaccharides, and polyphenols (PPs). Bacterial communities were predominantly composed of thermophilic Firmicutes (74.6–98.2%), with significant contributions from Actinobacteria, Chloroflexi, and Bacteroidetes, while fungal communities were dominated by Ascomycota and Basidiomycota. Although the addition of MPs reduced overall microbial richness and diversity, high-temperature conditions selectively enriched key organic matter-degrading taxa, suggesting a functional trade-off where thermal pressure favors specialized degradative capacity over generic community complexity. Co-occurrence network analysis revealed that high-temperature composting combined enhanced microbial functional connectivity and metabolic redundancy for both MPs and organic component transformation, promoting the proliferation of polysaccharides and lignocellulose-decomposing bacteria and fungi. These findings provide mechanistic insights into biodegradable MP degradation during thermophilic composting and establish a theoretical foundation for designing efficient MP remediation strategies in sludge treatment systems. Future studies are warranted to evaluate these findings under field-scale composting conditions and explore the integration of microbial inoculants to optimize the removal efficiency of biodegradable MPs.