The molecular transformation of microplastic-derived dissolved organic matter regulates the bioavailability of...

The release of dissolved organic matter (MP-DOM) derived from conventional microplastics (MPs) has received widespread attention. However, systematic research on the molecular characteristics of MP-DOM during the biodegradation of conventional MPs and its impact on conventional MPs bioavailability remain unclear. In this study, biodegradation incubation experiments of polyethylene microplastics (PE-MPs) were conducted at 7, 45, and 115 days to fill this gap. The results demonstrate that MP-DOM inhibits the bioavailability of PE-MPs by regulating the molecular transformation network of aromatic compounds within the plastisphere communities. Specifically, the aromatic compounds in MP-DOM are gradually converted into more easily degradable components by enriched aromatic-degrading bacteria, leading to continuous decreases in specific ultraviolet absorbance at 254 nm (SUVA₂₅₄), abundance-weighted average oxygen content (Owa), and mass-weighted average molecular weight (MWwa) of MP-DOM. Consequently, the biodegradation rate of PE-MPs significantly decreased from 0.41 μm/day to 0.15 μm/day, primarily because the transformation of MP-DOM alters the abundance and enzyme activity of plastisphere community. Notably, the relative abundance of Bacillus sp. PE-1, which has the ability to biodegrade PE-MPs, significantly declined from 7.51% to 1.07%, leading to increased recalcitrance of conventional MPs. These findings reveal a novel self-inhibitory mechanism of conventional MPs, indicating that existing models treating degradation as a linear process may seriously underestimate the environmental persistence of conventional MPs.