Divergent Shifts in Sediment Dissolved Organic Matter Stability and Microbial Carbon Metabolic Potential Induced by Biodegradable and Conventional Microplastics

Abstract Biodegradable polymers are promoted as alternatives to conventional plastics, but their effects on sediment dissolved organic matter (DOM) and microbial communities remain unclear. Here, we estimated impacts of two nonbiodegradable (low-density polyethylene (LDPE) and poly(ethylene terephthalate) (PET)) and two biodegradable (polylactide (PLA) and poly(butylene succinate) (PBS)) polymers on DOM composition and microbial functional profiles using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and metagenomic sequencing. The tested nonbiodegradable polymers shifted DOM toward recalcitrance. In contrast, biodegradable polymers exhibited divergent trajectories: PLA shifted from labile at day 30 to recalcitrant by day 60, whereas PBS showed the opposite trend toward instability. Microbial communities shifted from high-abundance Deltaproteobacteria (∼12%) at day 30 to Alphaproteobacteria (∼16%) and Actinomycetia (∼15%) at day 60. Glycolysis genes were less abundant under LDPE and PET, while fatty acid β-oxidation genes were enriched under LDPE, PET, and PLA. Spearman correlation and Mantel tests revealed that MP-driven DOM changes were associated with microbial community restructuring and bacterial diversity. This study highlights that biodegradable polymers cannot be assumed to have negligible sediment ecological impacts simply because of their degradable classification.