Biodegradability of microplastics reshapes surface biofilm microbial community structure and nitrogen cycling...

Environmental impacts of microplastics (MPs) in aquatic ecosystems have been extensively studied, limited attention has been given to how their material types affect surface biofilm development and related nutrient cycling. This experimental study involving three types of MPs biodegradable polylactic acid (PLA), non-biodegradable polyethylene (PE), and polyvinyl chloride (PVC) revealed that the PLA surface bioflims had a higher content of chlorophyll a (Chl a), and there are significant differences in the microbial community structure among the three groups of MPs. The PLA group enriched Niveispirillum and Flavobacterium, which are involved in the nitrogen cycle, and were positively associated with increased microbial diversity and community structural shifts at day 55. In contrast, the PE and PVC groups enriched Sediminibacterium, a genus with pollutant-degradation capabilities. Analysis of nitrogen cycling genes revealed that the PLA group had consistently high levels of the nitrite reductase gene (nirS) while the PVC group showed a significant increase in the copper-containing nitrite reductase gene (nirK) during the mid-stage of the experiment. Functional prediction analysis also revealed that PLA group showed enrichment in energy metabolism pathways such as glycolysis, indicating that surface microbes preferentially utilize sugars as carbon and energy sources. In contrast, the PVC group showed higher reliance on amino acid metabolism, with enriched biosynthesis pathways of L-tryptophan and L-ornithine. The PE group had strong organic pollutant degradation, as surface microbes adapt to hydrophobic conditions by decomposing complex organics. Our results reveal that biodegradable PLA and non-biodegradable PE/PVC exert divergent effects on the development and ecological functions of surface biofilms, highlighting the key role of MP biodegradability in mediating these outcomes.