Microplastic density significantly impacts the abundance, diversity, and composition of active carbon-cycling...

Microplastics pose serious ecological threats, and numerous studies have investigated the alterations in aquatic microbial communities induced by microplastics. Nevertheless, there is lack of information about microplastic-induced shift in microbial carbon-cycling functions, which is associated with primary production. In this study, we employed ¹³C-labelled sodium bicarbonate (NaH¹³CO₃) as the substrate and utilized DNA stable isotope probing (DNA-SIP) to investigate the variations in the diversity and composition of active carbon-cycling bacteria in the presence of polyethylene, polypropylene and polystyrene microplastics in river water. Comparing to the minor change in water physicochemical variables and whole bacterial community structure, microplastics significantly modified the composition of active carbon-cycling bacterial communities. The abundance of the active carbon-cycling bacteria increased from 9.40% (no microplastics) to 13.76% (polyethylene), 41.98% (polystyrene) and 62.09% (polypropylene). Notably, the relative abundance of Cyanobiaceae increased significantly, indicating a higher risk of eutrophication induced by microplastics. The diversity and composition of the active carbon-cycling bacteria were strongly correlated with the density difference between microplastics and water (Δρ); more active carbon-cycling bacteria, a higher abundance of Cyanobiaceae and more up-regulation of enzymes in Calvin cycle and TCA cycle were observed in treatments with microplastics that had a larger density difference from water. Our findings for the first time evidenced the remarkable impacts of microplastics on carbon-cycling functions in freshwater ecosystem by shifting the abundance, diversity and composition of the active carbon-cycling bacteria, potentially modifying carbon flux and aggravating eutrophication.