Significant impacts of microplastic density on the abundance, diversity and composition of the active carbon-cycling bacteria in water as identified by DNA-based stable isotope probing (DNA-SIP)

Microplastics are reported to pose a serious ecological threat, and many studies have investigated the change of aquatic microbial communities in the presence of microplastics. Nevertheless, there is lack of information about the change in microbial carbon-cycling functions induced by microplastics, which is associated with primary production. In this study, we used 13C-labelled sodium bicarbonate (NaH13CO3) as the substrate and explored the change in diversity and composition of the active carbon-cycling bacteria in the presence of different microplastics (polyethylene, polypropylene and polystyrene) by DNA stable isotope probing (DNA-SIP). Comparing to the slight change in water physicochemical variables and whole bacterial community structure, microplastics significantly altered the composition of the 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). Particularly, the relative abundance of Cyanobiaceae significantly increased, hinting at higher risks of eutrophication in microplastics contaminated water. Additionally, the diversity and composition of the active carbon-cycling bacteria and carbon cycling processes were strongly correlated with the density difference between microplastics and water; more active carbon-cycling bacteria, higher abundance of Cyanobiaceae and more up-regulation of enzymes in Calvin cycle and TCA cycle were found in treatments with lighter or heavier microplastics. Our findings for the first time evidenced the remarkable impacts of microplastics in carbon-cycling functions in freshwater ecosystem by shifting the abundance, diversity and composition of the active carbon-cycling bacteria, potentially altering carbon flux and aggravating eutrophication.