Polylactic acid microplastics facilitate nitrogen removal in freshwater sediments by modulating carbon-nitrogen coupling

It is widely recognized that biodegradable microplastics (BMPs) can enhance denitrification in freshwater sediments by releasing dissolved organic matter (DOM) as carbon sources for denitrifying microorganisms. However, it was largely unclear how the BMPs mitigated the denitrification process. Herein, a 45-day incubation was first conducted to evaluate the effects of polylactic acid microplastics (PLA-MPs) on nitrogen transformations in sediments. Results demonstrated that 0.05 % and 0.5 % (w/w) PLA-MPs significantly enhanced denitrification, thereby promoting nitrogen removal. To further identify the underlying mechanism, a subsequent 10-day anaerobic incubation was performed with 0.5 % PLA-MPs and 0, 1, 5, and 10 mg/L NO-N as the denitrification substrate. In the presence of PLA-MPs, total nitrogen removal was significantly increased, accompanied by increased abundances of denitrifying bacteria and functional genes encoding nitrate/nitric oxide reductase. Moreover, the abundance of the ANME-2d mcrA gene representing the nitrate-dependent denitrifying anaerobic methane oxidation was also increased. These findings indicate that PLA-MPs degradation, supported by the upregulation in esterase and lactate dehydrogenase, as well as the presence of polylactic acid oligomer, produces low-molecular-weight labile DOM and CH to serve as electron donors for nitrate reduction. Overall, the results demonstrated that the coupling between PLA-MPs degradation and multiple heterotrophic denitrification pathways facilitated nitrogen removal and shed light on understanding the role of BMPs in the biogeochemical cycling of elements.