Impact of aged and virgin microplastics on sedimentary nitrogen cycling and microbial ecosystems in estuaries

Microplastics (MPs) entering the environment undergo complex weathering (aging) processes, however, the impacts of aged MPs on estuarine nitrogen cycling and microbial ecosystems remain largely unknown. In this study, a 50 days microcosm experiment was conducted to investigate the response of sedimentary nitrogen (N) transformation processes, N₂O emission and microbial communities to virgin and aged MPs (PE and PS) exposure. We found that aged MPs influenced sediment nitrogen turnover more rapidly and profoundly than virgin MPs and showed type and dose-response effect. During the first 10 days, higher concentration (3 % by weight of sediment) aged MPs (both PS and PE) treatments significantly promoted denitrification (ANOVA, P < 0.05), while virgin MPs treatments had weak effect on denitrification, compared with the control (P > 0.05). Moreover, higher concentration aged PS-MPs remarkably enhanced N₂O emission on the 10th day, while N₂O was consumed in the control. After 50 days incubation, there was an overall increase in nirK gene abundance exposed to MPs, and nosZ gene copies in aged PS treatments were around twice that in the control based on qPCR (P < 0.05). The function prediction also showed significant elevation of relative abundance of denitrification and DNRA relevant genes in bacterial community. In addition, aged PS treatment (3 %) recruited specific bacterial and archaeal assemblies, with Sedimenticolaceae, Lentimicrobiaceae, SCGC_AAA011-D5, SG8-5, Lokiarchaeia, and Odinarchaeia selectively enriched in the treatment. Our study highlighted that virgin and aged MPs had different impact on sediment nitrogen cycling, and the ecological risks of aged MPs should be concerned since all MPs eventually get weathered when they enter the environment.