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Differential effects of conventional and biodegradable microplastics on carbon and nitrogen transformation and greenhouse gas emissions in Cd-contaminated riparian sediments: Responses of microbial community structure
Summary
"Biodegradable" plastics — often marketed as the eco-friendly choice — may actually make toxic cadmium pollution worse in river sediments, according to new research. In lab experiments, these plastics made cadmium more available to living things, disrupted helpful bacteria, and boosted emissions of greenhouse gases like CO2 and methane more than regular plastics did. This suggests that the shift toward "biodegradable" plastics isn't automatically better for the environment, and could indirectly affect human health through contaminated waterways, the food chain, and worsening climate impacts.
Although microplastics (MPs) and heavy metals widely coexist in river ecosystems, their interactive effects on key biogeochemical processes, microbial community structure and function in sediments remain unclear. Therefore, this study conducted a microcosm experiment to investigate the impacts of co-exposure to four types of MPs (including two conventional and two biodegradable MPs) at different concentrations (0.3% and 3%) with cadmium (Cd) on Cd bioavailability, enzyme activities, key functional gene abundances, microbial community structure and function, and greenhouse gas emissions in sediments. Results showed that biodegradable MPs (BMPs) had stronger overall effects than conventional MPs (CMPs). Relative to the control, BMPs more significantly enhanced Cd mobility and toxicity, with the F1 fraction increasing by 11.13%-18.14%. BMPs also reduced microbial diversity and richness, accelerated sediment carbon and nitrogen turnover, enhanced the activities of related enzymes, and significantly altered the abundances of functional genes involved in carbon and nitrogen transformation. Furthermore, greenhouse gas emissions significantly increased under BMP exposure, with CO cumulative emissions rising by 29.3%-165.3%, CH by 29.3%-121.2%, and NO by 52.8%-279.7%, leading to a substantial rise in global warming potential. The significant enrichment of Bacteroidota and Pseudomonadota as core microbial taxa under co-contamination conditions reflects microbial community adaptation to combined stress. Moreover, high-concentration MPs markedly weakened microbial co-occurrence network complexity, potentially due to induced higher Cd biotoxicity. Correlation and path analyses revealed that MPs indirectly affected sediment greenhouse gas emissions through biological (microbial diversity and enzyme activities) and abiotic factors (Cd bioavailability and sediment properties). Collectively, these findings advance our understanding of the ecological processes underlying MPs and heavy metal co-contamination and are expected to provide a scientific basis for accurate risk assessment and sustainable management of sediment environments.