Differential effects of sulfide-induced transformation of biodegradable and conventional microplastics on sedimentary CO2 and CH4 emissions: Underlying microbiome-mediated mechanisms.

The accumulation of microplastics (MPs) in sediments presents serious ecological risks. Although sulfidation is a key aging process in anoxic environments, its impact on sedimentary CO2 and CH4 emissions and underlying microbiome-mediated mechanisms remains unclear, particularly for biodegradable versus conventional MPs. Sediment incubation experiments with pristine and sulfidation-aged polyethylene (PE) and polylactic acid (PLA) revealed distinct carbon-related greenhouse gases patterns driven by material-specific microbial responses. Compared to controls, pristine PLA significantly enhanced cumulative CO2 and CH4 emissions by 4.47- and 2.59-fold, respectively, accelerating the CH4 emission peak due to its rapid carbon release. Conversely, sulfidation-aged PLA (PLA-S) reversed this trend, reducing CO2 emissions by 61.5%. This suppression was linked to an enriched microbiome (e.g., Acidobacteriota, ester-hydrolyzing Myxococcota) adapted to acidic stress, nitrogen fixation, and pathogenicity, likely diverted carbon flows. In contrast, sulfidation-aged PE (PE-S) exhibited surface oxidation, which led to a 36.7% increase in CH4 emissions, along with higher dissolved organic carbon (DOC) and microbial metabolic quotient (qCO2). This shift correlated with the enrichment of alkane-degrading Methylomirabilota and Bacillota, potentially converting plastic-derived carbon into methane. These findings emphasize the necessity of considering MPs' natural aging (e.g., sulfidation) and material types (degradable vs. conventional) when assessing their ecological risks and roles in CO2 and CH4 emissions, revealing key microbiome mechanisms linking MPs to the global carbon cycle.