Coupling of sulfate reduction and dissolved organic carbon degradation accelerated by microplastics in blue carbon ecosystems

Microplastics have increasingly accumulated in sulfate- and organic matter-rich mangrove ecosystems, yet their effects on microbially mediated carbon and sulfur cycling in sediments remains poorly understood. In this study, we performed a 70-day anaerobic microcosm experiment to examine the effects of polylactic acid (PLA) microplastics with different sizes on sulfate reduction and dissolved organic carbon (DOC) degradation in mangrove sediments. Our results demonstrated that millimeter-scale PLA (mm-PLA) more effectively enhanced sulfate reduction, sulfur isotope fractionation, reduced sulfide production, and carbon dioxide (CO₂) emission compared to micrometer-scale PLA (m-PLA). These results suggested that mm-PLA had a more pronounced impact on the carbon and sulfur cycles. Integrated 16S rRNA gene amplicon sequencing and metagenomic analyses revealed that mm-PLA preferentially enriched key functional microorganisms, including acetate-producing bacteria (e.g., Acetobacteroides), completely oxidizing sulfate-reducing bacteria (e.g., Desulfobacter), and incompletely oxidizing sulfate-reducing bacteria (e.g., Desulfobulbus). These microorganisms exhibited higher abundances and greater genetic potential for carbon metabolism and sulfate reduction under mm-PLA treatment. Their relative abundances showed positive correlations with sulfate reduction rates, sulfur isotope fractionation, and CO₂ emission, identifying them as crucial drivers of coupled carbon-sulfur cycling. Furthermore, the synergistic interactions among Acetobacteroides, Desulfobacter, and Desulfobulbus facilitated the oxidation of sediment-derived DOC, highlighting significant implications for carbon sequestration in blue carbon ecosystems.