Microplastics drive the reconfiguration of microbial sulfur cycling pathways in seagrass bed sediments

Microplastics (MPs) pollution threatens marine biogeochemical cycles, but its impact on the sediment sulfur cycle remains unclear. A 112-day microcosm incubation experiment was conducted to investigate the effects of three common MPs, polylactic acid (PLA), polyethylene (PE), and polystyrene (PS), on sulfur speciation, microbial communities, and functional genes in seagrass bed sediments using integrated amplicon sequencing and metagenomics. MPs significantly altered sediment sulfur speciation, with PLA inducing the strongest shifts, including 111.2% accumulation of total inorganic sulfate (TIS) and a 163.3% increase in TIS/Sulfide ratios, indicative of enhanced sulfur oxidation, while PE and PS promoted sustained sulfide accumulation. Distinct polymer-specific changes in sulfur-cycling bacteria communities were observed, with PLA suppressing the dominant Bradymonas (31.3% decrease) while enriching heterotrophic Sulfitobacter (26.5% increase), PE driving a transition towards autotrophic pathways with Thiohalomonas increasing by 272.8%, and PS selectively enriching generalist sulfur-oxidizing genera such as Roseovarius and Methyloceanibacter . Metagenomic analysis highlighted a shift from assimilatory biosynthetic pathways to dissimilatory energy-generating processes. These findings suggest that MPs intensify sulfide stress and disrupt sulfur metabolism, thereby reducing sediment biogeochemical stability and potentially impairing carbon burial and ecosystem resilience. These results provide critical insights into the ecological consequences of MP exposure on biogeochemical cycles in seagrass bed sediments. • Biodegradable PLA caused more sulfur disruption than conventional plastics. • PLA enhanced sulfur oxidation, whereas PE and PS promoted sustained sulfide accumulation. • PLA suppressed Bradymonas by 31% while PE enhanced Thiohalomonas by 273%. • MPs reprogrammed sulfur metabolism from biosynthetic to energy-generating processes.