Biological responses to microplastic–metal co-exposure in a tropical coral: Extracellular polymeric substance restructuring and microbial community shifts

Polyethylene microplastics (PE-MPs) are increasingly recognized as environmental particles capable of interacting with chemical contaminants in marine systems, yet their combined influences on coral extracellular polymeric substances (EPS) and associated microbiomes remain insufficiently characterized. In this study, we examined the responses of the scleractinian coral Goniopora columna to PE-MP and Cu co-exposure (PE-MPs, 50 mg/L; Cu, 0-300 μg/L) during a 7-day laboratory experiment. Co-exposure was associated with increased EPS aggregation and the formation of PE-MP-Cu-EPS hetero-aggregates. Increasing Cu concentrations corresponded with more negative EPS surface charge, enhanced metal-functional group associations, and shifts in EPS-derived organic matter toward more humified fractions. These physicochemical changes coincided with restructuring of EPS-associated microbial communities, including increased relative abundance of taxa such as Methylobacter luteus and reduced representation of others such as Wukongibacter baidiensis. Overall, the results indicate that PE-MP-Cu co-exposure is associated with measurable changes in coral-derived EPS characteristics and microbial community composition under short-term laboratory conditions.