Exploring the Role of Polystyrene Microplastics in Cu Binding in Sea Surface Waters: An Experimental Perspective for Future Research

The present study investigates the role of microplastics (MPs) (polystyrene (PS) microbeads) in copper (Cu) binding within the sea surface microlayer (SML) and underlying water (ULW). A mesocosm experiment was conducted, with both SML and ULW samples obtained daily, comparing mesocosms containing MPs with those free of them. The SML enrichment in dissolved Cu (Cu-D) and the Cu-complexing capacity (LT) were found to be significantly higher in the MP-treated mesocosms, with stability values of Cu-ligand complexes (logK′) being higher in the SML of MP treatments. Significant differences in Cu-D and LT between control and MP treatments were found in SML and ULW across treatments and over time. Cu-D was negatively correlated with transparent exopolymer particles (TEPs) in the ULW of both treatments, while LT was positively correlated with TEPs in the SML of MP treatments. Experimental data indicate that the co-existence of TEPs and MPs favors Cu binding with organic matter in the SML, suggesting that MPs may enhance this process. The impact of MPs on dissolved Cu complexation is probably attributed to the production of organic ligands, via enhanced TEP production, without excluding direct adsorption onto biofilm-coated MPs. The present study provides insight into the role of microplastics in Cu cycling in marine surface waters, focusing on the microenvironment of the SML.