Biofilm facilitates metal accumulation onto microplastics in estuarine waters

In aquatic environments, plastic debris accumulates chemical pollutants from the surrounding water, potentially altering the fate of xenobiotics in these ecosystems. The effects of biofouling on the potential for plastic to sorb environmental pollutants remain poorly understood. In this study, we test the hypothesis that concentrations of metals are directly related to biofilm accumulation on microplastics submerged in natural estuarine waters. Two types of pre-production plastic pellets (polylactic acid (PLA) and low-density polyethylene (LDPE)) and glass pellets, were suspended for up to 28 days in an urbanized estuary (San Francisco Bay, California) to investigate how biofilm affects the accumulation of metals on these materials. During the initial weeks of the experiment, biofilm growth differed between locations, but after 28 days, PLA and LDPE had similar amounts of biofilm at the two field sites. Biofilm was the only significant predictor variable for Ba, Cs, Fe, Ga, Ni and Rb, and simple regressions of these metals after one month of submersion predicted much of the variability in the data (respective adjusted R² values: 0.46, 0.90, 0.86, 0.81, 0.87, 0.90; p < 0.001). For other metals influenced by location or substrate material, multivariate analysis showed that increases in metal concentrations were predicted by increases in biofilm for Cu, Pb, Al, K, U, Co, Mg (p < 0.001) and Mn (p < 0.01). This work highlights the role of biofilm in facilitating metal accumulation on plastic debris and contributes to current understanding of the underlying processes that influence the behavior of microplastics as aquatic contaminants.