Extracellular polymeric substances in green alga facilitate microplastic deposition

Microplastics (MPs) are globally ubiquitous in sediments and surface waters. Interactions between biota and MPs are complex and influence their fate and effects in the environment. Once MPs enter aquatic systems, they are colonized by biofilms that may form from the excretion of extracellular polymeric substances (EPS) from microalgae. Biofilm accumulation may change the density of the MPs, contributing to their transport to the sediments. Furthermore, benthic plantivores may consume biofilm laden MPs allowing them to enter the food web. Thus, it is crucial to understand the role algae plays in the vertical transport of MPs in the aquatic environment. In this study, Chlamydomonas was cultured with MPs at different concentrations (0-0.4 mg/mL), and temperatures ranging from 2.5 to 32.5 °C to understand the deposition dynamics and impacts of MPs on EPS production and algal density. Temperatures ranging up to 25 °C increased algal density and MPs deposition. However, at 32.5 °C, algal density and MPs deposition declined. The quantity of MPs also affected algal cell density and EPS production. MPs concentration from 0 to 0.4 mg/mL increased EPS production at all temperatures. Similarly, an increase in algal cell density and MPs deposition occurred when MPs concentration was raised to 0.3 mg/mL. Algal cultures exposed to 0.3-0.4 mg/mL of MPs had a decrease in algal cell density, with no corresponding decline in EPS production. At certain conditions, MPs can facilitate biofilm formation by stimulating EPS production, which can increase cell density thereby expediting MPs transport to the sediment.