Microalgal colonization of microplastics in experimental mesocosms across the Iberian peninsula

Microplastics (MPs) can be colonized by a wide range of species (“plastisphere”). Even though plastisphere research mostly focused on heterotrophic bacteria, the presence of epiplastic microalgae has been repeatedly documented. However, studies investigating the colonization of MPs by microalgae are still limited, and mainly performed in marine environments. In this study, we assessed periphyton growth and diversity on two different plastic polymers (i.e., high-density polyethylene - HDPE, polyethylene terephthalate - PET) in freshwater mesocosms distributed across five locations in the Iberian Peninsula. Our results showed that colonization occurred in a range of diverse freshwater ecosystems, since we observed biofouling of the MP surfaces in all conditions, regardless of the sites and the plastic polymer type. The amount of biomass developed on substrates differed based on the polymer type, with higher biomass developed on PET substrate compared to HDPE. We observed a rich community of microalgae on both substrates (242 species), but we did not observe species-specificity in colonization of the different plastic polymers. Indeed, local species pool rather than polymeric composition seems to be the determinant factor defining community diversity.