Microalgal colonization of microplastics in experimental mesocosms across a biogeographical gradient

Microplastics (MPs) constitute suitable substrates for the formation of biofilms. Since local conditions and environmental factors influence the community composition of biofilm, biogeography plays an important role in the composition of MP-colonizing communities. However, there is still controversy as to whether substrate-specific properties or environmental factors prevail in determining microalgal assemblages on plastic debris. In this study, we assessed periphyton growth on two different plastic polymers in freshwater mesocosms distributed across five locations covering a variety of bioclimatic regions in the Iberian Peninsula. In each location, pellets of high-density polyethylene, polyethylene terephthalate or a mix of the two polymers were deployed inside 2L transparent bottles suspended from surface rafts inside three different 1000L mesocosms. We measured water temperature, pH, conductivity, chlorophyll-a and nutrients in each mesocosm. After one month, MPs were collected and biomass and algal species composition were assessed. Microalgae colonized all the different plastic substrates in all sites with a mean abundance of 288±36 µg cm-2, with the highest biomass reported for PET (329±70 µg cm-2). Significant differences in the biomass development for the different polymers were highlighted for diatoms, which represent on average one of the taxa with higher biomass (29% of total biomass). Our results suggest that different plastic polymers may determine different colonizing conditions for specific taxa and may affect freshwater primary productivity.