Microalgal colonization of microplastic substrates: results from a mesocosm experiment across biogeographical zones

A variety of organisms can colonize microplastic surfaces (or “plastisphere”) through biofouling processes. Heterotrophic bacteria tend to be the focus of plastisphere research, however, the presence of epiplastic microalgae within the biofilm has been repeatedly documented. Despite the importance of biofouling for microplastics in aquatic systems, data about this process are still scarce, especially for freshwater ecosystems. Here, our goal was to evaluate the biomass development of biofilms on plastic substrates and determine whether plastic surfaces exert a strong enough selection to drive species sorting, overcoming other niche defining factors driven by spatial patterns. We added microplastic pellets of high-density polyethylene (HDPE), polyethylene terephthalate (PET) and a mix of the two polymers in 15 mesocosms of 5 different locations of the Iberian Peninsula (totally 45 samples) and after one month we evaluated species composition and biomass of microalgae developed on plastic surfaces. Results show that all the different plastic substrates in all sites were colonized by microalgae with mean biomass of 3.05±0.40 mg cm-2, with PET having the highest value (3.63±0.83 mg cm-2). Microplastics supported the growth of many different species of microalgae (242 species), with some cosmopolite species that were able to colonize all different geographic regions and plastic surfaces. We found several species colonizing only one substrate, but with very low recurrence. Local species pools and different environmental conditions, mainly linked to biogeography, seem to prevail in driving community sorting on plastic surfaces.