Toxic plastisphere: How the characteristics of plastic particles can affect colonization of harmful microalgae and adsorption of phycotoxins

Microplastics (MP) are suitable substrates for the colonization of harmful microalgal cells and the adsorption of their lipophilic compounds including phycotoxins. Moreover, such interactions likely change as physical-chemical characteristics of the MP surface are gradually modified during plastic degradation in aquatic environments. Using a combination of innovative laboratory experiments, this study systematically investigated, for the first time, the influence of various MP characteristics (polymeric composition, shape, size, and/or surface roughness) on its capacity to carry both living harmful algal cells and dissolved phycotoxins. Cell colonization by the dinoflagellate Prorocentrum lima started early (within 24 h) on particles of all shapes tested. However, cell colonization was much more intense on polystyrene ∼800 µm microspheres (0.63-46.4 cells mm²; mean=11.7) and 500 × 1000 μm cuboid fragments (0.64-28.3 cells mm²; mean=7.0), compared to polypropylene 11,000 × 50 µm microfibers of equivalent surface area (0.01-0.64 cells mm²; mean=0.28), which were probably too narrow and light to interact with these benthic cells. Similar to lipophilic pollutants, adsorption of the diarrhetic toxin okadaic acid (OA) was greater on smaller MP particles (50 µm), attaining up to 8.0 pg mm² after 168 h of exposure. Moreover, in the short term (24 h), OA adsorption was significantly higher on aged MP, whose surface was modified following common degradation processes (abrasion, UV-photodegradation or microbial biodegradation), relative to virgin particles. During benthic P. lima blooms, the presence of aged MP covered by toxic cells and/or their dissolved compounds are expected to make diarrhetic toxins available to a greater diversity of organisms.