Effects of microbial-colonized microplastic beads on the sea urchin Paracentrotus lividus

Plastics are widespread in the environment where they are accumulated in oceans and sediments and are fragmented by physical, chemical and biological factors into pieces millimeter/micrometer in size. They also harbour microbial communities which contribute to their transformation and represent the so-called “plastisphere”. In this work we have investigated the effects of fluorescent polystyrene microbeads colonized by microbial communities on the sea urchin Paracentrotus lividus. We have exposed animals to microplastics (MPs) incubated in natural seawater from a polluted coastal site in the Gulf of Naples. The microbial community attached to the beads was characterized by SEM and DNA sequencing analyses. SEM images revealed a thick biofilm attached to the beads and sequencing revealed in majority members of Proteobacteria and Bacteriodetes, with also Cyanobacteria and Planctomycetes. Beads-internalization was examined in different tissues of sea urchins exposed to colonized MPs compared to control animals treated with virgin MPs. The redox status of the microparticles-incorporating tissues was assessed by measuring a series of biomarkers, including the activity of catalase and the levels of the total antioxidant activity and the stress mediator, nitric oxide. In addition, a special focus was placed on the sea urchin immune cells, coelomocytes, which are considered the sentinels of environmental stress. The total number, the cell type and the oxidative stress status of coelomocytes, including measurements of reactive oxygen species, nitric oxide and antioxidant activity levels, were examined in sea urchins treated with colonized MPs compared to control animals. Overall data indicate that microbial colonization plays a significant role in the impact of polystyrene microbeads on sea urchin in terms of uptake and stress-response. These findings provide novel insights into ecological toxicity of MPs on aquatic organisms.