Low diversity and potentially harmful taxa of microbial eukaryotes in chronically polluted marine sediments: the...

Marine coastal areas under high anthropogenic pressures typically display high chemical contamination, which can impact benthic ecosystems at different levels of biological organization, in turn affecting the overall ecosystem functioning. Microbial eukaryotes are key components of the marine food webs and crucial for ecosystem functions, yet their diversity in relation to chemical contamination is still largely unknown. Here, we analyzed by 18S rRNA gene metabarcoding the microbial eukaryotes in marine sediments collected in one of the most polluted marine sites in Europe (Bagnoli-Coroglio, Mediterranean Sea), facing both historical industrial activities and urban sewage discharges. All investigated benthic sites displayed very low diversity of microbial eukaryotes. Sites with the highest levels of chemical hazard also showed overrepresentation of bloom-forming microalgae and potentially harmful microbial eukaryotes, including parasitic or pathogenic protists. Some of these taxa may be either of terrestrial or marine origin, yet they indicate that chronic pollution can shape benthic microbial eukaryotic assemblages, potentially producing impacts that go beyond those directly attributable to chemical contamination and can last for long time scales. These findings should be carefully considered to support a more holistic assessment of the ecological risks in contaminated coastal areas and for defining adequate monitoring and environmental remediation and restoration actions.