Eukaryotic phototrophs of the plastisphere in the Southeastern Black Sea

Plastics, as artificial substrates in water, are rapidly colonized by microorganisms, forming the plastisphere. Eukaryotic phototrophs of the plastisphere were studied on 46 seafloor plastic samples, representing 11 polymer types, in the Southeast Black Sea during March and July 2022. The plastisphere included 55 species from Heterokontophyta phylum (incl. Coscinodiscophycea (4), Mediophyceae (3), Bacillariophyceae (45), Dictyophyceae (1)), and Haptophyta (1). Common species were Bacillaria paxillifera, Tabularia parva, Cocconeis scutellum, and Licmophora flabellata. Octactis speculum and Emiliana huxleyi were newly reported for the plastisphere, and Thalassiosira curviseriata was recorded for the first time in the Black Sea. Two potentially harmful algae and five bloom-forming species were identified. Shannon Diversity Index values ranged from 0 to 2.9, with the highest diversity on PE/PVA and PES samples and the lowest on PE. Habitat preferences showed 34 species associated with plankton-benthic or benthic microalgae. PERMANOVA analysis revealed that polymer type explained 11.9 %-15.8 % of species composition variation. Transparent and black plastics hosted the most species, while multicolor plastics had the fewest. This study highlights that diverse microbial communities, particularly diatoms, colonize plastics in the Black Sea. Plastics can transport harmful species, posing ecological risks by disrupting benthic ecosystems, nutrient cycles, and ecosystem services like carbon storage and sediment stabilization. Our findings contribute understanding of the ecological and economic impacts of plastic debris at the seafloor.