Community and three-dimensional structures of microbial biofilms on submerged marine plastics in temperate coastal waters

To clarify the community compositions and three-dimensional (3D) structure of microbial biofilms on marine plastics, five types of plastic and glass plates were incubated in temperate coastal waters (Sagami Bay, Japan). Metabarcoding analysis showed that both prokaryotic and eukaryotic community structures on plastics and glass were largely different compared to ambient seawater, although similar communities were observed on different types of substrates except for polyethylene. Among the plastic-associated communities, Flavobacteriales (33.5–54.7%) were the most dominant prokaryotes, and microbial eukaryotes were dominated by Exogenida (suctorian ciliates) (74.5–80.3%), particularly the genus Ephelota, followed by Naviculales (pennate diatoms) (17.7–23.3%). Confocal fluorescence microscopy visualized the 3D layered structure of plastic biofilms mainly composed of bacteria and Navicula-like diatoms with the abundant and spatially heterogeneous distribution of the stalked suctorian ciliates. Bacteria and diatoms were found from the inner to outer layers of biofilms with abundant bacterial cells on the plastic surface, whereas the cell bodies of suctorian ciliates were observed exclusively in the outermost layer. Furthermore, chlorophyll autofluorescence was detected abundantly in the cell bodies of the ciliates, indicating their active grazing on diatoms. These combined taxonomic and structural results suggest the formation and developmental processes of microbial biofilms and biological interactions (synergistic and predatory relationships) on marine plastics. The present study indicates the proliferation of suctorian ciliates, which have been reported to cause serious damage to aquaculture, on plastics, implicating the potential ecological roles of marine plastic debris as a dispersal vector transporting harmful hitchhiking microorganisms.