Early eukaryotic plastisphere in a boreal fjord: Identifying colonization drivers and uncovering novel parasite-diatom co-occurrence.

Marine plastic debris provide mobile surfaces for colonizing microbes, but the ecological implications of plastic biofilms, i.e. plastisphere, are not yet fully understood. This study focuses on the initial colonization dynamics of eukaryotes, an understudied but essential component of the plastisphere. We incubated pristine and weathered plastics (PE and PET), and an inert control (glass), in a boreal fjord in mid-Norway for 4-weeks per season, across two sites and over five consecutive seasons. Eukaryotic community composition was assessed by 18S rRNA metabarcoding. Multivariate analysis of community composition revealed that the environmental context (season and site) in which a plastic particle first enters the marine environment drives initial colonization dynamics. Substrate type did not affect community composition. Using generalized linear latent variable models (GLLVM), we identified individual taxa with higher occurrence probability on certain plastics compared to glass, but these were not consistent across sites and seasons, indicating context-dependent or stochastic polymer affinity. The GLLVM framework also revealed residual co-occurrence patterns (after accounting for season, site and substrate), with a strong Syndiniales-Bacillariophyceae signal emerging. Further studies are needed to determine whether this co-occurrence reflects a direct parasite-host interaction and if this potential relationship carries ecological implications, as both Syndiniales and diatoms are considered key players in marine food webs. The continuous flow of plastic debris reaching the marine environment means plastisphere communities are expanding. Understanding the ecological dynamics within these novel plastic communities is critical to fully assess the impact of plastic pollution on marine ecosystems.