A comprehensive pan-European exploration of fungal plastisphere dynamics along river-to-sea continuums

Microplastics (MPs) provide persistent substrates facilitating microbial transport across ecosystems. Since most marine plastic debris originates from land and are transported through rivers, a key concern is the dispersal of freshwater fungi into marine environments. Here, we characterized fungal communities colonizing MPs and immersed pristine microplastics (pMPs), as well as those in the surrounding waters, along the river-to-sea continuum in nine major European rivers during the Tara Microplastics expedition. DNA metabarcoding of the V9 region of the 18S rRNA gene showed fungi representing 4.6% of the total sequence reads and 6.2% of total richness among microeukaryotes, ranking in the top five microeukaryotic groups. Distinct fungal communities on MPs and pMPs compared to surrounding waters highlight clear differentiation between plastic-associated and ambient fungi in both marine and freshwater systems. Artificially immersed pMPs (1 month) resembled ambient water communities more than floating MPs. The latter exhibited high variability in fungal composition, suggesting complex and heterogeneous colonization patterns along the river-to-sea continuum. We also revealed a clear gradient in fungal community structure from freshwater to marine environments, with strong but incomplete segregation of plastisphere composition. Putative pathogenic fungi appeared enriched on MPs exclusively in freshwater, supporting limited transfer of these taxa toward marine-influenced environments.