Beyond bacteria: the overlooked role of fungi in plastisphere carbon cycling.

Plastics introduce a new and abundant source of organic carbon into aquatic carbon cycles, fragmenting and degrading in the environment and acting as host for microbial colonization that forms the plastisphere. While bacteria and fungi are known members of plastisphere communities, the role of fungi remains poorly characterized. Fungi are universal degraders that assimilate carbon into biomass, retain it in the biological cycle and contribute to marine snow, trophic transfer and sedimentary burial. But it is still unclear which fungal taxa colonize plastics, how fungi adhere to plastic surfaces, and through which mechanisms fungi degrade polymers to drive the mobilization of plastic-derived carbon within aquatic ecosystems. In coastal environments, which serve as the entry point of land-based plastics to the sea, fungal communities may differ across salinity gradients, habitats (water column vs sediments), or plastic types. Plastics become entrapped in coastal and marine plants, but it is unclear whether rooting zones act as hotspots of microbial activity that foster degradation. To address these questions, polyethylene (PE) and polylactic acid (PLA) plastics were deployed for two months in a coastal estuary of western Denmark across freshwater, brackish, and marine sites. Plastics were placed in the water column, sediment, and rhizosphere across sites. In total, about 900 fungal isolates were obtained from the exposed plastics, and an enrichment experiment yielded an additional 250 strains. The fungi were identified by Sanger sequencing, while metagenomics provided community-level profiles. SEM imaging documented fungal colonization and surface alteration. Current and future work includes NanoSIMS approaches to trace carbon assimilation and transport in cultures. This work reveals the remarkable diversity of plastisphere fungi and will track their potential to mediate carbon transformation and recycling in coastal ecosystems.