Size-fractionated fungal communities in the sunlit ocean

Marine fungi play key roles in organic matter cycling, yet their distribution across particle size fractions remains understudied. We analyze 18S rDNA data from four size fractions (0.8-5, 5-20, 20-180, and 180-2000 μm) collected across the global sunlit ocean. Here, we show fungal diversity and relative abundance decline with increasing particle size. Fungal community structure is influenced by eukaryotic diversity and chlorophyll levels. Fungi co-occur with other eukaryotes, especially zooplankton, hinting at potential predator-prey interactions. Generalist fungi dominate smaller fractions, while specialists dominate larger fractions, likely due to stronger microenvironmental selection. Co-occurrence networks are dominated by positive interactions and driven by fungal specialists. Dispersal limitation emerges as the main ecological process shaping community assembly. Our findings reveal strong niche differentiation among marine fungi along the particle continuum and emphasize the role of particle size and biological interactions in structuring fungal diversity and biogeography.