Chytrids Are Everywhere: Diatom-Parasitic Chytrids Dominate Pelagic Fungal Communities Across 50 Reservoirs

Aquatic fungi play an important role in the material cycling, yet the factors structuring the spatial distribution of aquatic fungal communities remain poorly understood. In this study, we investigated fungal community composition across 50 reservoirs in Japan using DNA metabarcoding. Chytridiomycota (chytrids) dominated fungal communities in many reservoirs and constituted a core component of fungal assemblage. Notably, diatom-parasitic taxa were detected in all reservoirs, indicating their ubiquitous occurrence. In contrast, rare and transient components were mainly composed of Dikarya (Ascomycota and Basidiomycota), likely influenced by stochastic processes, such as episodic inputs from terrestrial habitats. Variation partitioning showed that environmental, spatial, and host-related factors each explained only a small proportion of variation in both the total fungal community and chytrids, with spatial effects being slightly stronger for total fungi, whereas environmental and host-related factors contributed relatively more to chytrid communities. Our results indicate reservoir fungal communities are characterized by a ubiquitous chytrid core and substantial unexplained variation reflecting stochastic and within-lake processes.