Low determinism in pelagic fungal community assembly across climate zones in Scandinavian lakes

In lakes, fungi play pivotal roles in biogeochemical processes, particularly in the decomposition of organic matter. However, these organisms have been historically underrepresented and taxonomically unresolved in previous research. In this study, we employ high-resolution sequencing to delve into the fungal diversity within two distinct Scandinavian lake datasets spanning a vast latitudinal gradient, diverse climatic zones ranging from nemoral to arctic, and a wide spectrum of nutrient conditions. Utilizing eukaryotic primers, we reveal that fungi contribute, on average, 2.07% in the Norwegian and 5.4% in the Swedish sequencing dataset, with remarkable spikes of up to 55% in forested lakes. Fungal-specific sequencing identified the prevalence of several fungal phyla, including Ascomycota, Rozellomycota, Basidiomycota, Chytridiomycota, Aphelidiomycota, and Mortierellomycota. Notably, our research uncovers a striking degree of variability in fungal communities across the studied lakes, defying correlations with measured environmental factors or geographic distance. Null model analyses suggest that deterministic processes do not consistently override the influence of ecological drift in shaping these biogeographic patterns. However, signals for dispersal limitations and mass effects could be detected. These findings challenge conventional paradigms of eukaryotic communities being strongly guided by deterministic processes and underscore the complexity of fungal community assembly processes across climate zones.