A glimpse into Oomycota diversity in freshwater lakes and adjacent forests using a metabarcoding approach.

Oomycota, a diverse group of fungus-like protists, play key ecological roles in aquatic and terrestrial ecosystems, yet their habitat-specific diversity and distribution remain poorly understood. This study investigates the diversity of two major Oomycota classes, Saprolegniomycetes and Peronosporomycetes, in two freshwater lakes and their adjacent forests in northeastern Germany. Using a combination of targeted metabarcoding and traditional isolation techniques, we analyzed samples from six habitats, including soil (forest), rotten leaves (forest and shoreline), sediments (shoreline), and surface waters (littoral and pelagic zones). Metabarcoding revealed 401 Oomycota OTUs, with Pythium, Globisporangium, and Saprolegnia as dominant genera. Culture-based methods identified 110 strains, predominantly from surface water and sediment, with Pythium sensu lato and Saprolegnia as the most frequent taxa. Alpha and beta diversity analyses highlighted distinct community structures influenced by lake and habitat type, with significant co-occurrence of Saprolegniomycetes and Peronosporomycetes across habitats. This study provides the first comprehensive metabarcoding-based exploration of Oomycota biodiversity in interconnected freshwater and terrestrial ecotones, uncovering previously unrecognized patterns of habitat-specific diversity.