Variations in the Sporulation Efficiency of Pathogenic Freshwater Oomycetes in Relation to the Physico-Chemical Properties of Natural Waters

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.

Host Resistance and Behavior Determine Invasion Dynamics of a Detrimental Aquatic Disease

Understanding the role of variation in host resistance and the multitude of transmission modes of parasites infecting hosts with complex behavioral interactions is essential for the control of emerging diseases. We used a discrete stage model to study the invasion dynamics of crayfish plague-an example of a detrimental disease-into a naïve host population that displays within-population variation in resistance of environmental infections and juvenile classes that are safe from contacts with adults. In the model, infection sources include four age classes of crayfish, contaminated carcasses, and free-dwelling zoospores. Disease transmission occurs via environment with a threshold infection density and through contacts, cannibalism, and scavenging of disease-killed conspecifics. Even if the infection is fatal, coexistence of the host and the parasite can be facilitated by variance of resistance and survival of the hiding juveniles. The model can be applied in the control of emerging diseases especially in crayfish-like organisms.

New Insights into the Morphological Diversity of Saprolegnia parasitica (Oomycota) Strains under In Vitro Culture Conditions

Saprolegnia parasitica Coker, 1923 is a primary fish pathogen and one of the most common water molds in freshwater ecosystems. In our study, nineteen strains of S. parasitica were isolated, identified, and characterized using morphological and genetic markers. On the basis of the abundance of zoosporangia, gemmae, the formation of gemma chains, and the induction of zoospore release, three morphotypes were differentiated. A species-level molecular identification of isolates was performed using the ITS 1 and 2 regions. A total of six genotypes were distinguished based on partial DNA sequences of the genes RNA polymerase II subunit B (RPB2) and serine hydroxymethyltransferase (SHMT). In five settings of in vitro culture conditions differing in the mineral content and the temperature of water and in the presence of a host or bait, we found that the addition of fish skin extract boosted the formation of asexual reproductive and persistent vegetative structures in cultures, whereas an unfavorable environment did not support the formation of these structures in vitro.