Biotrophic parasitoid infections of Ulnaria as a model for host–pathogen dynamics and interactions

Abstract Diatoms are responsible for at least 30% of the global net photosynthesis, but their interactions with pathogens are much less understood as compared to terrestrial plants. As a first step towards addressing this knowledge gap, Ulnaria ulna was extensively sampled over the past 4 years from Iceland. Interestingly, three different obligate biotrophic parasitoids were found, which varied in their presence both spatially and temporally. While in the natural setting, oomycete pathogens dominated in the samples, they were regularly outperformed by a chytrid species when incubating samples in climate chambers, suggesting that the restricted space in Petri dishes, in combination with a lack of water turbulence, created an environment favouring the species. This might be in line with their asexual reproduction strategies, as the chytrid produces large amounts of rather weakly swimming zoospores, while an oomycete pathogen of the genus Miracula produces fewer zoospores that swim vigorously after hatching from cysts, and another oomycete pathogen of the genus Lagena produces comparatively few zoospores that are formidable swimmers. Apart from observational data, microscopic pictures and phylogenetic reconstructions are given for all three species encountered, and the chytrid species is introduced as Jonasia blauvikensis , a new species and genus in Halomycetaceae . For all three pathogens, host/pathogen co-cultures have been established as a basis for future research on the interactions of diatoms with obligate parasitoids.