The impact of chytrid–phytoplankton interactions on plankton population dynamics

• Applicant: Dr. Jürgen Strassert
• Subject Area: Organismic Interactions, Chemical Ecology and Microbiomes of Plant Systems ; Ecology and Biodiversity of Animals and Ecosystems, Organismic Interactions
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 432453260

Chytrids (Chytridiomycota) are unicellular fungi for which saprotrophic as well as numerous parasitic lineages are known. Whereas some lineages parasitize other fungi, plants or animals, causing for instance global amphibian declines, most chytrid diversity and abundance is found in association with phytoplankton, including diverse groups such as diatoms, dinoflagellates or cyanobacteria. Phytoplankton-infecting chytrids are assumed to have a significant impact on the planet’s carbon cycle by terminating algal blooms and can be particularly abundant in the Arctic ocean and sea ice. They constitute a valuable food source for zooplankton and channel carbohydrates and nutrients into the food web by degrading phytoplankton cells, making them more accessible to herbivores. Previous studies found Arctic chytrids to episodically reach high abundances, but it remains unknown how those fluctuating occurrences relate to different habitats, phylogenetic and functional chytrid diversity and to the composition of the microbial community. Acidification of oceans and freshwaters may also impact chytrid infection dynamics in phytoplankton populations by altering the parasites’ virulent potential. To date, it remains elusive to which extent chytrids are able to switch between parasitic and saprotrophic lifestyles, which would enhance their functional role as phytoplankton degraders. In this project, I aim to reveal abiotic and biotic factors that shape the biogeography of Arctic chytrids and their possible interactions with phyto- and zooplankton by analysing metabarcoding and metatranscriptome datasets. The chytrids’ importance in Arctic food webs will be determined by quantifying their abundances and carbohydrate/peptide metabolising activities. I will further use single-cell transcriptomics of diverse marine and freshwater chytrid species and their eukaryotic phytoplankton hosts that exist in culture to identify parasite-specific virulence genes and possible host defence mechanisms. Additional cultivation experiments will allow me to quantify the effect of reduced pH on chytrid infection prevalence. By experimentally adding chytrid zoospores to living and heat-killed host cells, I will also investigate chytrid lifestyle determination. The results will expand our current understanding of how chytrids influence phytoplankton population dynamics, closing profound knowledge gaps about the base of Arctic marine food webs.

• DFG Programme: Research Grants