(Re)discovering the diversity of chytrid parasites of phytoplankton

Fungal parasites of phytoplankton are ubiquitous and constitute an integral component of aquatic ecosystems. Despite the growing evidence that they have profound effects on ecosystem, functioning via top-down control of phytoplankton blooms and by providing alternative nutrient flows, they remain largely understudied. This is mainly because they are difficult to identify and as a consequence frequently overseen. Recently, environmental DNA surveys have uncovered a large amount of “dark matter” aquatic fungal diversity, with fungal sequences of phylum Chytridiomycota (hereafter referred to as chytrids) often dominating the fungal community in the pelagic zone of lakes. These findings support the results of more than a century of microscopy-based studies on aquatic chytrids and thus a substantial part of these “dark matter fungi” certainly belong to phytoplankton parasitic fungi. Up to date they remain, however, largely invisible for the molecular ecologist because so far, only a tiny proportion of validly described phytoplankton associated fungi is represented in molecular databases. This severely limits the use of environmental DNA and other molecular approaches to study their biogeography, ecological interactions and host specificity. In this talk, I will discuss methodologies to identify, isolate and cultivate parasitic chytrids. I will further demonstrate how a targeted approach, i.e. cultivation, single cell isolation, in situ baiting approach, can illuminate the ecological role of a substantial part of “dark matter” chytrid diversity in the pelagic zone, using oligo-mesotrophic lake Stechlin (Germany) as a case study.