Comparison of sterol and fatty acid profiles of chytrids and their hosts reveals trophic upgrading of nutritionally inadequate phytoplankton by fungal parasites

Chytrids are ubiquitous fungal parasites in aquatic ecosystems, infecting representatives of all major phytoplankton groups. They repack carbon from inedible phytoplankton hosts into easily ingested chytrid propagules (zoospores), rendering this carbon accessible to zooplankton. Grazing on zoospores may circumvent bottlenecks in carbon transfer imposed by the dominance of inedible or poorly nutritious phytoplankton (mycoloop). We explored qualitative aspects of the mycoloop by analysing lipid profiles (fatty acids, sterols) of two chytrids infecting two major bloom‐forming phytoplankton taxa of contrasting nutritional value: the diatom Asterionella formosa and the filamentous cyanobacterium Planktothrix agardhii. The polyunsaturated fatty acid composition of chytrids largely reflected that of their hosts, highlighting their role as conveyors of otherwise inaccessible essential lipids to higher trophic levels. We also showed that chytrids are capable of synthesizing sterols, thus providing a source of these essential nutrients for grazers even when sterols are absent in their phytoplankton hosts. Our findings reveal novel qualitative facets of the mycoloop, showing that parasitic chytrids, in addition to making carbon and essential lipids available from inedible sources, also upgrade their host's biochemical composition by producing sterols de novo, thereby enhancing carbon and energy fluxes in aquatic food webs