Fungal parasites of a toxic inedible cyanobacterium provide food to zooplankton

During the end of spring and throughout summer, large-sized phytoplankton taxa often proliferate and formdense blooms in freshwater ecosystems. In many cases, they are inedible to zooplankton and prevent efficient trans-fer of energy and elements to higher trophic levels. Such a constraint may be alleviated by fungal parasite infectionson large-sized phytoplankton taxa like diatoms and filamentous cyanobacteria, as infections may provide zooplank-ton with a complementary food source in the form of fungal zoospores. Zoospores have been shown to supportsomatic growth of large filter feeding zooplankton species. Here, we tested if selectively feeding zooplankton, morespecifically rotifers, also can use fungal zoospores as a food source. Our results show that chytrid fungal parasites canindeed support population growth of rotifers (Keratella sp.). Specifically,inculturesofaninediblefilamentous cyano-bacterium (Planktothrix rubescens), Keratella populations rapidly declined, while in Planktothrix cultures infected withchytrids, Keratella population growth rate equaled the growth observed for populations fed with a more suitablegreen algal diet (Chlorella sorokiniana). Feeding of Keratella on zoospores was furthermore indicated by a reducednumber of zoospores during the last sampling day. These findings not only imply that rotifers may survive onzoospores, but also that the zoospores can support high rotiferpopulationgrowthrates. We thus show that fungalparasites of inedible cyanobacteria can facilitate grazers by providing them alternative food sources. Together, theseresults highlight the important role that parasites may play in the aquatic plankton food web