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 form dense blooms in freshwater ecosystems. In many cases, they are inedible to zooplanktonand prevent efficient transfer of energy and elements to higher trophic levels. Such a constraint may be alleviated by fungal parasite infections on large-sized phytoplankton taxa like diatoms and filamentous cyanobacteria, as infections may provide zooplanktonwith a complementary food source in the form of fungal zoospores. Zoosporeshave been shown to support somatic growth of large filter feeding zooplanktonspecies. Here, we tested if selectively feeding zooplankton, more specifically rotifers, also can use fungal zoosporesas a food source. Our results show that chytrid fungal parasites can indeed support population growthof rotifers(Keratella sp.). Specifically, in cultures of an inedible filamentous cyanobacterium ( Planktothrixrubescens), Keratella populations rapidly declined, while in Planktothrixcultures infected with chytrids, Keratella population growthrate equaled the growth observed for populations fed with a more suitable green algal diet ( Chlorella sorokiniana). Feeding of Keratella on zoosporeswas furthermore indicated by a reduced number of zoosporesduring the last sampling day. These findings not only imply that rotifersmay survive on zoospores, but also that the zoosporescan support high rotifer population growthrates. We thus show that fungal parasites of inedible cyanobacteria can facilitate grazers by providing them alternative food sources. Together, these results highlight the important role that parasites may play in the aquatic plankton food web.