The potential of zooplankton in constraining chytrid epidemics in phytoplankton hosts.

Abstract Fungal diseases threathen natural and man-made ecosystems. Chytridiomycota(chytrids) infect a wide host range, including phytoplanktonspecies that form the basis of aquatic food webs and produce roughly half of Earth's oxygen. However, blooms of large or toxic phytoplanktonform trophic bottlenecks as they are inedible to zooplankton. Chytrids infecting inedible phytoplanktonprovide a trophic link to zooplanktonby producing edible zoosporesof high nutritional quality. By grazing chytrid zoospores, zooplanktonmay induce a trophic cascadeas a decreased zoosporedensity will reduce new infections. Conversely, fewer infections will not produce enough zoosporesto sustain long-term zooplanktongrowth and reproduction. This intricate balance between zoosporedensity necessary for zooplanktonenergetic demands (growth/survival), and the loss in new infections (and thus new zoospores) due to grazing was tested empirically. To this end, we exposed a cyanobacterial host ( Planktothrixrubescens) infected by a chytrid (Rizophydium megarrhizum) to a grazer density gradient (the rotiferKeratella cf. cochlearis). Rotiferssurvived and reproduced on a zoosporediet, but the Keratella population growth was limited by the amount of zoosporesprovided by chytrid infections, resulting in a situation where zooplanktonsurvived but was restricted in their ability to control disease in the cyanobacterial host. We subesequently developed and parameterized a dynamical food-chain model using an allometric relationship for clearance rateto theoretically assess the potential of different-sized zooplanktongroups to restrict disease in phytoplanktonhosts. Our model suggests that smaller-sized zooplanktonmay have a high potential to reduce chytrid infections on inedible phytoplankton. Together, our results point out the complexity of tri-way interactions between hosts-parasites-grazers, and highlight that trophic cascadesare not always sustainable and may depend on the grazer's energetic demand.