Abstract Fungal diseases threaten natural and man‐made ecosystems. Chytridiomycota (chytrids) infect a wide host range, including phytoplankton species that form the basis of aquatic food webs and produce roughly half of Earth's oxygen. However, blooms of large or toxic phytoplankton form trophic bottlenecks, as they are inedible to zooplankton. Chytrids infecting inedible phytoplankton provide a trophic link to zooplankton by producing edible zoospores of high nutritional quality. By grazing chytrid zoospores, zooplankton may induce a trophic cascade, as a decreased zoospore density will reduce new infections. Conversely, fewer infections will not produce enough zoospores to sustain long‐term zooplankton growth and reproduction. This intricate balance between zoospore density necessary for zooplankton energetic demands (growth/survival), and the loss in new infections (and thus new zoospores) because of grazing was tested empirically. To this end, we exposed a cyanobacterial host (Planktothrix rubescens) infected by a chytrid (Rizophydium megarrhizum) to a grazer density gradient (the rotifer Keratella cf. cochlearis). Rotifers survived and reproduced on a zoospore diet, but the Keratella population growth was limited by the amount of zoospores provided by chytrid infections, resulting in a situation where zooplankton survived but were restricted in their ability to control disease in the cyanobacterial host. We subsequently developed and parameterized a dynamical food‐chain model using an allometric relationship for clearance rate to assess theoretically the potential of different‐sized zooplankton groups to restrict disease in phytoplankton hosts. Our model suggests that smaller‐sized zooplankton may have a high potential to reduce chytrid infections on inedible phytoplankton. Together, our results point out the complexity of three‐way interactions between hosts, parasites, and grazers and highlight that trophic cascades are not always sustainable and may depend on the grazer's energetic demand.

中文翻译：

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浮游动物在抑制浮游植物宿主中壶菌流行方面的潜力

摘要 真菌病害威胁着自然和人造生态系统。壶菌门（壶菌）感染的宿主范围很广，包括构成水生食物网基础并产生地球大约一半氧气的浮游植物物种。然而，大型或有毒浮游植物的大量繁殖会形成营养瓶颈，因为它们不能被浮游动物食用。感染不可食用浮游植物的壶菌通过产生高营养品质的可食用游动孢子，为浮游动物提供营养联系。通过捕食壶菌游动孢子，浮游动物可能会引发营养级联反应，因为游动孢子密度降低会减少新感染。相反，较少的感染不会产生足够的游动孢子来维持浮游动物的长期生长和繁殖。浮游动物能量需求（生长/生存）所需的游动孢子密度之间的这种错综复杂的平衡，并且通过经验测试了由于放牧而导致的新感染（以及新的游动孢子）的损失。为此，我们将被壶菌（Rizophydium megarrhizum）感染的蓝藻宿主（Planktothrix rubescens）暴露于食草密度梯度（轮虫 Keratella cf. cochlearis）。轮虫在游动孢子饮食中存活并繁殖，但喀拉特藻种群增长受到壶菌感染提供的游动孢子数量的限制，导致浮游动物存活但在蓝藻宿主中控制疾病的能力受到限制。随后，我们使用清除率的异速生长关系开发并参数化了动态食物链模型，以从理论上评估不同大小的浮游动物群在限制浮游植物宿主疾病方面的潜力。我们的模型表明，小型浮游动物可能有很大的潜力减少不可食用的浮游植物的壶菌感染。总之，我们的结果指出了宿主、寄生虫和食草动物之间三向相互作用的复杂性，并强调营养级联并不总是可持续的，可能取决于食草动物的能量需求。