Microbial parasites have only recently been included in planktonic food web studies, but their functional role in conveying dietary energy still remains to be elucidated. Parasitic fungi (chytrids) infecting phytoplankton may constitute an alternative trophic link and promote organic matter transfer through the production of dissemination zoospores. Particularly, during proliferation of inedible or toxic algal species, such as large Cyanobacteria fostered by global warming, parasites can constitute an alternative trophic link providing essential dietary nutrients that support somatic growth and reproduction of consumers. Using phytoplankton-parasites associations grown under laboratory controlled conditions we assessed the fatty acids and biochemical composition of species with different nutritional quality and followed the metabolic pathway from the algal host and their parasites zoospores using compound-specific stable isotope analysis. This study demonstrated that chytrids are trophic upgraders able to retain essential nutrients that can be transferred to upper trophic levels both in terms of organic matter quantity and nutritional quality. Through the production of zoospores, nutritionally important long-chain polyunsaturated fatty acids that can be consequently assimilated by consumers. We conclude that parasitism at the base of aquatic food webs may represent a crucial trophic link for dietary nutrients and essential biomolecules alternative to herbivory or bacterivory, which can be particularly crucial during the proliferation of inedible or nutritionally inadequate algal species fostered by climate change.

微生物寄生虫直到最近才被包括在浮游食物网研究中，但是它们在传递膳食能量中的功能性作用尚待阐明。感染浮游植物的寄生真菌（chytrids）可能构成替代性的营养联系，并通过产生传播的游动孢子来促进有机物的转移。特别是在不可食或有毒的藻类繁殖（例如因全球变暖导致的大型蓝细菌）繁殖期间，寄生虫可构成替代的营养纽带，提供必需的饮食营养，以支持消费者的身体生长和繁殖。使用在实验室控制条件下生长的浮游植物-寄生虫协会，我们评估了具有不同营养质量的物种的脂肪酸和生化组成，并使用化合物特异性稳定同位素分析跟踪了藻类宿主及其寄生虫游动孢子的代谢途径。这项研究表明，糜藻是营养的改良剂，能够保留必需的营养素，无论从有机质含量还是营养质量上来说，营养素都可以转移到营养较高的水平。通过产生游动孢子，营养上重要的长链多不饱和脂肪酸可被消费者吸收。