Integrating effects of chytrid parasitism into carbon flows in pelagic food webs

在我们对食物网的理解中，寄生实际上仍然被忽视了。最近的证据表明，寄生虫的感染可以建立替代的营养联系，并影响食物网络上的碳转移。在水生生态系统中，可食性差或有毒的浮游植物在水生生态系统中的优势导致营养瓶颈，传统上认为这会扰乱大规模的碳流。有人建议通过真菌寄生虫感染浮游植物来绕过这种瓶颈，方法是重新包装从藻类中无法获得的碳，并将其作为可食用的游动孢子提供给浮游动物消费者，建立一个被称为霉菌的营养循环。除了直接转移碳，新的证据表明，壶菌感染间接影响生态系统中的其他生物成分，这可能反过来改变营养相互作用。因此，龟裂寄生虫的感染可能会以其他方式调节水生食物网和碳转移，而不是简单地通过菌丝体输送碳。相反，在壶菌感染下观察到的不可食用浮游植物大小的减少使浮游植物更容易被浮游动物食用，因此，壶菌流行病可能会通过食草动物食物链加强碳向消费者的转移。此外，在感染条件下观察到的异养菌生物量的大幅增加，消费者可以利用，这导致了一种假设，即chytrid感染进一步促进了通过微生物环向消费者的碳转移。通过使用由菊苣-蓝藻宿主-寄生虫系统、浮游动物消费者和异养细菌群落组成的人工组装的食物网，该项目将旨在经验性地测试菊苣寄生对碳通量的这种间接影响。实验中的食物网将被操纵，以排除、调节或组合单个食物网组件，并比较感染存在和不存在的情况。将进行实验，通过测试浮游动物对受感染和未受感染浮游植物的捕食能力的差异，来解开寄生虫对食草动物食物链的影响，通过量化其对营养转移的贡献，并描述其与感染严重程度的关系，来解开寄生虫对食草动物食物链的影响，以及iii)微生物环，通过研究其假定的被chytrid感染的增强，以及其与小的菌丝体贡献相结合的潜在协同作用。最后，在该项目的实验部分产生的数据将被用来生成一个模型，该模型首次综合了壶菌感染对个别食物链的直接和间接影响。这将改善我们对远洋食物网底部营养转移的预测，以及寄生在水生生态系统大规模碳循环中的影响。

Parasitism remains virtually neglected in our understanding of food webs. Recent evidence indicates that infection by parasites can establish alternative trophic links and affect carbon transfer up the food web. In aquatic ecosystems, the dominance of poorly edible or toxic phytoplankton in aquatic ecosystems leads trophic bottlenecks which are traditionally considered to disrupt large-scale carbon flows. Infection of phytoplankton by fungal parasites has been proposed to circumvent such bottlenecks by repacking inaccessible carbon from algae and rendering it available to zooplankton consumers as edible zoospores, establishing a trophic loop, called the mycoloop. In addition to directly transferring carbon, new evidence indicates that chytrid infection indirectly affects other biotic components in the ecosystem, which may in turn alter trophic interactions. Infection by chytrid parasites may thus modulate aquatic food webs and carbon transfer in other ways than simply conveying carbon through the mycoloop. Instead, reductions in inedible phytoplankton size observed under chytrid infection render phytoplankton more edible to zooplankton and, thereby, chytrid epidemics may enhance carbon transfer to consumers through the herbivore food chain. In addition, substantial increases in biomass of heterotrophic bacteria observed under infection conditions, which can be utilized by consumers, leads to the hypothesis that chytrid infection further facilitates carbon transfer to consumers through the microbial loop. By using an artificially-assembled food web consisting of a chytrid-cyanobacterium host-parasite system, a zooplankton consumer and a heterotrophic bacterial community, this project will aim at empirically testing such indirect effects of chytrid parasitism on carbon fluxes. The experimental food web will be manipulated to exclude, modulate, or combine individual food web components and compare scenarios of presence and absence of infection. Experiments will be conducted to disentangle parasite-mediated effects on: i) the herbivore food chain, by testing for differences in the ability of zooplankton to graze on infected and uninfected phytoplankton, ii) the mycoloop, by quantifying its contribution to trophic transfer and describing its relation with changing infection severity, and iii) the microbial loop, by investigating its putative enhancement by chytrid infection and its potential synergistic effects in combination with small mycoloop contributions. Lastly, the data produced in the experimental part of this project will be used to generate a model that, for the first time, integrates both direct and indirect effects of chytrid infection on individual food chains. This will improve our predictions of trophic transfer in the base of pelagic food webs and the impact of parasitism in large-scale carbon cycles in aquatic ecosystems.