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

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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.