Role of phytoplankton parasites in trophic transfer

Microparasites are overlooked in pelagic environments as most studies have so far been limited to infectious diseases in macro-organisms and in particular on plants or animals of economical interest, such as fishes and shellfish. Recent molecular surveys highlight a wide variety of eukaryotic parasites in the picoplanktonic size fraction, mainly recognized as chytrid fungi. Chytrids are adapted to pelagic life as they are characterized by a complex life cycle consisting of an infective phase attached to the host from which are produced free swimming zoospores that are released into the environment for new host hunting. By suppressing phytoplankton growth, parasitic chytrids can cause a decrease in primary production. In contrast, by feeding on its algal host, chytrids can convert algal biomass into edible, nutrient-rich spores that are efficiently grazed by zooplankton. The current proliferation of harmful and toxic algal species worldwide can constitute a trophic dead end for consumers due to their inadequate quality and potential toxicity. Parasitism can be fostered by such high host abundance and during inedible or nutritionally inadequate algal blooms parasites can constitute an alternative promoting trophic transfer within the planktonic food web, both in terms of organic matter quantity and nutritional quality. Considering that parasites are not only infectious agents, but also occupy various niches in the plankton with more functional and ecological characteristics than previously thought, this research aims to shed light on phytoplankton fungal parasite (chytrids) interactions within the planktonic food web and their role in ecosystem functioning. By combining laboratory experiments with artificial food webs and mathematical modeling, we will investigate how chytrids drive the transfer of the organic matter through the dispersion of their infectious propagules (i.e. spores). The final outcome will be a food web network model including the phytoplankton parasites allowing following trophic trajectories through parasites. We will also calculate ecological indices quantifying the direct and indirect effects of phytoplankton parasites on ecosystem processes such as organic matter recycling and trophic transfer, which are considered to be indicators of stability and resilience of food webs in the context of current global change scenarios.

This research could demonstrate, for the first time, that chytrids, kingdom fungi, ameliorate dietary energy for zooplankton because they can, in contrast to their host (algae), synthesize polyunsaturated fatty acids ('trophic upgrading') that consequently increase somatic growth and reproduction of zooplankton. This research clearly showed that chytrids, as intermediate trophic links, make energy from otherwise inedible (size constraints) and nutritionally inapt diets (e.g., cyanobacteria) available for metazoan consumers and help them increase their fitness. In this research project, we could also make methodological advancements within the field of trophic ecology via the development of compound-specific stable isotopes to demonstrate that chytrids convert precursor fatty acids to nutritionally higher quality polyunsaturated fatty acids conductive for somatic growth and reproduction of zooplankton. Taken together, the 'unseen' role of chytrids provides an as yet unknown crucial dietary link to aquatic animals and their survival.