Abstract
Parasitism is arguably the most commonly occurring consumer strategy. However, only a few food web studies assess how well stable isotopes reflect the trophic position of parasitic consumers and results are variable. Even fewer studies have measured the nutrient transfer by parasitic consumers, hindering an assessment of their role in nutrient transfer through food webs. Here we used a food chain consisting of a diatom as host, a chytrid as its parasitic consumer and a rotifer as the predatory consumer of the chytrid, to assess the trophic position of all three food-chain components using their natural 13C and 15N isotope signatures, and to measure the nitrogen transfer from the host via the chytrid to the rotifer by tracing 15N of a labelled host up the food chain. Additionally, we measured the carbon to nitrogen (C:N) ratios of all food-chain components. Natural isotope abundance results showed no clear 15N enrichment in the chytrid or rotifer relative to the primary producer. However, estimates of nitrogen transfer indicated that about 14% of host nitrogen was transferred per day from host to chytrid during infection epidemics and that some of this nitrogen was also transferred onward to the rotifer. Moreover, C:N ratios decreased with trophic level, suggesting that the chytrid provided a high-quality food source to the rotifer. In conclusion, our results support the “mycoloop”. The mycooloop proposes that chytrid infections allow the transfer of nutrients bound in large, inedible phytoplankton to zooplankton through the production of edible transmission spores, thereby rerouting nutrients back into the food web.
Highlights
Food webs describe the transfer of energy and nutrients through ecosystems based on trophic interactions
While the main focus of our study was on stable isotope results, we assessed population growth rates of uninfected and infected Synedra, chytrids and rotifers exposed to either uninfected or infected Synedra cultures to evaluate whether the overall performance of populations was similar between comparable treatments or between experiments
Our results show that there were only small differences in Synedra population growth rates between treatments performed in glass Erlenmeyer flasks (i.e. Synedra non-exposed to rotifers) or in plastic well plates (i.e. Synedra exposed to rotifers) and, we consider that Synedra growth rate is not affected by container type
Summary
Food webs describe the transfer of energy and nutrients through ecosystems based on trophic interactions. Even though parasitic consumers dominate food web links (Lafferty et al 2006), only few studies assess the trophic position of phytoplankton parasitic consumers and their role in nutrient transfer from host via parasitic consumer to zooplankton (Kagami et al 2007). Fungal parasitic consumers of phytoplankton, have been proposed to add previously unrecognized trophic links and to change the flow of carbon (C) in aquatic ecosystems (Grami et al 2011; Kagami et al 2014; Rasconi et al 2014). Epidemics of parasitic chytrids on phytoplankton can increase host genetic diversity (Gsell et al 2013), decrease uni-algal bloom size and duration, allowing competing species to thrive [see references in (Frenken et al 2017a)] and thereby play a role in the seasonal succession of phytoplankton (Van Donk and Ringelberg 1983). Physical and chemical factors, together with grazing, have been considered to be the major controlling factor of phytoplankton populations, and most aquatic food web models still exclude parasitic consumers (Frenken et al 2017a)
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