Trophic position, elemental ratios and nitrogen transfer in a planktonic host–parasite–consumer food chain including a fungal parasite

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.

Characterizing the “fungal shunt”: Parasitic fungi on diatoms affect carbon flow and bacterial communities in aquatic microbial food webs

“…The viral shunt has been estimated to liberate 2 to 10% of photosynthetic C to the DOC pool during viral-induced cell lysis ( 81 ), thereby stimulating C transfer from phytoplankton to free-living bacteria ( 82 ) and thus microbe-mediated remineralization ( 83 , 84 ). As opposed to the viral shunt, we suggest that parasitic chytrids shunt photosynthetic C to the particulate organic C pool since we demonstrated that C is most efficiently transferred to attached sporangia, and the therein developed zoospores, which, as shown previously, are grazed by micro- and mesozooplankton ( 23 – 27 ). Moreover, chytrid infections accelerated diatom decay, stimulating bacterial colonization (by potentially opportunistic bacteria that thrive on decaying organic particles).…”

“…A recent concept, called mycoloop, describes parasitic chytrids as an integral part of aquatic food webs ( 22 ). Energy and organic matter are thereby transferred from large, often inedible phytoplankton to chytrid zoospores, which are consumed by zooplankton ( 23 – 27 ). Hence, parasitic chytrids establish a novel trophic link between phytoplankton and zooplankton.…”

Genomic and metabolic adaptations of biofilms to ecological windows of opportunities in glacier-fed streams

“…CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made consequences for carbon and nutrient cycling as mediated by the fungal shunt 33,34 . The possibility of fungal parasitism on algae in epilithic biofilms further underlines the role of photoautotrophs as the foundation of a complex food web in GFSs as a typically energy-limited ecosystem.…”

“…Furthermore, our results hint at the existence of a more cryptic interaction in epilithic biofilms between parasitic fungi Chytridiomycetes and algae (i.e., Ochrophyta). Fungal parasitism on pelagic algae has been recently reported to be more important than expected, even with consequences for carbon and nutrient cycling as mediated by the fungal shunt 33,34 . The possibility of fungal parasitism on algae in epilithic biofilms further underlines the role of photoautotrophs as the foundation of a complex food web in GFSs as a typically energy-limited ecosystem.…”

Stable nitrogen isotope analysis of amino acids as a new tool to clarify complex parasite-host interactions within marine food webs

“…Parasitism is a widespread lifestyle, with ~40% of all organisms having at least one parasitic life-stage. Accounting for parasitic relationships, ~75% of food web interactions involve a parasite (Dobson et al 2008) causing increases in linkage density (Lafferty et al 2008;Sánchez Barranco et al 2020), food chain length (Amundsen et al 2009), and connectance (Dunne et al 2013). Contributions of parasite biomass to an ecosystem can be substantial and even exceed that of top predators (Kuris et al 2008;Preston et al 2013).…”