Infection of filamentous phytoplankton by fungal parasites enhances herbivory in pelagic food webs

Chytrid fungal parasites are ubiquitous in aquatic ecosystems and infect a wide array of aquatic organisms, including all phytoplankton groups. In addition to their role as parasites, chytrids serve as food to zooplankton, thereby establishing an alternative trophic link between primary and secondary production in pelagic food webs, the so-called mycoloop. We hypothesized that, in addition to the mycoloop, chytrid infection facilitates grazing of filamentous phytoplankton by rendering it more edible to zooplankton consumers through infection-induced fragmentation. We undertook grazing assays to compare the ability of the key zooplankter Daphnia to graze on a filamentous cyanobacterium in the presence or absence of chytrid infection. A near doubling in mean clearance rates was consistently recorded when Daphnia were fed with infected cultures of the cyanobacterium as compared to uninfected ones. Infected filaments were shorter than noninfected ones, indicating that infection-induced fragmentation undermines resistance of filamentous phytoplankton to grazing. We propose an extended conceptualization of the mycoloop that includes both direct effects (i.e., transfer via grazing of chytrid zoospores) and indirect effects (i.e., trophic upgrading and facilitated grazing on phytoplankton via fragmentation) of chytrid infection on trophic transfer at the base of pelagic food webs.