Parasitic chytrids: their effects on phytoplankton communities and food-web dynamics

Abstract

Many phytoplankton species are susceptible to fungal parasitism. Parasitic fungi of phytoplankton mainly belong to the Chytridiomycetes (chytrids). Here, we discuss the progression made in the study of chytrids that parasitize phytoplankton species. Specific fluorescent stains aid in the identification of chytrids in the field. The established culturing methods and the advances in molecular science offer good potential to gain a better insight into the mechanisms of epidemic development of chytrids and coevolution between chytrids and their algal hosts. Chytrids are often considered to be highly host-specific parasites, but the extent of host specificity has not been fully investigated. Chytrids may prefer larger host cells, since they would gain more resources, but whether hosts are really selected on the basis of size is not clear. The dynamics of chytrids epidemics in a number of studies were partly explained by environmental factors such as light, temperature, nutrients, pH, turbulence and zooplankton grazing. No generalization was made about the epidemic conditions; some state unfavorable conditions for the host growth support epidemic development, while others report epidemics even under optimal growth conditions for the host. Phytoplankton is not defenseless, and several mechanisms have been suggested, such as a hypersensitivity response, chemical defense, maintaining a high genetic diversity and multitrophic indirect defenses. Chytrids may also play an important role in food webs, because zoospores of chytrids have been found to be a good food source for zooplankton

Parasitic chytrids could promote copepod survival by mediating material transfer from inedible diatoms

“…The mechanism behind this is not clear, but might be due to transferring nutrients and carbon from the less edible diatoms to copepods via the small fungal zoospores and bacteria. This evidence also supports the existence of a ''mycoloop'' (Kagami et al, 2007a), suggesting that inedible algae and herbivores are connected via abundant zoospores of chytrids. …”

“…In lakes, diatoms are often infected by parasitic chytrids (Van Donk & Ringelberg, 1983;Ibelings et al, 2004;Kagami et al, 2007a). Chytrids infections seem to be most common in large phytoplankton species that are fairly resistant to grazing by zooplankton (Sommer, 1987;Kagami et al, 2007a).…”

“…Chytrids infections seem to be most common in large phytoplankton species that are fairly resistant to grazing by zooplankton (Sommer, 1987;Kagami et al, 2007a). Parasitic chytrids of phytoplankton play an important role in aquatic food webs (Kagami et al, 2007a;Gleason et al, 2008).…”

Multi-year assessment of coastal planktonic fungi reveals environmental drivers of diversity and abundance

“…and Guinardia striata) compared with the other years in the study (Supplementary Figure 2). Chytrids are presumed to be host specific, but the relationship is not yet fully understood (Kagami et al, 2007). Dinomyces arenysensis is a chytrid parasite of dinoflagellates that was originally isolated from an Alexandrium minutum bloom, and subsequently shown in laboratory assays with different phytoplankton species to have a broad host range but generally within the genus Alexandrium (Lepelletier et al, 2014).…”

“…It is important to note that no other abundant Chytridiomycota OTUs were recorded in 2012 at Station L4. Freshwater diatoms have been shown to defend against chytrid infections via a range of mechanisms (Kagami et al, 2007). Analogous explanations for the loss of the Chytrid OTU14 in 2012 at Station L4 include diatom hosts developing anti-parasite defense mechanisms, such as the production of harmful chemicals (Pohnert et al, 2007), or changes within host populations towards more parasiteresistant genotypes (Gsell et al, 2013).…”