Plankton parasites swim into the spotlight

Posted on September 21, 2011

Recent studies suggest that parasites of plankton have an important and hitherto unrecognised influence on the structure of aquatic food webs.  Chytrids are a widespread group of primitive fungi that infect large-bodied plant plankton.  Although such plankton cells are too big to be eaten by common zooplankton such as Daphnia (water fleas), chytrid parasitism moderates the biomass of large phytoplankton, thus making more nutrients available to small phytoplankton and their animal consumers.  In an interesting twist, it now seems that Daphnia graze very efficiently on the free-living spores produced by reproducing chytrids.   As a result, a food chain exists that connects  dissolved nutrients, large plant plankton, fungi and zooplankton.  This pathway can have both direct and indirect effects on ecosystem structure.  The direct effect is a potential boost in the  production of zooplankton as a result of their consumption of fungal spores.  The indirect effect comes about because the predation of chytrid spores by zooplankton can lower the incidence of parasitism, thus increasing the biomass of large phytoplankton and reducing the flow of material to zooplankton.  What is the balance of these direct and indirect effects on production and how does the chytrid link influence food web dynamics?  These questions were explored using models of lake food webs that included or excluded fungi and/or the chytrid link.  The modellers drew on parameter values from the literature and assumed that nutrient (phosphorus) levels limited plankton growth.  The results showed that the effect of fungal parasites on zooplankton production is always positive because it reduces the biomass of inedible large plankton and diverts nutrients to small edible plant cells.  In contrast, the net effect of the chytrid food chain on zooplankton can be positive or negative depending on the nutrient status of the water body, being positive in eutrophic (high nutrient) systems and negative in oligotrophic (low-nutrient) lakes.  In summary, it seems likely that parasitic aquatic fungi play a key role in material transfer, especially in productive  ecosystems.

Reference:  Miki, T., Takimoto, G. & Kagami, M.  2011.  Roles of parasitic fungi in aquatic food webs: a  theoretical approach.  Freshwater Biology 56, 1173–1183.