Fungal parasites of a toxic inedible cyanobacterium provide food to zooplankton

Abstract

During the end of spring and throughout summer, large‐sized phytoplankton taxa often proliferate and form dense blooms in freshwater ecosystems. In many cases, they are inedible to zooplankton and prevent efficient transfer of energy and elements to higher trophic levels. Such a constraint may be alleviated by fungal parasite infections on large‐sized phytoplankton taxa like diatoms and filamentous cyanobacteria, as infections may provide zooplankton with a complementary food source in the form of fungal zoospores. Zoospores have been shown to support somatic growth of large filter feeding zooplankton species. Here, we tested if selectively feeding zooplankton, more specifically rotifers, also can use fungal zoospores as a food source. Our results show that chytrid fungal parasites can indeed support population growth of rotifers (Keratella sp.). Specifically, in cultures of an inedible filamentous cyanobacterium (Planktothrix rubescens), Keratella populations rapidly declined, while in Planktothrix cultures infected with chytrids, Keratella population growth rate equaled the growth observed for populations fed with a more suitable green algal diet (Chlorella sorokiniana). Feeding of Keratella on zoospores was furthermore indicated by a reduced number of zoospores during the last sampling day. These findings not only imply that rotifers may survive on zoospores, but also that the zoospores can support high rotifer population growth rates. We thus show that fungal parasites of inedible cyanobacteria can facilitate grazers by providing them alternative food sources. Together, these results highlight the important role that parasites may play in the aquatic plankton food web.

Horizontal and vertical diversity jointly shape food web stability against small and large perturbations

“…We experimentally tested the effect of horizontal and vertical diversity on the stability of a freshwater plankton food web representative of Dutch ditches. These two experiments, each lasted for 21 days, were performed in 900‐mL glass jars, filled with 500 mL WC medium (Guillard & Lorenzen ; Frenken et al ) and contained in a water bath at constant temperature (19.9 ºC ± 0.8 ºC) and a light regime of 12 h : 12 h (light : dark). The light intensity at the surface (measured with a LI‐COR LI‐250A, LI‐COR Biosciences, Lincoln, USA) was 120 μmol m −2 s −1 and was created using Ceramalux® Philips 430 Watt High Pressure Sodium Non‐Cycling Lamps.…”

Phytoplankton growth and stoichiometric responses to warming, nutrient addition and grazing depend on lake productivity and cell size

“…Moreover, for the low productivity community, C:P ratios of the smaller size fraction and N:P of the larger size fraction were reduced by grazing at ambient nutrient levels, suggesting that cells in the low productivity lake can effectively take up recycled P from grazing (Figures 3 and 4; Elser & Urabe, 1999). Grazing effects were less apparent in the medium and high productivity communities, which were dominated by filamentous cyanobacteria that are largely inedible and presumed to be of poor nutritional quality to zooplankton (Frenken et al, 2018;Urabe & Waki, 2009). Although not a typical grazer of the pelagic zone in lakes, we used D. magna because it can be cultured with ease and is a generalist grazer that allowed us to standardize grazing pressure in our experiments.…”

“…<30 µm; Figure 1). Grazing effects were less apparent in the medium and high productivity communities, which were dominated by filamentous cyanobacteria that are largely inedible and presumed to be of poor nutritional quality to zooplankton (Frenken et al, 2018;Urabe & Waki, 2009). Moreover, for the low productivity community, C:P ratios of the smaller size fraction and N:P of the larger size fraction were reduced by grazing at ambient nutrient levels, suggesting that cells in the low productivity lake can effectively take up recycled P from grazing (Figures 3 and 4; Elser & Urabe, 1999).…”

Pretreatment Method for DNA Barcoding to Analyze Gut Contents of Rotifers

“…Choanoflagellate, HNF species, has the habitat selection characteristic of being attached to phytoplankton species, and consequently it is expected that rotifers can eat Choanoflagellate indirectly in the process of eating phytoplankton, or select it as their food source directly [55]. In the case of Meira nashicola, which is a kind of yeast-like fungi species, although whether or not M. nashicola exists in aquatic ecosystem needs further research, it is considered a valid result of Asplanchna gut contents because parasitic fungus on phytoplankton, such as cyanobacteria, are known to feed on rotifers as alternative food sources [56]. So, when the limitations related to the detection of bacteria and HNF will be resolved, rotifer gut contents can be analyzed by pretreating with alcohol and commercial bleach as we recommend in the present study.…”