LAUSR

The accumulation of pollen grains in freshwater ecosystems may have a significant impact on nutrient dynamics, despite the extracellular wall of pollen grains hampering bacterial degradation and digestion by invertebrates. This is because pollen grains may be readily degraded by saprotrophic fungi (Chytridiomycota). We tested in the laboratory whether free swimming chytrid zoospores may be grazed by zooplankton, transferring pollen-derived nutrients to zooplankton. We conducted grazing and growth experiments with daphnids (Daphnia pulex and D. similis), tree pollen grains from the pine family (Pinus and Cedrus), and a chytrid (Boothiomyces sp.). Both daphnid species grazed on zoospores, but not on pollen. Both daphnid species grew better in the presence of chytrid. Higher growth rates were associated with chytrid zoospores, rather than pollen or bacteria. The nutritional quality of zoospores was higher than that of the pollen and comparable with that of the green algae Scenedesmus in terms of carbon to phosphorus ratios and fatty acid composition. The percentage of phosphorus from pollen grains used by chytrids was estimated to be 30%–60%, indicating that these organisms used the pollen efficiently. The growth efficiencies of daphnids that ingested zoospores were about 17%–47% and 6%–18%, in terms of carbon and phosphorus, respectively, reflecting substantial growth rates even at low-food concentrations. Hence, the release of chytrid zoospores transferring inedible pollen to zooplankton functions as a “mycoloop.” In freshwater systems, saprotrophic chytrid-dependent mycoloops may play an important role in shaping aquatic food webs, especially where pollen deposition is significant.