Grazing on Microcystis aeruginosa and degradation of microcystins by the heterotrophic flagellate Diphylleia rotans

Cyanobacterial toxins can cause damage in aquatic ecosystems worldwide, as well as the poisoning of livestock, plants and humans when ingested in large amounts. Although many studies investigated grazing of harmful cyanobacteria by metazoan plankton, grazing of cyanobacteria by hetertophic flagellates is largely unexplored. This laboratory study investigated grazing of toxic Microcystis aeruginosa by the heterotrophic flagellate Diphylleia rotans isolated from a Saudi hypertrophic lake. D. rotans was able to feed on M. aeruginosa with estimated ingestion (10.2 to 16.5 prey flagellate −1 d−1) and specific growth rates (0.71 to 0.99 d−1) differed with the increase in the initial density of the flagellate. Grazing increased microcystin production within Microcystis cells during first two days of incubation, and afterwards induced the release of these toxins into the medium. The concentrations of releasing microcystin were strongly reduced in grazing-treated cultures compared to controls, indicating the degradation of microcystins by D. rotans growing under axenic conditions. Taken these results, D. rotans can play an important role in the reduction of Microcystis biomass and microcystin toxins, and thus could be used as a safe bioagent for the biocontrol of harmful algal blooms in aquatic environments.