Kill and eat your predator: a winning strategy of the planktonic flagellate Prymnesium parvum

Abstract

Interactions between the toxic and mixotrophic haptophyte Prymnesium parvum and the heterotrophic dinoflagellate Oxyrrhis marina were investigated in P-limited semi-continuous cultures and nutrient-replete batch culture experiments. When exposed to 100 × 10 3 cells ml -1 of P- limited P. parvum, starved O. marina initially ingested the algae, but ingestion was low (0.07 cells grazer -1 h -1 ) compared to the O. marina ingestion rate when the Cryptophyte Rhodomonas sp. was offered as food (2.75 cells grazer -1 h -1 ). Microscopic observations showed that low ingestion is due to toxic effects of P. parvum on the dinoflagellate. Cells of O. marina lost their normal cell shape and became rounded, hyaline and finally lysed. Rounded and partly lysed O. marina cells were rapidly attacked by several P. parvum cells, which formed larger aggregates around the remains of the O. marina cells. This was accompanied by phagotrophic ingestion of particulate material originating from disintegrating O. marina. Cell-free culture medium lysed O. marina cells, although to a lower degree compared to the effect of algal suspensions. O. marina mortality was not only reduced by diluting P. parvum, but also by increasing the dinoflagellate concentration. This clearly indicates that the toxin is removed from the system by its action, presumably by binding to the membrane. Under conditions where toxic effects were not apparent (nutrient-replete batch cultures, low cell concentra- tions, dim light), P. parvum, after an initial lag period, was rapidly ingested and sustained growth of O. marina at rates comparable to those estimated for Rhodomonas sp. Toxicity of P. parvum is thus a key factor in determining the interaction with protozoan grazers. If toxicity is low, P. parvum is a suit- able prey for O. marina. At high toxicity levels, however, O. marina is rapidly killed and ingested by P. parvum, thus reversing the normal direction of grazing interactions between protozoa and algae.