Food uptake in the harmful alga Prymnesium parvum mediated by excreted toxins

Toxin production is widespread among aquatic microalgae, suggesting a functional advantage for organisms producing toxic compounds. However, the biological role of algal toxin production is only vaguely understood. Here, we show that excretion of a toxic substance in the phagotrophic phytoflagellate Prymnesium parvum (Prymnesiophyceae) constitutes a mechanism to immobilize and seize motile prey. Feeding frequency of P. parvum in dilute batch cultures was low when fed the motile prey Heterocapsa rotundata (dinoflagellate). However, dense cultures caused immobilization of H. rotundata cells, thereby allowing P. parvum to feed on them. In contrast, when fed a nonmotile prey—the diatom Thalassiosira pseudonana —feeding frequency was high, even in dilute P. parvum cultures. We could demonstrate that feeding frequency of P. parvum on H. rotundata was positively correlated with the measure of the toxic effect causing immobilization and lysis of prey cells. The fact that the toxic effect on H. rotundata was found in cell‐free filtrate of P. parvum cultures suggests that immobilization and lysis of prey cells were caused by the excretion of toxins.