Reactive oxygen species are linked to the toxicity of the dinoflagellate Alexandrium spp. to protists

Abstract

Short-term experiments were conducted to examine the response of the ciliate Tia- rina fusus and the heterotrophic dinoflagellate Polykrikos kofoidii to 3 strains in the Alexandrium tamarense species complex, each with a different paralytic shellfish toxin (PST) content. Both pro- tist species fed on all 3 Alexandrium strains, but significant mortality occurred within 24 h of initial exposure to high densities of each dinoflagellate isolate. Protist mortality was not related, how- ever, to the PST content of the Alexandrium strains, indicating a different mechanism of toxicity. Exposure of T. fusus to cell-free culture filtrates or cell extracts did not cause significant ciliate mortality. In contrast, significant mortality occurred when ciliates were separated physically from a live Alexandrium sp. culture by a 10 µm nylon mesh, suggesting that the toxicity is dependent upon the viability of the Alexandrium spp. cells but does not require physical contact or ingestion. Addition of antioxidant compounds significantly increased the survival of both protist species when exposed to Alexandrium, suggesting that reactive oxygen species and/or the secondary compounds produced by ROS-induced lipid peroxidation are involved in the toxicity of Alexan- drium spp. to ciliates and heterotrophic dinoflagellates. This mechanism of toxicity is previously unknown for Alexandrium spp. and may play an important role in bloom dynamics and toxin transfer within the food web.