Toxicity in Peridinium aciculiferum—an adaptive strategy to outcompete other winter phytoplankton?

Freshwater dinoflagellates may form dense blooms during winter in ice-covered lakes. Unlike their marine counterparts, freshwater dinoflagellates are rarely considered to be potential toxin producers. Here we tested whether the winter species Peridinium aciculiferum produces a toxin(s) and investigated the potential adaptive function of that toxin, i.e., predator defense or inhibition of competitors (allelopathy). Using traditional toxicity bioassays (Artemia toxicity test and hemolytic activity assay), we detected the production of a toxic substance by P. aciculiferum cells from both the field and from laboratory cultures. Cultures deprived of phosphorus and in stationary phase showed highest toxicity. Potential predators, such as Daphnia galeata (Cladocera) and Eudiaptomus graciloides (Copepoda), were apparently not harmed by P. aciculiferum toxicity. However, the naturally cooccurring competitor Rhodomonas lacustris (Cryptophyceae) was killed by P. aciculiferum. An allelopathic substance(s) caused the cells of R. lacustris to form blisters and subsequently lyse. We concluded that our results support the hypothesis that P. aciculiferum is allelopathic, but not that toxins serve as predator defense. We therefore suggest that allelopathy may be an adaptive strategy of winter dinoflagellates, which could allow them to outcompete other phytoplankton species and thereby dominate the algal biomass. Freshwater dinoflagellates are generally considered to be nontoxic and harmless algae despite the fact that their marine counterparts may be extremely toxic and can have substantial negative effects on the systems they occur in. In freshwater phytoplankton communities, dinoflagellates are a common component and may dominate the biomass completely during late summer in temperate lakes. In addition, dinoflagellates can also form extensive blooms under ice during late winter and early spring, a phenomenon that has received little scientific attention. The adaptive function of toxin production in phytoplankton is still debated. Possible functions may be repulsion of grazers, inhibition of cooccurring phytoplankton species (allelopathy), release of degradation products, or storage of products (Turner and Tester 1997). Regardless of which evolutionary forces have led to toxin production in marine di1 Present address: Limnology, Department of Ecology, Ecology Building, Lund University, SE-223 62 Lund, Sweden (Karin. [email protected]).