Golden Alga (Prymnesium parvum) in Texas

Workshop Abstracts

Kill Your Enemies and Eat Them: The Role of Prymnesium Toxins

Edna Graneli

Marine Sciences Department, University of Kalmar, SE-39182 Kalmar, Sweden

Abstract.--The haptophyte Prymnesium parvum is known to produce a set of highly potent exotoxins commonly called prymnesins. These toxins have been shown to have several biological effects, including ichthyotoxic, neurotoxic, cytotoxic, hepatotoxic and hemolytic activity towards a range of marine organisms. Toxic incidents of the haptophyte Prymnesium parvum have been known since the end of the 19th century. Since then, toxic blooms have been reported from brackish water localities in Europe, the Middle East, Ukraine, China and U.S.A. These blooms have affected coastal marine ecosystems heavily, and caused economic problems for commercial aquaculture. Therefore, it is important to understand the selective forces leading to bloom formation of this species. The ability of a specific phytoplankton species to become dominant and form blooms in natural environments is, apart from its competitive ability, also dependent on mortality losses. Grazing by herbivorous zooplankton is considered a major loss factor, preventing the development of phytoplankton blooms. Adaptations of algae to escape grazing would therefore directly favor the ecological success of that particular species. Several studies have shown that Prymnesium-species are able to diminish or completely avoid grazing by excretion of toxins into the water. Another important aspect in bloom formation is the ability to out-compete co-occurring algal species for nutrients. Over the last few years strong evidence has accumulated that Prymnesium spp. are able to kill not only their grazers but also other algal species, a process called allelopathy. Killing the nutrient-competing phytoplankton species enables Prymnesium to freely utilize limiting resources. Mixotrophy, i. e., the capability to ingest bacteria, other algae and even potential grazers, also contributes to the bloom-forming ability of Prymnesium spp. Allelopathy, mixotrophy and grazer deterrence increase dramatically when Prymnesium spp. cells are grown under N or P deficiency, and so does toxicity. On the other hand, if cells are grown in a medium with high amounts of N and P in balanced proportions, allelopathy, mixotrophy, grazer deterrence and toxicity decrease in intensity or cease completely. Usually additions of Prymnesium filtrates from nutrient deficient cultures have almost the same strong effect on grazers and other plankton cells as Prymnesium cells grown together with their target. This suggests that the toxins and the allelopathic/grazer deterring compounds are the same substances. In conclusion, it seems that toxin production in Prymnesium spp. works not only as a defense mechanism, but also, by killing competitors, improve the competitive ability of Prymnesium under conditions of severe nutrient depletion. Also, it seems that stress in general, rather than solely P- or N-limitation, is the cause for an increase in toxin production. Prymnesium toxins are poor in N and P, but have a high C content. Toxin production might be a way to store excess organic carbon, made available in photosynthesis under nutrient stress. This is thus similar to the way e.g. lipids or carbohydrates are produced in excess by most "normal" phytoplankton cells when there is not enough N or P available to build up material for cell division (DNA, proteins, etc).