The Role of Allelopathy for Harmful Algae Bloom Formation
Strong evidence has accumulated on the last years that some algal species are able to kill not only their grazers but also other algal species, a process called allelopathy. Killing the nutrient-competing phytoplankton species enable these species to freely utilize limiting resources such as nitrogen and phosphorus. While for some algal species, like e.g. the flagellate Prymnesium sp., the allelochemicals seem to be the same substances as their toxins, for some other algal species they are not. Alexandrium spp. are among the latter case: their internal toxins (such as saxitoxins) are not able to inhibit the growth of other algal species. However, these species by producing other substances than their internal toxins also cause allelopathic effects. Emphasis is placed here on the flagellate species Prymnesium parvum; which is not only able of allelopathy but mixotrophy as well. 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, but decrease in intensity or cease completely if cells are grown with high amounts of N and P in balanced proportions. 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. It seems that toxin production in Prymnesium spp. works not only as a defense mechanism, but also, by killing competitors, improve the algae competitive ability under conditions of severe nutrient depletion. We can assume thus that a consequence of the increased input of N and P to aquatic ecosystems is provoking an unbalanced nutrient situation for Prymnesium spp., as well as many of the other HAB species producing toxins, to growth but ideal to produce toxins instead.
KeywordsHABs allelopathy phytoplankton nutrients toxins
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