On the allelochemical potency of the marine dinoflagellate Alexandrium ostenfeldii against heterotrophic and autotrophic protists

Three strains of the marine dinoflagellate Alexandrium ostenfeldii of different geographic origin were tested for their short-term deleterious effects on a diversity of marine protists. All A. ostenfeldii strains were capable of eliciting an apparent allelochernical response, but the various protistan target species were differentially affected. Protists that were negatively affected by exposure to cells of A. ostenfeldii and associated extracellular metabolites comprised both autotrophs (Rhodomonas sp., Dunaliella salina, Thalassiosira weissflogii) and heterotrophs (Oxyrrhis marina, Amphidinium crassum, Rimostrombidium caudatum). Observed effects included immobilisation (e.g. of O. marina), morphological changes (e.g. in D. salina) and/or aberrant behaviour (e.g. of R. caudatum), mainly as preliminary stages of cell lysis. Immobilization and lytic effects against O. marina were strongly dependent on A. ostenfeldii cell concentrations. Effects also differed substantially among strains and different batch cultures of the same strain. Values of EC50, defined as the A. ostenfeldii cell concentration causing lysis of 50% of O. marina cells, ranged from 0.3 to 1.9 x 10(3) mL(-1), depending on the A. ostenfeldii strain. The autotrophic dinoflagellate Scrippsiella trchoidea reacted to exposure to A. ostenfeldii cells by formation of temporay (ecdysal) cysts, whereas, in contrast, the flagellates Emaliania huxleyi and Prymnesium parvum and the ciliate Strombidium sp. were relatively refractory or even unaffected. As long as cells did not lyse, the fluorescence field of target autotrophs, estimated by pulse-amplitude modulation fluorometry, did not significantly change during the first 3 h of incubation, suggesting that allelochemicals produced by A. ostenfeldii caused no shortterm negative effects oil the photosynthetic apparatus. Overall, the allelochemical responses of target species showed no obvious relationship to cell quota or extracellular concentrations of either toxic inacrog,clic innues (spirolides) or letralydropurine neuroloxins (saxitoxill and analogues) produced by various strains of A. ostenfeldii. Instead, the poteng of A. osteifieldii, eliciting immobilization and lytic speries-specific responses in potential piedato?s and competitors, is consistent with the existence of all alleloclienzical meckam m unrelated to the bioactiviy of known phycoloxins of the genus A lexandrium.