Abstract
Members of the marine dinoflagellate genus Alexandrium are known to exude allelochemicals, unrelated to well-known neurotoxins (PSP-toxins, spirolides), with negative effects on other phytoplankton and marine grazers. Physico/chemical characterization of extracellular lytic compounds of A. tamarense, quantified by Rhodomonas salina bioassay, showed that the lytic activity, and hence presumably the compounds were stable over wide ranges of temperatures and pH and were refractory to bacterial degradation. Two distinct lytic fractions were collected by reversed-phase solid-phase extraction. The more hydrophilic fraction accounted for about 2% of the whole lytic activity of the A. tamarense culture supernatant, while the less hydrophilic one accounted for about 98% of activity. Although temporal stability of the compounds is high, substantial losses were evident during purification. Lytic activity was best removed from aqueous phase with chloroform-methanol (3:1). A “pseudo-loss” of lytic activity in undisturbed and low-concentrated samples and high activity of an emulsion between aqueous and n-hexane phase after liquid-liquid partition are strong evidence for the presence of amphipathic compounds. Lytic activity in the early fraction of gel permeation chromatography and lack of activity after 5 kD ultrafiltration indicate that the lytic agents form large aggregates or macromolecular complexes.
Highlights
Phytoplankton in aquatic systems face various survival challenges, some of which result from grazer attack, pathogens and competition with other microalgae
After storage of diluted (1:10) cell-free culture supernatant in 20 mL glass vials under normal culture conditions (15 °C), lytic activity rapidly declined, with an almost 50% reduction after 4 days and a complete disappearance after 7 days (Figure 1)
Lytic activity was reestablished by shaking the containers vigorously before removal of stored supernatant
Summary
Phytoplankton in aquatic systems face various survival challenges, some of which result from grazer attack, pathogens and competition with other microalgae. Many HAB species are regarded as rather poor exploitation competitors in terms of growth rate and/or resource uptake capabilities [1]. The hypothesis evolved that a number of HAB species may gain dominance or at least maintain co-existence by production of bioactive compounds, secondary metabolites that affect growth or elicit other physiological responses in other organisms. Such allelochemicals may be targeted to exclude competitors from exploiting limited resources (interference competition), as well as to avoid/reduce predation.
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