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THE ALLELOPATHIC EFFECTS OF TOXIN-PRODUCING CYANOBACTERIA ARE PH-DEPENDENT
Abstract
Cyanobacteria are the oldest prokaryotes with worldwide distribution. Some of them are able to produce secondary metabolites (including cyanotoxins), which can have different effects on the other phytoplankton species. It is speculated that production of cyanotoxins is an adaptation mechanism of the cyanobacteria to survive in diverse habitats inhibiting the growth of the competing aquatic biota. In certain environmental conditions, bloom forming cyanobacteria may exert rapidly their allelopathic potential and change the ecological balance in aquatic ecosystems. In this study, we investigated the allelopathic effects of two cyanobacterial strains, Cylindrospermopsis raciborskii and Microcystis aeruginosa (Cyanobacteria) on the green algae Chlamydomonas asymmetrica and Scenedesmus obtusiusculus (Chlorophyta), which may co-occur in freshwater ecosystems. We have evaluated the growth, morphological changes and production of cyanotoxins (cylindrospermopsin and microcystin) in single and mixed co-cultures grown under different pH (pH=7 and pH=9). Our results showed that Cylindrospermopsis raciborskii and Microcystis aeruginosa significantly inhibit the growth of Chlamydomonas asymmetrica and Scenedesmus obtusiusculus at alkaline conditions (pH=9) with more pronounced effects on Chlamydomonas asymmetrica. The growth inhibition was accompanied by increased production of cyanotoxins (cylindrospermopsin and microcystin) measured by ELISA. Interestingly, at neutral conditions (pH=7) the green algae Chlamydomonas asymmetrica and Scenedesmus obtusiusculus were dominating, and produced cyanotoxins in the mixed cultures were in concentrations similar to those in the single cyanobacterial cultures. No significant changes of the morphological characters of the green algae were observed in both experimental pH conditions. These data suggest that toxin production and allelopathic activity of cyanobacteria are pH-dependent.
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