The importance of morphological versus chemical defences for the bloom-forming cyanobacterium Microcystis against amoebae grazing
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Amoebae grazing can be an important loss factor for blooms of the common cyanobacterium Microcystis. Some Microcystis strains seem to be protected against amoebae grazing, but it is unclear whether this is achieved by their colony morphology or biochemically. These factors were investigated in grazing experiments using two Microcystis-grazing amoebae (Korotnevella sp. and Vannella sp.) and two Microcystis strains with differing colony morphology (aeruginosa and viridis morphotype) and different sensitivity to amoebae grazing. Amoebae did not increase in density and failed to reduce the growth rate of cultures of the amoebae insensitive viridis strain, irrespective of whether the Microcystis strain was colonial or unicellular. This suggests that the extended mucilage matrix surrounding viridis colonies is not the main defence mechanism against amoebae grazing. At the same time, the growth rate of both unicellular and colonial cultures of the amoebae-sensitive aeruginosa strain was heavily reduced by the growing amoebae. The addition of filtered viridis-conditioned medium to aeruginosa cultures significantly decreased both amoebae growth and its effect on aeruginosa growth rates, which indicates that extracellular compounds constitutively produced by viridis are at least partially responsible for their insensitivity to amoebae grazing. These results demonstrate the potential importance of chemical interactions between lower trophic levels (protists) for Microcystis bloom dynamics.
KeywordsMicrocystis Amoebae Grazing Cyanobacteria Biochemicals Colony morphology
We sincerely thank Stina Drakare from the Swedish University of Agricultural Sciences for providing us with Microcystis bloom samples from Lake Mälaren in Sweden and Karen Soenen, Lancelot Blondeel and Ahmed Abdul Jabbar for technical assistance during the experiments. Two anonymous reviewers gave valuable and highly appreciated comments that have lead to a considerable improvement of this paper. P.V. is a postdoctoral research fellow with the Research Foundation – Flanders (FWO). This research was financially supported by the BELSPO (Belgian Science Policy) project B-BLOOMS2.
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