Effects of <i>Daphnia</i> exudates and sodium octyl sulphates on filament morphology and cell wall thickness of <i>Aphanizomenon gracile</i> (Nostocales), <i>Cylindrospermopsis raciborskii</i> (Nostocales) and <i>Planktothrix agardhii</i> (Oscillatoriales)

<p>Grazing is recognized as one of the selective factors shaping the morphology and physiology of cyanobacteria. A recent study has shown that the filamentous cyanobacterium <i>Aphanizomenon gracile</i> strain SAG 31.79 thickened in the presence of <i>Daphnia</i> (Cladocera) and its exudates. The aims of our study were: (1) to determine whether this type of response to <i>Daphnia</i> cues is common for other strains of <i>A. gracile</i>, and other species of filamentous cyanobacteria, (2) to test whether the response is due to nutrients recycled by <i>Daphnia</i>, or kairomone induced, and (3) whether it is related to toxin production. Prior to the experiment, cyanobacterial strains were inspected using chromatographic methods for the presence of two toxins, cylindrospermopsin (CYN) and three homologues of microcystin (MC-RR, MC-YR, MC-LR). HPLC analyses showed that all strains were free of cylindrospermopsin, whereas microcystins were detected only in one strain (<i>Planktothrix agardhii</i>). We then tested whether <i>Daphnia</i> exudates can cause thickening of cyanobacterial filaments, which would suggest the morphological changes in cyanobacterial filaments are caused by recycled nutrients. Cyanobacteria were also exposed to sodium octyl sulphate (a commercially available <i>Daphnia</i> kairomone). Transmission electron microscopy (TEM) was used to check whether <i>Daphnia</i> exudates and sodium octyl sulphate trigger thickening of cyanobacterial cell walls, which would be a defence mechanism against grazing. The TEM analysis revealed no significant effect of either <i>Daphnia</i> exudates or kairomone (sodium octyl sulphate) on the cell wall thickness of cyanobacteria. However, our study showed that <i>Daphnia</i> exudates triggered filament thickening in nostocalean cyanobacteria, while filaments of the oscillatorialean strain <i>P. agardhii</i> did not show this response. It was also demonstrated that sodium octyl sulphate alone can also cause filament thickening, which suggests that this might be a specific defence response to the presence of grazers.</p