Allelopathic activity of cyanobacteria on green microalgae at low cell densities

Allelopathic interactions are thought to play an important role in phytoplankton ecology. Many cyanobacterial genera have been shown to produce compounds with allelopathic activity; however, most studies have used high cyanobacterial cell densities, typical of bloom situations. We investigated whether low cell densities of cyanobacteria exhibit allelopathic activity. Twenty-two cyanobacterial strains were cultured in laboratory conditions, and the effects of their exudates were tested on the microalgae Ankistrodesmus falcatus and Chlorella vulgaris. We found that the exudates from one strain of Cylindrospermopsis raciborskii caused an enhancement of A. falcatus growth, followed by a marked decrease in growth rates, and that exudates from one strain of Oscillatoria sp. strongly inhibited C. vulgaris, as confirmed by dose-response assays. After the removal of the stressors, the microalgae grew at normal rates. The responsible compounds showed low sensitivity to heat and proteinase treatments. However, the majority of the strains did not exhibit appreciable allelopathic activity. Allelopathy may thus be a less frequent process in cyanobacteria at low cell-densities.