Allelopathic interactions between microcystin-producing and non-microcystin-producing cyanobacteria and green microalgae: implications for microcystins production

Most mixed culture studies on the allelopathic interactions between toxic and nontoxic cyanobacteria with phytoplankton species rarely investigate the role of microcystins (MC) production and regulation in the course of the studies. This study investigated the interactions between intact cells of toxic (Microcystis aeruginosa (Kutzing) Kutzing) and nontoxic (Microcystis panniformis Komarek et al.) cyanobacteria with those of green algae (Monoraphidium convolutum (Corda) Komarkova-Legnerova and Scenedesmus acuminatus (Largerheim) Chodat) as well as the effects of their respective crude extracts (5 and 10 mu g.L-1) on their growth under controlled conditions. M. aeruginosa and M. panniformis were able to significantly (p < 0.05) inhibit the growth of the green algae with M. convolutum being the most affected. The green alga S. acuminatus in return was able to inhibit the growth of the both cyanobacteria. In response to the presence of a competing species in the growth medium, M. aeruginosa significantly increased its MC production per cell with the progression of the experiment, having the highest concentration at the end of the experiment. On the other hand, the extracts of the cyanobacteria had no significant inhibitory effect on the green algal strains investigated, while those of the green algae also had significant inhibitory effect on the growth of M. aeruginosa. In conclusion, both cyanobacterial and green algal strains investigated were negatively affected by the presence of competing species. M. aeruginosa responded to the presence of green algae by increasing its MC production. The green algal strains significantly inhibited the growth of M. aeruginosa.