Antialgal activity of a hepatotoxin-producing cyanobacterium, Microcystis aeruginosa

Antimicrobial activity of toxin produced by a freshwater bloom-forming cyanobacterium Microcystis aeruginosa has been studied. When tested against certain green algae, cyanobacteria, heterotrophic bacteria and fungi, the toxin inhibited growth of only green algae and cyanobacteria. The toxin has been partially purified employing Thin layer chromatography (TLC) and High-performance liquid chromatography (HPLC) techniques and appears to be microcystin-LR (leucine-arginine). Both crude and purified toxins showed toxicity to mice, the clinical symptoms in test mice being similar to those produced by hepatotoxin. Purified toxin at a concentration of 50 mug ml(-1) caused complete inhibition of growth followed by cell lysis in Nostoc muscorum and Anabaena BT1 after 6 days of toxin addition. Addition of toxin (25 mug ml(-1)) to the culture suspensions of the Nostoc and Anabaena strains caused instant and drastic loss of O-2 evolution. Furthermore a marked reduction (about 87%) in the (CO2)-C-14 uptake was also observed at a concentration of 50 mug ml(-1). Besides its inhibitory effects on photosynthetic processes, M. aeruginosa toxin (50 mug ml(-1)) also caused 90% loss of nitrogenase activity after 8 h of its addition. Experiments performed with C-14-labelled toxin indicate that the toxin uptake by cyanobacterial cells occurs both in light and dark. These results demonstrate that the toxin is strongly algicidal and point to the possibility that it may have an important role in establishment and maintenance of toxic blooms of M. aeruginosa in freshwater ecosystems. The relative significance of the hepatotoxic effect and the algicidal effect of the toxin is discussed with reference both to survival and dominance of M. aeruginosa in nature.