VARIATION OF MICROCYSTINS, CYANOBACTERIAL HEPATOTOXINS, IN ANABAENA SPP AS A FUNCTION OF GROWTH STIMULI

Cyanobacterial hepatotoxins, microcystins, are specific inhibitors of serine/threonine protein phosphatases and potent tumor promoters. They have caused several poisonings of animals and also pose a health haza rd for humans through the use of water for drinking and recreation. Di fferent strains of the same cyanobacterial species may variously be no ntoxic, be neurotoxic, or produce several microcystin variants. It is poorly understood how the amount of toxins varies in a single strain. This laboratory study shows the importance of external growth stimuli in regulating the levels and relative proportions of different microcy stin variants in two strains of filamentous, nitrogen-fixing Anabaena spp. The concentration of the toxins in the cells increased with phosp horus. High temperatures (25 to 30 degrees C), together,vith the highe st levels of light studied (test range, 2 to 100 mu mol m(-2) s(-1)), decreased their amount. Different structural variants of microcystins responded differently to growth stimuli. Variants of microcystin (MCYS T)-LR correlated with temperatures below 25 degrees C, and those of MC YST-RR correlated with higher temperatures. Nitrogen added into the gr owth medium and increasing temperatures increased the proportion of mi crocystin variants demethylated in amino acid 3. All variants remained mostly intracellular. Time was the most important factor causing the release of the toxins into the growth medium, Time, nitrogen added int o the growth medium, and light fluxes above 25 mu mol m(-2) s(-1) sign ificantly increased the concentrations of the dissolved toxins. Accord ing to the results, it thus seems that the reduction of phosphorus loa ds in bodies of water might play a role in preventing the health hazar ds that toxic cyanobacterial water blooms pose, not only by decreasing the cyanobacteria but also by decreasing their toxin content.