A GROWTH ADVANTAGE FOR MICROCYSTIN PRODUCTION BY MICROCYSTIS PCC7806 UNDER HIGH LIGHT

Batch cultures of both Microcystis PCC7806 and a mcyA(-) knockout mutant (MT) of PCC7806 were cultured at three different light intensities and five media treatments, so as to vary cellular N:C ratios and concentrations and sampled daily over 5 d for analysis of microcystin concentration, cell numbers, and residual nitrate in the growth medium. A competitive survival advantage was noted at a high-light level (37 mu mol photons . m(-2) . s(-1)), where the toxic strain survived while the nontoxic strain became chlorotic. A strong correlation (r(2) = 0.91, P < 0.001, N = 22) between microcystin concentration and growth rate was observed at high-light conditions. No advantage was observed at optimal or low-light conditions, and media composition had no significant effect on the relationship between toxicity and survival at high-light conditions. These data suggest a possible role for microcystin in protection against photooxidation.