• growth advantage;
  • light;
  • microcystin;
  • Microcystis PCC7806

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 μmol photons · m−2 · s−1), where the toxic strain survived while the nontoxic strain became chlorotic. A strong correlation (r2 = 0.91, < 0.001, = 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.