Does Haematococcus pluvialis need to sleep?

Light intensity has been identified as one of the main factors affecting the synthesis of astaxanthin in Haematococcus pluvialis, but most experiments have been carried out under unnatural continuous light, which may constitute an additional source of stress to the cells. In order to isolate the role of light intensity and light:dark cycles in astaxanthin synthesis, H. pluvialis was cultivated in continuous light and with a circadian light:dark cycle of 12 h:12 h. Different initial cellular densities were used for both light regimes in order to establish different ratios of light per cell. Light:dark cycles produced higher cellular densities while continuous light produced faster nitrogen consumption and higher final astaxanthin concentrations. Under continuous light, Haematococcus pluvialis showed two different rates of astaxanthin accumulation: small amounts of astaxanthin were accumulated in the first 3 days of culture, while cells were still dividing, and massive accumulation of astaxanthin was triggered as a response to the cessation of cell division after nitrogen depletion. The generation of oxidative stress under continuous light could be responsible for the observed initial accumulation of astaxanthin that would be further exacerbated under nutrient limitation. The results confirm that light:dark cycles are optimal for the production of vegetative green cells of H. pluvialis, while continuous illumination might be used to accelerate astaxanthin accumulation rate once nitrogen is depleted from the culture medium.