A freshwater green microalga, Haematococcus pluvialis, has been cultivated as a source of natural astaxanthin, a ketocarotenoid. Astaxanthin in microalgal cells is located in the cytoplasmic lipid globules. The biflagellate and motile cells of

H. pluvialis transform into resting cyst cells, the aplanospores, and develop a distinct red color due to astaxanthin accumulation. After maturation, the cysts germinate, releasing flagellated cells (Margalith, 1999). The transformation of vegetative micro­algal cells to astaxanthin-accumulating resting cells could be achieved by subjecting the microalgal culture to environmental and nutritional stress, for example, nitrogen and phosphorus limitation, increases in culture temperature, increases in the salinity of the culture medium, and exposure of the culture to high irradiance (Del Campo et al., 2007). The astaxanthin content of Haematococcus cells can go up to 3%, mak­ing them an attractive source of the carotenoid pigment.

The accumulation of astaxanthin in Haematococcus cells in the resting phase necessitates a two-phase cultivation protocol where in the first phase the microalga is grown under optimal growth conditions to achieve high biomass yields, and then the green biomass is subjected to nutritional and environmental stress in a second phase to induce cyst (aplanospore) formation and the accumulation of astaxanthin (Del Campo et al., 2007). Complete outdoor cultivation of Haematococcus has not been feasible due to its high sensitivity to contamination and extreme environmen­tal conditions during the growth phase. Commercial production of Haematococcus biomass is generally carried out in closed photobioreactors, or it combines closed photobioreactors and open ponds where the first stage of biomass generation is car­ried out in closed photobioreactors, followed by a short residence period of culture in open ponds for the second phase of induction of astaxanthin accumulation (Olaizola and Huntley, 2003; Cysewski and Lorenz, 2004; Del Campo, 2007).

Astaxanthin has major commercial application in aquaculture as a source of pigmentation for salmon, trout, and red sea bream (Lorenz and Cysewski, 2000; Guerin et al., 2003; Cysewski and Lorenz, 2004), and the market is dominated by synthetic astaxanthin. Natural astaxanthin from Haematococcus is not competitive with synthetic astaxanthin for aquaculture applications due to high production costs (Guerin et al., 2003; Olaizola, 2003). Therefore, the economic viability of large — scale cultivation of Haematococcus to produce natural astaxanthin for aquaculture applications alone may not be feasible, but finding high-value markets is important. Human nutraceuticals have emerged as the high-value market for natural astaxan — thin from Haematococcus. Several in vitro and in vivo studies have demonstrated the beneficial health effects of Haematococcus-derived natural astaxanthin (Guerin et al, 2003; Olaizola, 2003; Kamath et al., 2008; Yuan et al., 2011). Haematococcus has been cleared by the U. S. FDA for application as an ingredient in dietary supple­ments for humans and has also been approved for human consumption in several European countries (Lorenz and Cysewski, 2000). This has paved the way for mar­keting Haematococcus biomass for application as a food supplement.