Haematococcus pluvialis (Chlorophyta) is a freshwater, uni cellular alga with a rather complex life cycle. Its ovoid vegetative cells are motile by way of two flagella and during growth, nonmotile cells (cysts) also occur. The cells are normally green but under stress conditions (nutrient deficiency, salinity, high temperatures in com bination with high irradiance), the green vegetative cells produce thicker walls and change to globular cysts with a great increase in cell volume and pigmen tation to orange red, due to an increased carotenoid deposition. When the condition becomes favorable for growth the cysts germinate, releasing a large number of new motile cells.

The Haematococcus strains grow slowly at around 25-28 °C, and are prone to contamination by other microalgae. Therefore, a two stage process is employed for biomass production. Vegetative green cells are usually produced in closed photobioreactors and then the culture is exposed to high irradiance in open sys tems under nutrition stress (usually nitrogen deficiency) to induce astaxanthin synthesis (up to 5% of dry weight) within 3-5 days. This pigment is the important natural colorant for salmonoid fish, shrimp, lobster, and crayfish. However, today, the production of astaxanthin is still restricted to that of a few hundred kilos, mainly addressed to the health food market. The actual pro duction costs are still too high to compete with synthetic equivalents.

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