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During light induction for astaxanthin formation in Haematococcus pluvialis, we substituted photoautotrophic induction for heterotrophic induction using acetate, both to prevent contamination by heterotrophs due to addition of organic carbon and to enhance carbon assimilation in the induced cells. Strong photoautotrophic induction was performed by N-deprivation of photoautotrophically grown Haematococcus cells followed by supplementation with bicarbonate (HCO(3)(-)) or CO(2). Bicarbonate-induced cells contained more astaxanthin than acetate-induced cells, and even further enhancement of astaxanthin accumulation was achieved by continuous CO(2) supply. The maximum astaxanthin content (77.2 mg g(-1) biomass, 3.4-fold higher than with heterotrophic induction) was obtained under conditions of 5% CO(2), yielding astaxanthin concentration and productivity of 175.7 mg l(-1) and 6.25 mg l(-1) day(-1), respectively. The results indicate that photoautotrophic induction is more effective than heterotrophic induction for astaxanthin synthesis in H. pluvialis.

We investigated the effect of carbon/nitrogen (C/N) ratio on astaxanthin synthesis in Haematococcus pluvialis during photoautotrophic induction by continuous input of both CO(2)-air mixture and intense light. When H. pluvialis was induced by constant irradiance induction at 200 micromol photon m(-2) s(-1), there was a positive correlation with astaxanthin content and C/N ratio, which was similar to the case for heterotrophic induction. Lower C/N ratios did not retard Haematococcus encystment, but did increase culture biomass, resulting in a decrease in astaxanthin production because of light limitation. However, induction using variable irradiance showed that reduction of astaxanthin production at low C/N ratios was successfully overcome by simply increasing the light intensity from 200 to 300 micromol photon m(-2) s(-1) to overcome the light limitation. This resulted in a greatly enhanced astaxanthin synthesis in proportion to cell density in cultures with low C/N ratios. Our results indicate that light intensity is more critical than C/N ratio in astaxanthin production by H. pluvialis during photoautotrophic induction.

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