The chlorophyte Haematococcus pluvialis accumulates large quantities of astaxanthin under stress conditions. Under either nitrogen starvation or high light, the production of each picogram of astaxanthin was accompanied by that of 5 or 3–4 pg of fatty acids, respectively. In both cases, the newly formed fatty acids, consisting mostly of oleic (up to 34% of fatty acids in comparison with 13% in the control), palmitic, and linoleic acids, were deposited mostly in triacylglycerols. Furthermore, the enhanced accumulation of oleic acid was linearily correlated with that of astaxanthin. Astaxanthin, which is mostly monoesterified, is deposited in globules made of triacylglycerols. We suggest that the production of oleic acid‐rich triacylglycerols on the one hand and the esterification of astaxanthin on the other hand enable the oil globules to maintain the high content of astaxanthin esters.

Haematococcus pluvialis under stress conditions overproduces the valuable red ketocarotenoid astaxanthin. Two proposed strategies for commercial production are under current analysis. One separates in time the production of biomass (optimal growth, green stage) and pigment (permanent stress, red stage), while the other uses an approach based on continuous culture under limiting stress at steady state. The productivities, efficiencies and yields for the pigment accumulation in each case have been compared and analyzed in terms of the algal basic physiology. The two-stage system indoors yields a richer astaxanthin product (4% of dry biomass) with a final astaxanthin productivity of 11.5 mg L(-1) day(-1), is more readily upscalable and amenable to outdoors production. Furthermore, each stage can be optimized for green biomass growth and red pigment accumulation by adjusting independently the respective ratio of effective irradiance to cell density. We conclude that the two-stage system performs better (by a factor of 2.5-5) than the one-stage system, and the former is best fit in an efficient mass production setup.

Green cells of Haematococcus pluvialisFlotow accumulate the ketocarotenoid astaxanthin under stress conditions, such as high irradiance, nutrient deficiency, high salinity, and high temperature. Though some photoprotective mechanisms have been suggested, the function of astaxanthin in red cysts is still questioned. We studied the role of astaxanthin in photoprotection by inducing its formation in logarithmically growing cultures by high irradiance, thus avoiding unrelated processes that can occur in H. pluvialis when carotenogenesis is induced by other stresses. On exposure to high irradiance, the green Haematococcus culture turned red as lipid globules loaded with astaxanthin esters were formed and concentrated at the periphery of the cell. During this phase of induction, the photosynthesis rates remained high, but the amount of the D1 protein of PSII was significantly reduced. The decline in D1 protein content stopped after 1 day; the level then increased, returning to normal after 5 days. The response of the D1 protein was indicative of a transitional phase in the acclimation of Haematococcus to high light. The formation and deposition of astaxanthin seemed to prevent further reduction in D1 protein level, thus enabling the cell to maintain PSII function and structural integrity. This result seems to be a clear indication of the light screening by astaxanthin, which absorbs light in the blue region, thus protecting the photosynthetic apparatus. When the cells recovered from the high light stress, the astaxanthin globules concentrated around the nucleus, indicating that the pigment also serves as a physicochemical barrier, protecting the replicating DNA from oxidation as the cells divide.

We have identified and isolated a cDNA encoding a novel acyl‐CoA:diacylglycerol acyltransferase (DGAT)1‐like protein, from the diatom microalga Phaeodactylum tricornutum (PtDGAT1). The full‐length cDNA sequences of PtDGAT1 transcripts revealed that two types of mRNA, PtDGAT1short and PtDGAT1long, were transcribed from the single PtDGAT1 gene. PtDGAT1short encodes a 565 amino acid sequence that is homologous to several functionally characterized higher plant DGAT1 proteins, and 55% identical to the putative DGAT1 of the diatom Thalassiosira pseudonana, but shows little homology with other available putative and cloned algal DGAT sequences. PtDGAT1long lacks several catalytic domains, owing to a 63‐bp nucleotide insertion in the mRNA containing a stop codon. Alternative splicing consisting of intron retention appears to regulate the amount of active DGAT1 produced, providing a possible molecular mechanism for increased triacylglycerol (TAG) biosynthesis in P. tricornutum under nitrogen starvation. DGAT mediates the last committed step in TAG biosynthesis, so we investigated the changes in expression levels of the two types of mRNA following nitrogen starvation inducing TAG accumulation. The abundance of both transcripts was markedly increased under nitrogen starvation, but much less so for PtDGAT1short. PtDGAT1 activity of PtDGAT1short was confirmed in a heterologous yeast transformation system by restoring DGAT activity in a Saccharomyces cerevisiae neutral lipid‐deficient quadruple mutant strain (H1246), resulting in lipid body formation. Lipid body formation was only restored upon the expression of PtDGAT1short, and not of PtDGAT1long. The recombinant yeast appeared to display a preference for incorporating saturated C16 and C18 fatty acids into TAG. Database Nucleotide sequence data are available in the GenBank/EMBL/DDBJ databases under accession number http://www.ncbi.nlm.nih.gov/nuccore/HQ589265, sequence to be released November 15 2011.