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Microalgae are a major natural source for a vast array of valuable compounds, including a diversity of pigments, for which these photosynthetic microorganisms represent an almost exclusive biological resource. Yellow, orange, and red carotenoids have an industrial use in food products and cosmetics as vitamin supplements and health food products and as feed additives for poultry, livestock, fish, and crustaceans. The growing worldwide market value of carotenoids is projected to reach over US$1,000 million by the end of the decade. The nutraceutical boom has also integrated carotenoids mainly on the claim of their proven antioxidant properties. Recently established benefits in human health open new uses for some carotenoids, especially lutein, an effective agent for the prevention and treatment of a variety of degenerative diseases. Consumers' demand for natural products favors development of pigments from biological sources, thus increasing opportunities for microalgae. The biotechnology of microalgae has gained considerable progress and relevance in recent decades, with carotenoid production representing one of its most successful domains. In this paper, we review the most relevant features of microalgal biotechnology related to the production of different carotenoids outdoors, with a main focus on beta-carotene from Dunaliella, astaxanthin from Haematococcus, and lutein from chlorophycean strains. We compare the current state of the corresponding production technologies, based on either open-pond systems or closed photobioreactors. The potential of scientific and technological advances for improvements in yield and reduction in production costs for carotenoids from microalgae is also discussed.

When grown photoautotrophically, Chlorella zofingiensis strain CCAP 211/14 accumulates a significant amount of valuable carotenoids, namely astaxanthin and lutein, of increasing demand for use as feed additives in fish and poultry farming, as colorants in food, and in health care products. Under standard batch-culture conditions, this microalgal strain exhibits high values of both growth rate (about 0.04 h(-1)) and standing cell population (over 10(11) cells l(-1), or 7 g dry weight l(-1)). Lutein, in a free (unesterified) form, was the prevalent carotenoid during early stages of cultivation (over 0.3 pg cell(-1), equal to 4 mg g(-1) dry weight, or 20 mg l(-1) culture), whereas esterified astaxanthin accumulated progressively, to reach a maximum (over 0.1 pg cell(-1), equal to 1.5 mg g(-1) dry weight, or 15 mg l(-1) culture) in the late stationary phase. A differential response of lutein and astaxanthin accumulation was also recorded with regard to the action of some environmental and nutritional factors. C. zofingiensis CCAP 211/14 represents a unique model system for analyzing the differential regulation of the levels of primary (lutein) and secondary (astaxanthin) carotenoids. Relevant also from the biotechnological viewpoint, this photosynthetic organism, with outstanding attributes for fast photosynthetic growth and carotenoid accumulation, might prove most valuable for its application to the mass production of either or both lutein and astaxanthin.

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~ ~~The effect of the nitrogen source on the activity of ferredoxin-dependent nitrite reductase has been studied in the cyanobacterium Anacystis nidulans. De novo synthesis of nitrite reductase occurred in the absence of an added nitrogen source, although enzyme activity was higher when the medium contained either NO; or NO,. The positive effect of NOT on nitrite reductase was also evident in tungstate-treated A. nidulans, which lacked an active nitrate reductase, indicating that the stimulatory effect was due to NOT itself and not to the NO, resulting from its intracellular reduction. NH,+ acted as a repressor, overriding any positive effect of NOT or NO,. Nitrite reductase synthesis was freed from NH,+ repression by L-methionine-DL-sulphoximine, an irreversible inhibitor of glutamine synthetase. NH,+ must therefore be metabolized through glutamine synthetase before repressing nitrite reductase. The kinetics of nitrate reductase and nitrite reductase development were similar in cells transferred from NH,+-to NOT-containing media, suggesting a coordinate regulation of synthesis.

The biochemical composition and fatty acid content of twelve strains of filamentous, heterocystous, nitrogen‐fixing cyanobacteria have been determined. When grown under diazotrophic conditions, protein, carbohydrate, lipid, and nucleic acids comprised 37–52%, 16–38%, 8–13%, and 8–11% of the dry weight, respectively. The presence of a combined nitrogen source resulted in an increase in the protein content of the cells and a decrease in the levels of lipids and carbohydrates, although biomass productivity was not affected significantly. Biochemical composition also changed during culture growth, with the highest levels of proteins and lipids occurring as the culture entered stationary phase, whereas the highest levels of carbohydrate and nucleic acids were found during the exponential phase. Total fatty acid levels in the strains assayed ranged between 3 and 5.7% of the dry weight. With regard to fatty acid composition, all strains showed high levels of polyunsaturated fatty acids (PUFAs) and saturated fatty acids (SAFAs), with values of 24–45% and 31–52% of total fatty acids, respectively, whereas the levels of monounsaturated fatty acids (MUFAs) were in general lower (11– 32%). Palmitic acid (16:0) was the most prevalent SAFA, whereas palmitoleic (16:1n‐ 7) and oleic acid (18:1n‐9) were the most abundant MUFAs in all the strains. Among PUFAs, γ‐linolenic acid (GLA, 18:3n‐6) was present at high levels (18% of total fatty acids) in Nostoc sp. (Chile) and at lower levels (3.6% of total fatty acids) in Anabaenopsis sp. The presence of GLA has not been previously reported in these genera of cyanobacteria. The rest of the strains exhibited high levels (12–35% of total fatty acids) of α‐linolenic acid (ALA, 18:3n‐3). Linoleic acid (18:2n‐6) was also present at a substantial level in most of the strains. Eicosapentaenoic acid (EPA, 20:5n‐3) was also detected in Nostoc sp. (Albufera). Some filamentous nitrogen‐fixing cyanobacteria therefore represent potential sources of commercially interesting fatty acids.