About 10 000 strains of microalgae have been isolated from natural habitats and are kept in numerous culture collections around the world (e.g., UTEX, Austin, USA; IAM, University of Tokyo, Japan; CCALA, Trebori, Czech Republic; ISE-CNR, Italy). However, to date, only a few microalgal strains, mostly of aquatic origin, have been cultivated in large-scale production systems of hundreds to thousands of liters. Basically, these strains belong to two groups of unicellular or filamentous photo-synthetic microorganisms - prokaryotic cyanobacteria and eukaryotic algae.
Arthrospira platensis is a planktonic filamentous cyano-bacterium composed of individual cells (about 8 mm in diameter), which grows in subtropical alkaline lakes with the temperature optimum above 35 °C. In productive cultures, Arthrospira is cultivated in shallow mixed ponds or semiclosed tubular photobioreactors. The growth medium contains inorganic salts with a high concentration of bicarbonate, keeping the pH value between 9 and 10. This cyanobacterium is the most cultivated photosynthetic prokaryote since its biomass is widely used as a health food, feed supplement, and as a source of fine chemicals. The total annual production of biomass reaches about 45001, 50% of which is in China. It contains proteins, polyunsaturated fatty acids, phycobiliproteins, carotenoids, polysaccharides, vitamins, and minerals.
Chlorella (green algae; Chlorophyta) is a cosmopolitan genus with small globular cells. It includes strains with a high temperature tolerance since some strains can grow between 15 and 40 °C. Chlorella strains grow auto-trophically in an inorganic medium as well as in mixotrophic and heterotrophic conditions (e.g., with addition of acetic acid and glucose). At present, auto-trophic production of Chlorella is carried out in open ponds, semiclosed tubular photobioreactors, or inclined cascades, since its high growth rate prevents contamination by other microalgae (e.g., in Japan, Czech Republic, and Germany). Processing of Chlorella cells requires effective centrifugation and mechanical disintegration of the cellulose cell wall.
Chlorella is the most cultivated eukaryotic alga since it is widely used as a health food and feed supplement, as well as in the pharmaceutical and cosmetics industry. It contains proteins, carotenoids, some immunostimulators, polysaccharides, vitamins, and minerals.
Dunaliella salina (Chlorophyta) and similar hypersaline strains have biflagellated, pear-shaped cells. They lack a rigid polysaccharide cell wall, having instead a thin elastic plasma membrane. Dunaliella produces ^-carotene in high amounts, up to 12% of dry matter. This micro-alga is a natural source of carotenoids for some shrimps. The high content of ^-carotene also makes Dunaliella attractive to biotechnologists for large-scale production in high-salt, nitrogen-deficient, inorganic media in shallow, open ponds under high solar radiation (>30 °C). Natural ^-carotene is marketed in various forms: ^-carotene extracts, Dunaliella powder for human use, and dried Dunaliella for feed coloration. In the cells, ^-carotene is usually accompanied by other carotenoids (lutein, neoxanthin, zeaxanthin, violaxanthin, and cryptoxanthin).
Haematococcus pluvialis (Chlorophyta) is a freshwater, unicellular 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 combination with high irradiance), the green vegetative cells produce thicker walls and change to globular cysts with a great increase in cell volume and pigmentation 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 systems 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 production costs are still too high to compete with synthetic equivalents.
A number of microalgae are used in aquaculture, for example, Isochrysis, Pavlova (Prymnesiophyceae), Monodus and Nannochloropsis (Eustigmatophyceae), and Tetraselmis (Prasinophyceae). Live microalgae play a key role in aquaculture, being the feed source for the larvae of mollusks, crustaceans, fish hatchings, or rotifers, which are used as fish feed. The biomass has a high content of polyunsaturated fatty acids (PUFAs). Various types of closed photobioreactors are used for their cultivation: polyethylene bags, cylinders, flat panels, horizontal tubular systems, and annular columns. Microalgal products for aquaculture represent one of the biggest global markets. The dried biomass is also used as a feed supplement in pellet form for adult fish.
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