Microalgae: A Renewable Source of Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 227

Phytosterols are absorbed only in trace amounts by the body but inhibit the absorption of
intestinal cholesterol, including recirculating endogenous biliary cholesterol, a key step
in cholesterol elimination.

9.8.3 Proteins and carbohydrates
Microalgae are good sources of protein (typically 25-40% of the dry weight) and are rich in
the essential amino acids (Volkman and Brown, 2006). The amino acid patterns found in
most species compare favourably with those of other food proteins. Most microalgae have
rather similar amino acid profiles; aspartate and glutamate are usually the most abundant
amino acids (8-12% of total amino acids) whereas levels of cystine, methionine, tryptophan
and histidine can be quite low (Volkman and Brown, 2006). Interestingly, the arginine
content in species of the green alga Tetraselmis , which is often used as an aquaculture feed,
is twice that of other species (Brown, 1991). The amino acid composition of microalgae
does not seem to be greatly affected by changes in light or nutrient conditions.
The carbohydrate content of microalgae can be quite variable, with values as low as 5%
and as high as 30% of dry weight in some species (Volkman and Brown, 2006). Sugars can
occur in a variety of chemical forms, but polysaccharides typically constitute 80-95% of the
total carbohydrate (Brown, 1991). Unlike the amino acids, the sugar composition of algal
polysaccharides varies greatly between species and with changing culture conditions. Glucose
is usually the dominant sugar (20-90% of sugars) reflecting the fact that glucan is the major
energy reserve in microalgae (Handa and Yanagi, 1969). Note that some species excrete
polysaccharides (exopolysaccharides, EPS) into the culture medium, especially when
nitrogen is limited as in the stationary growth phase. Microalgal exopolysaccharides have a
range of potential applications, such as adhesives as well as gelling, flocculant and thickening
agents for the glue, oil, paper, paint, textile, cosmetic, food, and beverage industries. Mancuso-
Nichols and co-workers (2009) have screened nearly 800 cultured microalgal strains across
18 algal taxonomic groups for their potential as a source of adhesives.
The red microalga Porphyridum is seen as a potential commercial source of polysaccha-
rides for the cosmetic, pharmaceutical and health food markets as gelling agents, thickeners,
stabilizers, and emulsifiers. The carbohydrate content of this marine species can be very
high (up to 57%) (Singh et al ., 2000) and it is present as a gel-like acidic heteropolymer
envelope (Arad et al ., 2010), the thickness of which varies with phase of growth and
environmental conditions. Such sulfated polysaccharides form thermally reversible gels
similar to agar and carrageenan more usually extracted from marine macroalgae.

9.8.4 Vitamins and antioxidants
Some microalgae can be excellent sources of vitamins and antioxidants, including ascorbic
acid (vitamin C), -tocopherol (vitamin A) and riboflavin (vitamin B 2 ) (Volkman and

Brown, 2006). Ascorbic acid is usually the major vitamin present and contents from 1 to
16 mg g -1 have been noted (Brown and Miller, 1992). A full list of vitamins and their
concentration ranges can be found in Volkman and Brown (2006). The importance of growth
conditions for production of

α

-tocopherol, paramylon and tyrosine from the green alga
Euglena gracilis was examined by Rodríguez-Zavala and co-workers (2010). By combining
different carbon and nitrogen sources as well as inducing a tolerable stress to the cell by
adding ethanol, it was possible to increase production of biomass and

α

α -tocopherol as well

as paramylon and some amino acids.