Microalgae: A Renewable Source of Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 226

acid (20:5n-3; EPA). This company also patented a process for producing arachidonic acid
(20:4n-6, ARA) from the fungus Mortierella alpina . Martek's products life's DHA™ and
life's ARA™ are found in about 99% of infant milk formulas produced in the United States,
as well as a range of supplements and functional foods. OmegaTech, also owned by Martek,
uses an algal-like protist, Schizochytrium (a thraustochytrid), to produce an oil (formerly
known as DHA Gold) that is used as a dietary supplement in food and beverages, health
foods, animal feeds and mariculture products.
There has been a recent resurgence of interest in the fatty acid content and composition
of microalgae as possible source material for the production of biodiesel (Chisti, 2007;
Stephens et al ., 2010). Biodiesel consists of the fatty acid methyl esters (FAME)
produced from natural plant oils, waste cooking oil or animal fats. Microalgae are being
reconsidered as a feedstock because of the high productivity per unit area and because
microalgae can be grown under conditions that are not favourable for agricultural crop
production.
Much of our information on novel lipids in microalgae has come from studies by
organic geochemists and chemical oceanographers trying to understand the origins of the
complex distributions of lipids found in marine sediments. More recently, the emphasis
has switched to understanding the distribution of biochemical pathways amongst the
various algal groups and to relating these to the evolution of biochemical pathways. For
example, while hydrocarbons are not usually abundant in microalgae, an exception is
found in some diatoms. The genera Haslea and Pleurosigma produce C 25 unsaturated
highly branched isoprenoid (HBI) alkenes while C 25 and C 30 alkenes are found in
Rhizosolenia . It is now clear from detailed studies of diatoms in culture and from
examination of well-dated sediment cores that the HBI biosynthetic pathways evolved
separately in two different diatom clusters, with the oldest being about 92 million years
ago (Sinninghe Damsté et al ., 2004 ).
Many unusual compounds have been identified that have little similarity to the lipids
produced by higher plants. These include unusual C 36 -C 40 unsaturated methyl and ethyl
ketones in some haptophyte microalgae, where the double bonds have trans geometry rather
than the more usual cis . Other compounds include saturated and unsaturated C 30 and C 32
alkyl 1,15-diols having a mid-chain hydroxyl group at C-15 that occur in eustigmatophyte
microalgae (Volkman, 2006). It has now been shown that these compounds are precursors
for aliphatic biopolymers (algaenans) synthesised by these algae. A structurally similar
group of saturated and monounsaturated C 28 -C 30 diols, but with the hydroxyl group at C-14,
has now been identified in diatoms of the genus Proboscia (Sinninghe Damsté et al ., 2003 ).
It is not known whether the structural similarity is accidental and arises from very different
biosynthetic pathways.
Microalgae are an excellent source of sterols, which can comprise up to 10% of the
dry weight of an alga. Extensive information on the diverse sterol compositions of
marine microalgae has been established over the past decades (Volkman, 1986, 2005;
Barrett et al ., 1995). Some microalgae contain simple distributions of sterols whereas
others can contain 20 or more sterols differing in number of carbon atoms, presence or
absence of methyl groups at C-4, patterns of side chain alkylation or positions of double
bonds. Some microalgae can contain high contents of cholesterol, more commonly
thought of as an animal sterol, or of sitosterol, more commonly associated with higher
plants (Volkman, 1986).
Microalgae are a potential source of the phytosterols and stanols added to margarine
and have the advantage that the algal oil also contains vitamins and carotenoids.