Microalgae: A Renewable Source of Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 224

this knowledge has been used to identify natural anti-foulants (Steinberg et al ., 1998 )
and natural sunscreens (Dunlap and Shick, 1998). It is now recognised that some of these
compounds are derived from microalgae and cyanobacteria ingested by these animals or
living symbiotically with them (Skulberg, 2004). Indeed, microalgae with their rich and
diverse biochemistries represent potential sources for new bioactive compounds that are
still largely untapped.
9.8.2 Lipids
Microalgae contain a surprisingly diverse range of lipids, many of which are restricted in
distribution to a few species or genera. Thus, they can be useful as chemotaxonomic markers
(Volkman et al ., 1998). More than 30years ago the US National Renewable Energy
Laboratory (NREL) initiated the Aquatic Species Program (ASP) as a result of the oil crisis
in the 1970s. Between 1978 and 1996 the ASP studied the feasibility of using algae grown
in ponds and using carbon dioxide from power plants to produce biodiesel. This study
provided considerable information about the lipid composition of microalgae and the effects
of different growth techniques and environmental conditions. Although the program ended
in 1995 it set the scene for renewed interest in algae as a source of biofuels and other
chemicals (Sheehan et al ., 1998). An unfortunate loss is that many of the strains that were
isolated as part of the ASP were not deposited in the main algal culture collections in the
United States and are thus no longer available.
One of the most important findings from the Aquatic Species Program is that although
nutrient stress causes lipid to increase in many strains (when expressed as a percentage of
the total biomass), this increase is generally accompanied by a decrease in total cell and
lipid productivity. One way to overcome this is to use a two-phase production system.
Rodolfi and co-workers (2009) demonstrated that with careful strain selection and a two-
phase cultivation system (a nutrient-sufficient phase to produce the inoculum followed by a
nitrogen-deprived phase to boost lipid synthesis) an oil production potential of more than
90 kg per hectare per day could be attained with the final lipid content reaching more than
60% of the cell biomass. This is the first report of an increase in both lipid content and real
lipid productivity achieved through nutrient deprivation in an outdoor culture.
There are now efforts to enhance oil production through genetic engineering. Wiffjels
and Barbosa (2010) consider that knowledge of the biosynthesis of triacylglycerols and their
accumulation in lipid bodies would open the way to inducing lipid accumulation in oil
bodies without having to apply a stress factor. Radokovits and co-workers (2010) reviewed
the potential of genetic engineering to improve microalgae as a biofuel platform for the
production of biohydrogen, starch-derived alcohols, diesel fuel, and alkanes. While in its
infancy at present, advances in the development of genetic manipulation tools pave the way
for significant future progress.
Already there are a number of biodiscovery programmes searching for new hyper-lipid
producing strains (Mutanda et al ., 2011). This, in turn, has led to efforts to develop methods
for rapid screening techniques for naturally high lipid producers (Dean et al ., 2010).
Both the lipid content and fatty acid profile is first and foremost dependent on the class
of microalgae, with members of the green algae (Chlorophyceae) containing a high
proportion of 16:0 and C 18 unsaturated fatty acids with relatively low levels (or absence) of
longer chain fatty acids. Of the green microalgal C 18 fatty acids, usually either 18:2(n-6) or
18:3(n-3) can dominate. A few species from the genera Polytoma , Botryococcus and
Ankistrodesmus have 18:1 as the major C 18 fatty acid. Other microalgal classes that have