52 Marine Macro- and Microalgae: An Overview
and is usually carried out in closed photobioreactors to minimise contamination. This is then followed by
a second stage, often in open ponds, in which the cells are ‘stressed’ (high light, higher temperatures) to
induce aplanospore and concomitant astaxanthin formation.
Lipids, fatty acids, and biofuels
Microalgae can accumulate high levels of lipids and are existing and potential future sources of a
number of fatty acids with applications in human and animal nutrition. The fatty acids of most interest
are the very long-chain polyunsaturated fatty acids arachidonic (AA; C20:4n-6), eicosapentaenoic acid
(EPA; C20:5n-3), docosahexaenoic (DHA; C22:6n-3). The high lipid content and productivity of some
microalgae has also made them attractive targets for the development of renewable biofuels (Griffiths and
Harrison 2009; Fon Sing et al. 2013). Lipid and fatty acid composition and metabolism in algae and their
manipulation has been extensively reviewed in recent years (Guschina and Harwood 2006; Harwood and
Guschina 2009; Cohen and Ratledge 2010; Guschina and Harwood 2013) in much greater detail than can
be covered here.
The total lipid content of microalgae ranges from about 10–50% of organic dry weight. Since the
original finding that nitrogen limitation increased the lipid content in Chlorella (Aach 1952), many
studies have shown that nutrient limitation, especially N limitation can lead to an increase in lipid
content, especially triacylglycerol (TAG) content in many species of microalgae. In diatoms, Si limitation
also leads to increased TAG formation. In most cases the increase in TAG is accompanied by reduced
growth. Some species of algae such as members of the genus Tetraselmis, usually have their highest
total lipid content in the exponential phase of growth and the lipid content declines in the stationary
phase (Mercz 1994). Tetraselmis spp. therefore tend to have higher lipid productivity than many other
microalgae (Barclay et al. 2005; Huerlimann et al. 2010). The effect of P limitation on lipid accumulation
varies. For example, in Phaeodactylum tricornutum and Chaetoceros sp. (Bacillariophyceae), Isochrysis
galbana (clone T-Iso) and Pavlova lutheri (Prymnesiophyceae), P-limitation increased the lipid content,
whereas in Nannochloris atomus (Chlorophyceae) and Tetraselmis sp., it decreased (Reitan et al. 1994).
Lipid content can be increased by providing additional CO 2 to algae cultures which are carbon limited
(Muradyan et al. 2004; Tang et al. 2011; Moheimani 2012). High lipid productivities also have been
obtained in a number of microalgae grown heterotrophically (Xiong et al. 2008) or mixotrophically (Li et
al. 2011; Kong et al. 2012; Zhao et al. 2012).
The fatty acid composition of microalgae varies between taxa and, to a lesser degree, also between
species (Borowitzka 1988b). Thus, the lipids of diatoms, haptophtes, and eustigmatophytes and the red
unicell Porphyridium cruentum are particularly rich in EPA, whereas the highest content of DHA is found
in thraustrochytrid algae such as Crypthecodinium cohnii and Schizochytrium spp., and the freshwater
trebouxiophyte Parietochloris incisa contains exceptionally high amounts of AA (Bowles et al. 1999;
Bigogno et al. 2002; Guschina and Harwood 2006). The content of these long-chain polyunsaturated fatty
acids (PUFAs) are affected by environmental factors and can be manipulated by growth conditions and the
composition of the medium (e.g., Carvalho et al. 2006), however these effects are very variable between
species and the results of studies on the same species are sometimes inconsistent making it difficult
to generalise. For example, Chrismadha and Borowitzka (1994) found that CO 2 addition reduced the
level of EPA in the cells of Phaeodactylum tricornutum, as did Carvalho and Malcata (2005) in Pavlova
lutheri, whereas Yongmanitchai and Ward (1991) found the opposite. These differences in the findings
might be due to culture conditions such as available light, or due to strain differences. However, despite
this reservation, some broad generalisations can be made. Nutrient limitations (especially N as well as Si
in diatoms) generally lead to an increase in non-polar storage lipids with a concomitant increase in C16
and C18 fatty acids (Arisz et al. 2000; Lynn et al. 2000). High light generally leads to the production of
TAG in many microalgae (Roessler 1990; Sukenik 1999) and higher temperatures increase the degree
of unsaturation (Materassi et al. 1980). However, in Parietochloris incisa FA synthesis, mainly as TAG,
was lower under high light in N replete and more so in N-starved cultures. For optimum lipid synthesis,
intermediate irradiances are required (Solovchenko et al. 2008). The red unicell Porphyridium cruentum
shows a similar response (Cohen et al. 1988).
and is usually carried out in closed photobioreactors to minimise contamination. This is then followed by
a second stage, often in open ponds, in which the cells are ‘stressed’ (high light, higher temperatures) to
induce aplanospore and concomitant astaxanthin formation.
Lipids, fatty acids, and biofuels
Microalgae can accumulate high levels of lipids and are existing and potential future sources of a
number of fatty acids with applications in human and animal nutrition. The fatty acids of most interest
are the very long-chain polyunsaturated fatty acids arachidonic (AA; C20:4n-6), eicosapentaenoic acid
(EPA; C20:5n-3), docosahexaenoic (DHA; C22:6n-3). The high lipid content and productivity of some
microalgae has also made them attractive targets for the development of renewable biofuels (Griffiths and
Harrison 2009; Fon Sing et al. 2013). Lipid and fatty acid composition and metabolism in algae and their
manipulation has been extensively reviewed in recent years (Guschina and Harwood 2006; Harwood and
Guschina 2009; Cohen and Ratledge 2010; Guschina and Harwood 2013) in much greater detail than can
be covered here.
The total lipid content of microalgae ranges from about 10–50% of organic dry weight. Since the
original finding that nitrogen limitation increased the lipid content in Chlorella (Aach 1952), many
studies have shown that nutrient limitation, especially N limitation can lead to an increase in lipid
content, especially triacylglycerol (TAG) content in many species of microalgae. In diatoms, Si limitation
also leads to increased TAG formation. In most cases the increase in TAG is accompanied by reduced
growth. Some species of algae such as members of the genus Tetraselmis, usually have their highest
total lipid content in the exponential phase of growth and the lipid content declines in the stationary
phase (Mercz 1994). Tetraselmis spp. therefore tend to have higher lipid productivity than many other
microalgae (Barclay et al. 2005; Huerlimann et al. 2010). The effect of P limitation on lipid accumulation
varies. For example, in Phaeodactylum tricornutum and Chaetoceros sp. (Bacillariophyceae), Isochrysis
galbana (clone T-Iso) and Pavlova lutheri (Prymnesiophyceae), P-limitation increased the lipid content,
whereas in Nannochloris atomus (Chlorophyceae) and Tetraselmis sp., it decreased (Reitan et al. 1994).
Lipid content can be increased by providing additional CO 2 to algae cultures which are carbon limited
(Muradyan et al. 2004; Tang et al. 2011; Moheimani 2012). High lipid productivities also have been
obtained in a number of microalgae grown heterotrophically (Xiong et al. 2008) or mixotrophically (Li et
al. 2011; Kong et al. 2012; Zhao et al. 2012).
The fatty acid composition of microalgae varies between taxa and, to a lesser degree, also between
species (Borowitzka 1988b). Thus, the lipids of diatoms, haptophtes, and eustigmatophytes and the red
unicell Porphyridium cruentum are particularly rich in EPA, whereas the highest content of DHA is found
in thraustrochytrid algae such as Crypthecodinium cohnii and Schizochytrium spp., and the freshwater
trebouxiophyte Parietochloris incisa contains exceptionally high amounts of AA (Bowles et al. 1999;
Bigogno et al. 2002; Guschina and Harwood 2006). The content of these long-chain polyunsaturated fatty
acids (PUFAs) are affected by environmental factors and can be manipulated by growth conditions and the
composition of the medium (e.g., Carvalho et al. 2006), however these effects are very variable between
species and the results of studies on the same species are sometimes inconsistent making it difficult
to generalise. For example, Chrismadha and Borowitzka (1994) found that CO 2 addition reduced the
level of EPA in the cells of Phaeodactylum tricornutum, as did Carvalho and Malcata (2005) in Pavlova
lutheri, whereas Yongmanitchai and Ward (1991) found the opposite. These differences in the findings
might be due to culture conditions such as available light, or due to strain differences. However, despite
this reservation, some broad generalisations can be made. Nutrient limitations (especially N as well as Si
in diatoms) generally lead to an increase in non-polar storage lipids with a concomitant increase in C16
and C18 fatty acids (Arisz et al. 2000; Lynn et al. 2000). High light generally leads to the production of
TAG in many microalgae (Roessler 1990; Sukenik 1999) and higher temperatures increase the degree
of unsaturation (Materassi et al. 1980). However, in Parietochloris incisa FA synthesis, mainly as TAG,
was lower under high light in N replete and more so in N-starved cultures. For optimum lipid synthesis,
intermediate irradiances are required (Solovchenko et al. 2008). The red unicell Porphyridium cruentum
shows a similar response (Cohen et al. 1988).