Microalgal Metabolism and their Utilisation 53
Growth-limiting conditions, especially N-limiting conditions in the light where continuing
photosynthetic carbon fixation result in an increased cellular C/N ratio mean that the photosyntheticallyfixed carbon cannot be metabolised to N-containing compounds such as proteins. Therefore, this
photosynthetically fixed carbon either must be stored or excreted from the cell. Clearly, there are
advantages in storing the energy rich organic carbon for later re-metabolism when growth conditions
improve (Pohl and Zurheide 1979; Harwood and Jones 1989), but the carbon storage product must be
in a form which does not negatively affect other cellular processes such as enzyme activities or osmotic
potential. Two main options are possible: storage as starch or storage in the form on neutral lipids (TAGs),
or a combination of both. For biofuels production, algae with a high content of TAG are desirable.
Saturated acyl moieties are preferred for this purpose because they require less energy to synthesise
than PUFA and provide more energy on oxidation (Cohen and Khozin-Goldberg 2010). However, some
algae such a P. incisa and P. cruentum also store appreciable amounts of PUFAs in the TAG. Cohen and
Khozin-Goldberg (2010) have hypothesised that this is so because these algae species live in ecological
niches subject to rapid short-term fluctuations such as in temperature or salinity where significant and
rapid alterations in the fatty acid and molecular species composition of chloroplast membrane lipids to
maintain membrane fluidity are required. In these situations, the TAG may also have a role as a ‘buffering’
agent, where the PUFA moieties are mobilised for the construction and rapid alteration of chloroplast
membranes.
The synthesis of TAGs under conditions of N-deprivation requires photosynthesis and an adequate
supply of inorganic carbon. Providing either additional CO 2 or bicarbonate leads to stimulation of TAG
accumulation (Gardner et al. 2012; White et al. 2013). For example, in D. salina, a 1-day long increase
in CO 2 concentration from 2% to 10% increased the total fatty acids on a dry weight basis by 30%
(Muradyan et al. 2004). This was due mainly to de novo fatty acid synthesis with the elongation and
desaturation of the fatty acids being inhibited, leading to an increase in the relative content of saturated
fatty acids at high CO 2 . In Dunaliella viridis however, the addition of CO 2 increased the total lipid content
only under N-limiting conditions (Gordillo et al. 1998).
The challenge to commercially viable production of lipid-rich algae for biofuels or PUFA-rich algal
lipids for nutritional use however is still a significant one, and will require the selection of strains best
suited for commercial-scale culture and the manipulation of the algal physiology and metabolism by the
use of appropriate culture management to maximise lipid productivity. Genetic modification of metabolic
pathways to enhance lipid productivity is also an option.
Optimising productivity of outdoor cultures
Commercial production of microalgae is generally carried out in outdoor cultures, mainly in open
raceway ponds, although there are a few producers using closed photobioreactors for the production
of very high value products. The most important limiting factors to growth and productivity are light,
carbon supply, oxygen, and temperature (Borowitzka 1998). Furthermore, the factors affecting growth
(biomass production) and those affecting the production of the desired products may not be the same. In
outdoor cultures, optimisation is complicated by the fact that all these parameters vary both over the day
and also over the year and, unlike most laboratory cultures, algae cultures outdoors are never in a steady
state. The level of control of the growth environment in large-scale cultures is limited. The provision of
optimum nutrient levels is relatively easy to achieve and inorganic C can be provided in the form of CO 2
or bicarbonate, although this is often uneconomical. Temperature generally cannot be regulated, but the
irradiance received by the algae cells can be managed to some degree.
The productivity of outdoor cultures is strongly correlated with the available irradiance and the
efficient use of the available irradiance is a key target for optimising culture productivity (Richmond
1996). In dilute cultures (i.e., cultures with low cell densities) at optimal growth conditions of optimum
temperature and non-limiting nutrients, the photosynthetic rate of the algae with relation to irradiance
follows the classical text-book light response curve (P/E curve). However, dilute cultures have a low
productivity as the productivity is a function of the specific growth rate (μ, time
–1
) times the cell density;
that is, Productivity = µ x cell density (g.L
–1
). Thus, in order to achieve high productivities, high cell
Growth-limiting conditions, especially N-limiting conditions in the light where continuing
photosynthetic carbon fixation result in an increased cellular C/N ratio mean that the photosyntheticallyfixed carbon cannot be metabolised to N-containing compounds such as proteins. Therefore, this
photosynthetically fixed carbon either must be stored or excreted from the cell. Clearly, there are
advantages in storing the energy rich organic carbon for later re-metabolism when growth conditions
improve (Pohl and Zurheide 1979; Harwood and Jones 1989), but the carbon storage product must be
in a form which does not negatively affect other cellular processes such as enzyme activities or osmotic
potential. Two main options are possible: storage as starch or storage in the form on neutral lipids (TAGs),
or a combination of both. For biofuels production, algae with a high content of TAG are desirable.
Saturated acyl moieties are preferred for this purpose because they require less energy to synthesise
than PUFA and provide more energy on oxidation (Cohen and Khozin-Goldberg 2010). However, some
algae such a P. incisa and P. cruentum also store appreciable amounts of PUFAs in the TAG. Cohen and
Khozin-Goldberg (2010) have hypothesised that this is so because these algae species live in ecological
niches subject to rapid short-term fluctuations such as in temperature or salinity where significant and
rapid alterations in the fatty acid and molecular species composition of chloroplast membrane lipids to
maintain membrane fluidity are required. In these situations, the TAG may also have a role as a ‘buffering’
agent, where the PUFA moieties are mobilised for the construction and rapid alteration of chloroplast
membranes.
The synthesis of TAGs under conditions of N-deprivation requires photosynthesis and an adequate
supply of inorganic carbon. Providing either additional CO 2 or bicarbonate leads to stimulation of TAG
accumulation (Gardner et al. 2012; White et al. 2013). For example, in D. salina, a 1-day long increase
in CO 2 concentration from 2% to 10% increased the total fatty acids on a dry weight basis by 30%
(Muradyan et al. 2004). This was due mainly to de novo fatty acid synthesis with the elongation and
desaturation of the fatty acids being inhibited, leading to an increase in the relative content of saturated
fatty acids at high CO 2 . In Dunaliella viridis however, the addition of CO 2 increased the total lipid content
only under N-limiting conditions (Gordillo et al. 1998).
The challenge to commercially viable production of lipid-rich algae for biofuels or PUFA-rich algal
lipids for nutritional use however is still a significant one, and will require the selection of strains best
suited for commercial-scale culture and the manipulation of the algal physiology and metabolism by the
use of appropriate culture management to maximise lipid productivity. Genetic modification of metabolic
pathways to enhance lipid productivity is also an option.
Optimising productivity of outdoor cultures
Commercial production of microalgae is generally carried out in outdoor cultures, mainly in open
raceway ponds, although there are a few producers using closed photobioreactors for the production
of very high value products. The most important limiting factors to growth and productivity are light,
carbon supply, oxygen, and temperature (Borowitzka 1998). Furthermore, the factors affecting growth
(biomass production) and those affecting the production of the desired products may not be the same. In
outdoor cultures, optimisation is complicated by the fact that all these parameters vary both over the day
and also over the year and, unlike most laboratory cultures, algae cultures outdoors are never in a steady
state. The level of control of the growth environment in large-scale cultures is limited. The provision of
optimum nutrient levels is relatively easy to achieve and inorganic C can be provided in the form of CO 2
or bicarbonate, although this is often uneconomical. Temperature generally cannot be regulated, but the
irradiance received by the algae cells can be managed to some degree.
The productivity of outdoor cultures is strongly correlated with the available irradiance and the
efficient use of the available irradiance is a key target for optimising culture productivity (Richmond
1996). In dilute cultures (i.e., cultures with low cell densities) at optimal growth conditions of optimum
temperature and non-limiting nutrients, the photosynthetic rate of the algae with relation to irradiance
follows the classical text-book light response curve (P/E curve). However, dilute cultures have a low
productivity as the productivity is a function of the specific growth rate (μ, time
–1
) times the cell density;
that is, Productivity = µ x cell density (g.L
–1
). Thus, in order to achieve high productivities, high cell