Alternative Green Biofuel from Microalgae: A Promising Renewable Resource 255
been found to have a major effect on the fatty acid synthesis of microalgae (Guschina and Harwood
2006; Dean et al. 2010; Sharma et al. 2012). It has been reported that as temperature increases there
is an increase in saturated fatty acids (Sushchik et al. 2003), whereas when temperature are lower the
accumulation of unsaturated fatty acids is induced (Hu et al. 2008). Some studies have also reported that
carbohydrate content increases with temperature (Oliveira et al. 1999; Sharma et al. 2012).
pH
Most of microalgae are sensitive to pH changes, its control being essential for achieving high growth
rates. As microalgae are able to capture CO 2 , there is a trend for pH to increase. Most microalgae (e.g.,
Nannochloropsis salina) usually grow faster at pH values between 8.0 and 9.0 (Bartley et al. 2014).
Nonetheless, some species withstand and thrive at lower (Chlorococcum littorale) and higher pH
(Arthrospira platensis) values. For example, A. platensis grows well at 11.5 but not at 7.0; however,
productivity decreases significantly at pH higher than 9 (Fontes et al. 1987; Kodama et al. 1993; Vieira
et al. 2004). In the case of high-density microalgal cultures in controlled systems using air enriched
with carbon dioxide (CO 2 from flue gas or in pure form), the concentration of dissolved CO 2 may be the
dominant factor determining the culture pH (Sanchez et al. 2003). On the other hand, the dissolved CO 2
is a result of balance between the mass transfer of CO 2 from gas to liquid phase and the consumption
of CO 2 by culture cells. In contrast, microalgae have shown to cause a rise at pH from 10 to 11 in open
ponds because of CO 2 mitigation (Oswald 1988). This increase in pH can be beneficial for inactivation
of pathogens in microalgal wastewater treatment, but can also inhibit microalgal growth. With elevated
CO 2 concentrations, pH drops down to pH 5, and with higher SOx concentrations pH values as low as 2.6
have been reported (Maeda et al. 1995), which can cause growth inhibition. With buffered medium, the
pH drop can be avoided, also preventing changes in the growth rate caused by SOx (Maeda et al. 1995).
Therefore, the pH control mechanisms need to be integrated with the aeration system by adding buffer to
the culture (Qiu et al. 2017).
Mixing
Mixing is an important factor that (i) promotes gas exchange in a microalgal culture, (ii) uniforms CO 2
distribution, (iii) prevents sedimentation of culture cells, (iv) optimises exposure to light and nutrients,
and (v) helps remove excess oxygen (Pulz 2001). A high concentration of dissolved oxygen is toxic to
microalgal cells, and continuous exposure to light generates oxygen radicals, which can lead to cell
damage and growth inhibition.
Harvesting
The term harvesting refers to the concentration of diluted algae suspensions to a thick paste and it may
account for up to 20–30% (Borodyanski and Konstantinov 2002; Mata et al. 2010) of the total production
costs. Microalgae with cell/filament sizes around 200 µm appear to be suitable candidates for cultivation
and harvesting, compared to other smaller microalgae with sizes ranging between 0.5 to 30 µm (Molina
et al. 2003; Waterbury 2006; Pereira et al. 2016). Microalgae with large cells or filaments can help
reduce the harvesting problem, as they may be harvested with relative ease by filtration. For example, the
cyanobacterium Arthrospira forms along spiral shape filament (trichome) with lengths varying, in most
cases, between 20–100 µm, naturally allowing the use of more cost-efficient, energy-efficient harvesting
methods (Benemann and Oswald 1996). However, the main drawback of using oleaginous microalgae for
biomass production is that they are generally unicellular and are in suspension, thus being very difficult
to harvest (Moreno 2008). Generally, a two-stage harvesting process, involving dewatering followed by
thickening, can be quite costly if an additional drying step is needed (Barros et al. 2015). As a universal
harvesting technique does not exist, intense research has been carried out to find suitable processes for
specific microalgae.
been found to have a major effect on the fatty acid synthesis of microalgae (Guschina and Harwood
2006; Dean et al. 2010; Sharma et al. 2012). It has been reported that as temperature increases there
is an increase in saturated fatty acids (Sushchik et al. 2003), whereas when temperature are lower the
accumulation of unsaturated fatty acids is induced (Hu et al. 2008). Some studies have also reported that
carbohydrate content increases with temperature (Oliveira et al. 1999; Sharma et al. 2012).
pH
Most of microalgae are sensitive to pH changes, its control being essential for achieving high growth
rates. As microalgae are able to capture CO 2 , there is a trend for pH to increase. Most microalgae (e.g.,
Nannochloropsis salina) usually grow faster at pH values between 8.0 and 9.0 (Bartley et al. 2014).
Nonetheless, some species withstand and thrive at lower (Chlorococcum littorale) and higher pH
(Arthrospira platensis) values. For example, A. platensis grows well at 11.5 but not at 7.0; however,
productivity decreases significantly at pH higher than 9 (Fontes et al. 1987; Kodama et al. 1993; Vieira
et al. 2004). In the case of high-density microalgal cultures in controlled systems using air enriched
with carbon dioxide (CO 2 from flue gas or in pure form), the concentration of dissolved CO 2 may be the
dominant factor determining the culture pH (Sanchez et al. 2003). On the other hand, the dissolved CO 2
is a result of balance between the mass transfer of CO 2 from gas to liquid phase and the consumption
of CO 2 by culture cells. In contrast, microalgae have shown to cause a rise at pH from 10 to 11 in open
ponds because of CO 2 mitigation (Oswald 1988). This increase in pH can be beneficial for inactivation
of pathogens in microalgal wastewater treatment, but can also inhibit microalgal growth. With elevated
CO 2 concentrations, pH drops down to pH 5, and with higher SOx concentrations pH values as low as 2.6
have been reported (Maeda et al. 1995), which can cause growth inhibition. With buffered medium, the
pH drop can be avoided, also preventing changes in the growth rate caused by SOx (Maeda et al. 1995).
Therefore, the pH control mechanisms need to be integrated with the aeration system by adding buffer to
the culture (Qiu et al. 2017).
Mixing
Mixing is an important factor that (i) promotes gas exchange in a microalgal culture, (ii) uniforms CO 2
distribution, (iii) prevents sedimentation of culture cells, (iv) optimises exposure to light and nutrients,
and (v) helps remove excess oxygen (Pulz 2001). A high concentration of dissolved oxygen is toxic to
microalgal cells, and continuous exposure to light generates oxygen radicals, which can lead to cell
damage and growth inhibition.
Harvesting
The term harvesting refers to the concentration of diluted algae suspensions to a thick paste and it may
account for up to 20–30% (Borodyanski and Konstantinov 2002; Mata et al. 2010) of the total production
costs. Microalgae with cell/filament sizes around 200 µm appear to be suitable candidates for cultivation
and harvesting, compared to other smaller microalgae with sizes ranging between 0.5 to 30 µm (Molina
et al. 2003; Waterbury 2006; Pereira et al. 2016). Microalgae with large cells or filaments can help
reduce the harvesting problem, as they may be harvested with relative ease by filtration. For example, the
cyanobacterium Arthrospira forms along spiral shape filament (trichome) with lengths varying, in most
cases, between 20–100 µm, naturally allowing the use of more cost-efficient, energy-efficient harvesting
methods (Benemann and Oswald 1996). However, the main drawback of using oleaginous microalgae for
biomass production is that they are generally unicellular and are in suspension, thus being very difficult
to harvest (Moreno 2008). Generally, a two-stage harvesting process, involving dewatering followed by
thickening, can be quite costly if an additional drying step is needed (Barros et al. 2015). As a universal
harvesting technique does not exist, intense research has been carried out to find suitable processes for
specific microalgae.