Alternative Green Biofuel from Microalgae: A Promising Renewable Resource 253
low fat content. The cultivation of Arthrospira sp. is possible at high salinity and high pH. Other studies
have reported the use of microalgae (e.g., Arthrospira) in animal feed (Belay et al. 1996), Arthrospira as
food additive (Erica 1996) and Porphyridium sp. as feed supplement for poultry (Ginzberg et al. 2000).
Moreover, cyanobacteria have been used as feed for aquaculture based on their nutritional and non-toxic
performance (Thajuddin and Subramanian 2005). Chlorella sp. has a high amount of chlorophyll among
various species of microalgae (Dring 1991; Nakanishi 2001; Deng et al. 2008). Microalgae such as
cyanobacteria, are also native producers of poly-β-hydroxybutyrate (PHB), providing a viable potential
alternative to plastic carry bags made from petroleum-derived polymers (Brandl et al. 1995).
Cultivation and harvesting
Cultivation
The main way to produce microalgae biomass is by cultivation, which can be carried out in two different
systems: open (e.g., raceway ponds) and closed (e.g., photobioreactors) systems. Several methods have
been developed for the optimization of cultivation (Li et al. 2008; Wang and Lan 2011), and harvesting
process for the production of biomass (Singh et al. 2011). Currently, industrial microalgae cultivation
system is mostly carried out in open ponds due to their low construction and capital costs. However,
it has some drawbacks such as lack of control of operational conditions, leading to water evaporation
and contaminations with unwanted species, inefficient exposure of microalgal cells to sunlight and
CO 2 , due to poor mixing, thus sustaining low biomass yields (Suh and Lee 2003). Although sunlight in
outdoor systems is free and abundant, seasonal variations of sunlight can significantly limit productivity
(Borowitzka 1999). On the other hand, the main advantages of using photobioreactors (i.e., closed
systems) are linked to a better control of the microalgal culture and its environment, large surface to
volume ratios, less water evaporation, better isolation from outside contaminations, and higher biomass
productivity.
Microalgae can be grown in various types of water like fresh, salt, waste, or brackish water and
with as well as without an organic carbon source. Depending upon their carbon metabolism, microalgae
can be divided into three types: (i) phototrophic, (ii) heterotrophic, and (iii) mixotrophic. Phototrophic
microalgae utilize light as energy source and CO 2 as an inorganic carbon source from atmosphere and flue
gases, whereas heterotrophic growth is directly independent of light, utilizing organic compounds (i.e.,
glucose, acetate, glycerol, fructose, sucrose, lactose, galactose, mannose, and corn powder hydrolysate
instead of sugars) as both energy and carbon sources (Mata et al. 2010). Mixotrophic microalgae are
capable of growing both photo- or heterotrophically. Most of the microalgal strains can shift from
phototrophic to heterotrophic growth, and some can grow mixotrophically (Carlsson et al. 2007). To
date, only phototrophic cultivation is technically and economically viable to cultivate microalgae in large
scale, preferentially using seawater (Matsunaga et al. 2005). High salinity helps prevent the contamination
of culture media, allowing seawater to be directly used, instead of depleting freshwater resources,
simultaneously mitigating CO 2 from flue gases and removing/or re-using nutrients from wastewater and
water returned to the growth facility. Several parameters are important to consider for microalgal growth,
namely light, nutrient availability, temperature, pH, and culture mixing.
Light
Light is a source of energy for microalgal growth to produce lipids, carbohydrates, and proteins from
CO 2 and H 2 O. Due to its simpler morphology, metabolism, and development, microalgae achieve
higher photosynthetic efficiency (PE) than terrestrial plants (Vasudevan and Briggs 2008). The use of
natural light sources for commercial microalgal production has the advantage of using sunlight freely
(Janssen et al. 2003). Light intensity plays an important role in microalgal photosynthesis, as exposure
of cells to long periods of high light intensity causes photoinhibition, which leads to the formation of
free radicals, causing photo oxidative damage (Rubio et al. 2003; Torzillo et al. 2003). Saturation light
intensity determines the light utilization efficiency and overall photosynthetic efficiency (Torzillo et
al. 2003). Sunlight within the range of 400–700 nm is photosynthetic active radiation (PAR), which
low fat content. The cultivation of Arthrospira sp. is possible at high salinity and high pH. Other studies
have reported the use of microalgae (e.g., Arthrospira) in animal feed (Belay et al. 1996), Arthrospira as
food additive (Erica 1996) and Porphyridium sp. as feed supplement for poultry (Ginzberg et al. 2000).
Moreover, cyanobacteria have been used as feed for aquaculture based on their nutritional and non-toxic
performance (Thajuddin and Subramanian 2005). Chlorella sp. has a high amount of chlorophyll among
various species of microalgae (Dring 1991; Nakanishi 2001; Deng et al. 2008). Microalgae such as
cyanobacteria, are also native producers of poly-β-hydroxybutyrate (PHB), providing a viable potential
alternative to plastic carry bags made from petroleum-derived polymers (Brandl et al. 1995).
Cultivation and harvesting
Cultivation
The main way to produce microalgae biomass is by cultivation, which can be carried out in two different
systems: open (e.g., raceway ponds) and closed (e.g., photobioreactors) systems. Several methods have
been developed for the optimization of cultivation (Li et al. 2008; Wang and Lan 2011), and harvesting
process for the production of biomass (Singh et al. 2011). Currently, industrial microalgae cultivation
system is mostly carried out in open ponds due to their low construction and capital costs. However,
it has some drawbacks such as lack of control of operational conditions, leading to water evaporation
and contaminations with unwanted species, inefficient exposure of microalgal cells to sunlight and
CO 2 , due to poor mixing, thus sustaining low biomass yields (Suh and Lee 2003). Although sunlight in
outdoor systems is free and abundant, seasonal variations of sunlight can significantly limit productivity
(Borowitzka 1999). On the other hand, the main advantages of using photobioreactors (i.e., closed
systems) are linked to a better control of the microalgal culture and its environment, large surface to
volume ratios, less water evaporation, better isolation from outside contaminations, and higher biomass
productivity.
Microalgae can be grown in various types of water like fresh, salt, waste, or brackish water and
with as well as without an organic carbon source. Depending upon their carbon metabolism, microalgae
can be divided into three types: (i) phototrophic, (ii) heterotrophic, and (iii) mixotrophic. Phototrophic
microalgae utilize light as energy source and CO 2 as an inorganic carbon source from atmosphere and flue
gases, whereas heterotrophic growth is directly independent of light, utilizing organic compounds (i.e.,
glucose, acetate, glycerol, fructose, sucrose, lactose, galactose, mannose, and corn powder hydrolysate
instead of sugars) as both energy and carbon sources (Mata et al. 2010). Mixotrophic microalgae are
capable of growing both photo- or heterotrophically. Most of the microalgal strains can shift from
phototrophic to heterotrophic growth, and some can grow mixotrophically (Carlsson et al. 2007). To
date, only phototrophic cultivation is technically and economically viable to cultivate microalgae in large
scale, preferentially using seawater (Matsunaga et al. 2005). High salinity helps prevent the contamination
of culture media, allowing seawater to be directly used, instead of depleting freshwater resources,
simultaneously mitigating CO 2 from flue gases and removing/or re-using nutrients from wastewater and
water returned to the growth facility. Several parameters are important to consider for microalgal growth,
namely light, nutrient availability, temperature, pH, and culture mixing.
Light
Light is a source of energy for microalgal growth to produce lipids, carbohydrates, and proteins from
CO 2 and H 2 O. Due to its simpler morphology, metabolism, and development, microalgae achieve
higher photosynthetic efficiency (PE) than terrestrial plants (Vasudevan and Briggs 2008). The use of
natural light sources for commercial microalgal production has the advantage of using sunlight freely
(Janssen et al. 2003). Light intensity plays an important role in microalgal photosynthesis, as exposure
of cells to long periods of high light intensity causes photoinhibition, which leads to the formation of
free radicals, causing photo oxidative damage (Rubio et al. 2003; Torzillo et al. 2003). Saturation light
intensity determines the light utilization efficiency and overall photosynthetic efficiency (Torzillo et
al. 2003). Sunlight within the range of 400–700 nm is photosynthetic active radiation (PAR), which