Alternative Green Biofuel from Microalgae: A Promising Renewable Resource 251
crops. Biodiesel is typically produced from vegetable oils such as rapeseed, canola, soybean, palm,
sunflower, mustard oil, and Karanja oil (Issariyakul and Dalai 2014). However, the use of food crops
for biodiesel has raised much debate and criticism involving food security concerns, for vegetable oils
compete with their use as food and require considerable use of arable lands and fresh water. The higher
prices of biodiesel compared to those of petrodiesel and the possible use of vegetable oils as food are
great limiting factors for their application in large scale ventures. Therefore, scientists and researchers
have focussed on waste materials as, for instance, animal fats (Meher et al. 2006), waste cooking or nonedible oils (Issariyakul and Dalai 2014) and free fatty acids containing oils (e.g., those from rice bran;
Srilatha et al. 2012) to prepare biodiesel. Nonetheless, these oils might not be appropriate as biodiesel due
to their high amount of saturated fatty acids. In addition, as most of them solidify at room temperature,
they cannot be used as fuel in a diesel engine in their original form (Leung et al. 2010).
Bioethanol, which is derived from edible biomass, primarily corn, soybeans, and sugarcane raised
concerns about the impact of first generation biofuel on food prices and increased deforestation (Cassman
and Liska 2007; Fargione et al. 2008). Therefore, attention has been diverted into second generation
feedstocks like lignocellulosic and Miscanthus biomass as well as agricultural and municipal waste
(Hattori and Morita 2010; Daroch and Mos 2011). Despite lignocellulosic feedstocks being cheaper than
first-generation feedstocks, they are more difficult to break down/or convert into small molecules like
ethanol than starch, sugar, or oils, and the technology to convert them into liquid fuels is at present not
cost-effective due to its resistance to saccharification (i.e., hydrolysis) usually caused by their high lignin
content. Consequently, there is currently a great effort to find alternative third generation feedstocks.
Fortunately, microalgae have been identified as a very good candidate for biofuel production (Schenk et al.
2008), being already cultivated as source of food-based and pharmaceuticals applications (Apt and Behrens
1999; Guedes et al. 2011; Virginie et al. 2012). Therefore, microalgae feedstocks are currently receiving
a lot of attention due to a multiplicity of reasons such as higher photosynthetic efficiencies and higher
growth rates, as compared to first and second-generation crops, ability of being cultivated on marginal
lands, continuous biomass production (i.e., day and night), the fact that they do not compete with food or
other crops, and the possibility of using saline and wastewater streams for biomass production (Schenk et
al. 2008). The other main advantages of microalgae-derived biofuels over the first and second-generation
biofuels are to produce considerably greater amounts of biomass including lipids, carbohydrates, and
proteins per hectare than any kind of terrestrial photoautotrophs (Chisti 2007). Therefore, the combined
potential of biofuel production, CO 2 fixation, and bio-treatment of wastewater emphasises the utilization
of microalgae as a promising feedstock for biodiesel, bioethanol, and biomethane, including bio-oil,
synthetic gas, as well as other valuable pharmaceutical and nutraceutical products.
Strengths and opportunities encompassing marine microalgae
Microalgae are a large and diverse group, ranging from unicellular to multi-cellular microorganisms,
including both prokaryotic microalgae (e.g., cyanobacteria) and eukaryotic microalgae such as diatoms
(Bacillariophyta), green (Chlorophyta), and red (Rhodophyta) algae. Microalgae are very important
from an ecological point of view, which are the food source for many animals and belonging to the
bottom of the food chain, being thus the main oxygen producers on earth. They were once considered
to be aquatic plants but are now classified separately because they lack true roots, stems, leaves, and
embryos. The simpler morphology of microalgae is perhaps one of their main advantages, as compared
to terrestrial crops, allowing them to grow faster. Photosynthetic growth of microalgae requires light,
water, and CO 2 to synthesise lipids, carbohydrates, nucleic acids, proteins, and other metabolites. The use
of a wide range of microalgae such as Phormidium, Arthrospira, Chlorella, Scenedesmus, Botryococcus,
and Chlamydomonas for treating domestic wastewater has been reported and efficacy of this method
is promising (Olguın 2003; Schulze et al. 2017a; Chinnasamy et al. 2010; Kong et al. 2010; Wang et
al. 2010). According to reported studies, to produce 1000 Kg of microalgal biomass, about 1800 Kg
CO 2 , 70 Kg N, 10 Kg P, and 8 Kg K are required (Chisti 2007; Wijffels and Barbosa 2010; Collet et al.
2011). Hence, microalgal biomass can contribute to the bio-fixation of atmospheric CO 2 , improving air
quality. Microalgae also exhibit other advantages when compared with terrestrial plants. Apart from
crops. Biodiesel is typically produced from vegetable oils such as rapeseed, canola, soybean, palm,
sunflower, mustard oil, and Karanja oil (Issariyakul and Dalai 2014). However, the use of food crops
for biodiesel has raised much debate and criticism involving food security concerns, for vegetable oils
compete with their use as food and require considerable use of arable lands and fresh water. The higher
prices of biodiesel compared to those of petrodiesel and the possible use of vegetable oils as food are
great limiting factors for their application in large scale ventures. Therefore, scientists and researchers
have focussed on waste materials as, for instance, animal fats (Meher et al. 2006), waste cooking or nonedible oils (Issariyakul and Dalai 2014) and free fatty acids containing oils (e.g., those from rice bran;
Srilatha et al. 2012) to prepare biodiesel. Nonetheless, these oils might not be appropriate as biodiesel due
to their high amount of saturated fatty acids. In addition, as most of them solidify at room temperature,
they cannot be used as fuel in a diesel engine in their original form (Leung et al. 2010).
Bioethanol, which is derived from edible biomass, primarily corn, soybeans, and sugarcane raised
concerns about the impact of first generation biofuel on food prices and increased deforestation (Cassman
and Liska 2007; Fargione et al. 2008). Therefore, attention has been diverted into second generation
feedstocks like lignocellulosic and Miscanthus biomass as well as agricultural and municipal waste
(Hattori and Morita 2010; Daroch and Mos 2011). Despite lignocellulosic feedstocks being cheaper than
first-generation feedstocks, they are more difficult to break down/or convert into small molecules like
ethanol than starch, sugar, or oils, and the technology to convert them into liquid fuels is at present not
cost-effective due to its resistance to saccharification (i.e., hydrolysis) usually caused by their high lignin
content. Consequently, there is currently a great effort to find alternative third generation feedstocks.
Fortunately, microalgae have been identified as a very good candidate for biofuel production (Schenk et al.
2008), being already cultivated as source of food-based and pharmaceuticals applications (Apt and Behrens
1999; Guedes et al. 2011; Virginie et al. 2012). Therefore, microalgae feedstocks are currently receiving
a lot of attention due to a multiplicity of reasons such as higher photosynthetic efficiencies and higher
growth rates, as compared to first and second-generation crops, ability of being cultivated on marginal
lands, continuous biomass production (i.e., day and night), the fact that they do not compete with food or
other crops, and the possibility of using saline and wastewater streams for biomass production (Schenk et
al. 2008). The other main advantages of microalgae-derived biofuels over the first and second-generation
biofuels are to produce considerably greater amounts of biomass including lipids, carbohydrates, and
proteins per hectare than any kind of terrestrial photoautotrophs (Chisti 2007). Therefore, the combined
potential of biofuel production, CO 2 fixation, and bio-treatment of wastewater emphasises the utilization
of microalgae as a promising feedstock for biodiesel, bioethanol, and biomethane, including bio-oil,
synthetic gas, as well as other valuable pharmaceutical and nutraceutical products.
Strengths and opportunities encompassing marine microalgae
Microalgae are a large and diverse group, ranging from unicellular to multi-cellular microorganisms,
including both prokaryotic microalgae (e.g., cyanobacteria) and eukaryotic microalgae such as diatoms
(Bacillariophyta), green (Chlorophyta), and red (Rhodophyta) algae. Microalgae are very important
from an ecological point of view, which are the food source for many animals and belonging to the
bottom of the food chain, being thus the main oxygen producers on earth. They were once considered
to be aquatic plants but are now classified separately because they lack true roots, stems, leaves, and
embryos. The simpler morphology of microalgae is perhaps one of their main advantages, as compared
to terrestrial crops, allowing them to grow faster. Photosynthetic growth of microalgae requires light,
water, and CO 2 to synthesise lipids, carbohydrates, nucleic acids, proteins, and other metabolites. The use
of a wide range of microalgae such as Phormidium, Arthrospira, Chlorella, Scenedesmus, Botryococcus,
and Chlamydomonas for treating domestic wastewater has been reported and efficacy of this method
is promising (Olguın 2003; Schulze et al. 2017a; Chinnasamy et al. 2010; Kong et al. 2010; Wang et
al. 2010). According to reported studies, to produce 1000 Kg of microalgal biomass, about 1800 Kg
CO 2 , 70 Kg N, 10 Kg P, and 8 Kg K are required (Chisti 2007; Wijffels and Barbosa 2010; Collet et al.
2011). Hence, microalgal biomass can contribute to the bio-fixation of atmospheric CO 2 , improving air
quality. Microalgae also exhibit other advantages when compared with terrestrial plants. Apart from