Alternative Green Biofuel from Microalgae: A Promising Renewable Resource 263
immobilized Clostridium acetobutylicum cells has been studied (Efremenko et al. 2012), which offers
promising multiple applications using immobilized biocatalysts for converting waste/microalgal biomass
into biofuel. Recently, Mychonastes afer PKUAC9 and Scenedesmus abundans PKUAC12 were tested
for saccharification followed by fermentative bioethanol production. S. abundans exhibited the highest
production of total glucose (5.730 g/L) and sugars hydrolysates (10.752 g/L) as compared to M. afer.
Chemo-enzymatic treated S. abundans was the best feedstock for fermentative production (0.103 g/g dry
weight) of bioethanol (Hui et al. 2013).
Biogas production
Biogas is the biofuel substitute for natural gas, being produced by anaerobic (i.e., in the absence of
oxygen/or in an oxygen-deficient environment) digestion or fermentation of biodegradable materials
such as cow manure, municipal waste, plant residues, and microalgal biomass. For example, as largescale production of microalgal-based biodiesel is still limited by the downstream costs of lipid extraction
(Collet et al. 2011), there is a need to explore other energy uses of spent microalgae biomass. The absence
of lignin makes microalgae ideal candidates for efficient biomethane production by fermentation in
biogas manufactures. Thus, microalgae are not only a possible source of biodiesel (Amaro et al. 2011),
but also as a source of other types of biofuels: bioethanol, biomethane, hydrogen, and butanol (Brennan
and Owende 2010; Varfolomeev et al. 2010; Parmar et al. 2011; Varfolomeev and Wasserman 2011). The
coupled process of microalgae cultivation and biogas production may be a better option than biodiesel
production alone (Collet et al. 2011). Upon oil extraction, the spent biomass can be used to produce biogas
and/or biofertilisers (Jasvinder and Gu 2010). In addition to being renewable, this would encourage
sustainable farming procedures, providing higher efficiencies and reduce production costs (Vergara et al.
2008). Generally, anaerobic digestion can be divided into four types: (i) hydrolysis (ii) acidogenesis, (iii)
acetogenesis, and (iv) methanogenesis. Methanogenic digestion is widely used in digestion of organic
waste. First, in hydrolysis, the complex materials are degraded into small particle such as soluble sugars,
which are later fermented to alcohols and acetic acid and/or volatile fatty acids and other by-products like
hydrogen and carbon dioxide, which are metabolized to biogas by methanogens. The biogas production
from this anaerobic digestion process is primarily affected by organic loadings, pH, temperature, and
retention time in reactors (Harun et al. 2010). Mainly long solid retention time and high organic loading
rate give significant results in terms of high methane yield (Chynoweth 2005). In addition, anaerobic
digestion can operate in either mesophilic (35ºC) or thermophilic (55ºC) conditions (Otsuka and Yoshino
2004). Biomethane production by anaerobic digestion of microalgal biomass has been reported for
freshwater and marine microalgae in various combinations (Aino et al. 2013). Net energy analysis of
two production systems for producing biodiesel and biogas from Nannochloropsis and Haematococcus
pluvialis, grown in a raceway in salt and freshwater, respectively, were studied (Luis and Tan 2011). Life
cycle assessment of the system has shown that biomethane production without greenhouse gas heating
would have a net energy ratio of 1.54, which is slightly lower than that of biomethane from ley crop
(1.78) (Aino et al. 2013). Biogas production from microalgae change due to variation in cellular lipid,
carbohydrate and protein content, cell wall structure, cultivation, and digestion temperature (Aino et
al. 2013). Alternatively, biomethane, as well as bio-oil, can be produced from Emiliania huxleyi via
direct pyrolysis (Wu et al. 1999), thus, making this coccolithophore a promising candidate for biofuel
production. Methanation of syngas produced from gasification of microalgal biomass is another route to
produce biofuels. Although microalgae offer a good potential for biogas production, commercial ventures
exploiting this possibility have not yet been implemented (Jasvinder and Gu 2010).
Acknowledgements
The authors would like to thanks financial assistance for a postdoctoral fellowship (ref: SFRH/
BPD/81882/2011) by Foundation for Science and Technology (FCT), Portugal. We are grateful for
financial support through project DINOSSAUR—PTDC/BBB-EBB/1374/2014 - POCI-01-0145-
immobilized Clostridium acetobutylicum cells has been studied (Efremenko et al. 2012), which offers
promising multiple applications using immobilized biocatalysts for converting waste/microalgal biomass
into biofuel. Recently, Mychonastes afer PKUAC9 and Scenedesmus abundans PKUAC12 were tested
for saccharification followed by fermentative bioethanol production. S. abundans exhibited the highest
production of total glucose (5.730 g/L) and sugars hydrolysates (10.752 g/L) as compared to M. afer.
Chemo-enzymatic treated S. abundans was the best feedstock for fermentative production (0.103 g/g dry
weight) of bioethanol (Hui et al. 2013).
Biogas production
Biogas is the biofuel substitute for natural gas, being produced by anaerobic (i.e., in the absence of
oxygen/or in an oxygen-deficient environment) digestion or fermentation of biodegradable materials
such as cow manure, municipal waste, plant residues, and microalgal biomass. For example, as largescale production of microalgal-based biodiesel is still limited by the downstream costs of lipid extraction
(Collet et al. 2011), there is a need to explore other energy uses of spent microalgae biomass. The absence
of lignin makes microalgae ideal candidates for efficient biomethane production by fermentation in
biogas manufactures. Thus, microalgae are not only a possible source of biodiesel (Amaro et al. 2011),
but also as a source of other types of biofuels: bioethanol, biomethane, hydrogen, and butanol (Brennan
and Owende 2010; Varfolomeev et al. 2010; Parmar et al. 2011; Varfolomeev and Wasserman 2011). The
coupled process of microalgae cultivation and biogas production may be a better option than biodiesel
production alone (Collet et al. 2011). Upon oil extraction, the spent biomass can be used to produce biogas
and/or biofertilisers (Jasvinder and Gu 2010). In addition to being renewable, this would encourage
sustainable farming procedures, providing higher efficiencies and reduce production costs (Vergara et al.
2008). Generally, anaerobic digestion can be divided into four types: (i) hydrolysis (ii) acidogenesis, (iii)
acetogenesis, and (iv) methanogenesis. Methanogenic digestion is widely used in digestion of organic
waste. First, in hydrolysis, the complex materials are degraded into small particle such as soluble sugars,
which are later fermented to alcohols and acetic acid and/or volatile fatty acids and other by-products like
hydrogen and carbon dioxide, which are metabolized to biogas by methanogens. The biogas production
from this anaerobic digestion process is primarily affected by organic loadings, pH, temperature, and
retention time in reactors (Harun et al. 2010). Mainly long solid retention time and high organic loading
rate give significant results in terms of high methane yield (Chynoweth 2005). In addition, anaerobic
digestion can operate in either mesophilic (35ºC) or thermophilic (55ºC) conditions (Otsuka and Yoshino
2004). Biomethane production by anaerobic digestion of microalgal biomass has been reported for
freshwater and marine microalgae in various combinations (Aino et al. 2013). Net energy analysis of
two production systems for producing biodiesel and biogas from Nannochloropsis and Haematococcus
pluvialis, grown in a raceway in salt and freshwater, respectively, were studied (Luis and Tan 2011). Life
cycle assessment of the system has shown that biomethane production without greenhouse gas heating
would have a net energy ratio of 1.54, which is slightly lower than that of biomethane from ley crop
(1.78) (Aino et al. 2013). Biogas production from microalgae change due to variation in cellular lipid,
carbohydrate and protein content, cell wall structure, cultivation, and digestion temperature (Aino et
al. 2013). Alternatively, biomethane, as well as bio-oil, can be produced from Emiliania huxleyi via
direct pyrolysis (Wu et al. 1999), thus, making this coccolithophore a promising candidate for biofuel
production. Methanation of syngas produced from gasification of microalgal biomass is another route to
produce biofuels. Although microalgae offer a good potential for biogas production, commercial ventures
exploiting this possibility have not yet been implemented (Jasvinder and Gu 2010).
Acknowledgements
The authors would like to thanks financial assistance for a postdoctoral fellowship (ref: SFRH/
BPD/81882/2011) by Foundation for Science and Technology (FCT), Portugal. We are grateful for
financial support through project DINOSSAUR—PTDC/BBB-EBB/1374/2014 - POCI-01-0145-