262 Marine Macro- and Microalgae: An Overview
and Wu 2003). Isopropanol showed virtually no negative effect on lipase activity and also the fatty acid
isopropyl esters improved the cold weather performance of the biofuel (Lee et al. 1995). Producing ethyl
esters rather than methyl esters is of considerable interest, as the extra carbon atom increases the heat
content and cetane number (Encinar et al. 2002).
Bioethanol production
Bioethanol is a biofuel that is obtained from starch-based (corn, wheat, barley) and sugar-based (sugar
beets, sugarcane) first generation feedstocks and has partially replaced petrol in some parts of the world,
for instance, Brazil and United States (Bai et al. 2008). Although bioethanol is easily produced via
fermentation, the use of food crops for its production impacts on food security and agricultural land
availability (Sun and Cheng 2002). Second generation feedstock such as lignocellulosic (i.e., agriculture
waste, forest residues, and others sources) biomass contains a complex mixture of carbohydrate
polymers from plant cell walls such as cellulose, hemicellulose, and lignin. In order to produce sugars,
lignocellulosic biomass must be pre-treated with acids or enzymes in order to breakdown these polymers
into smaller sized compounds, thus increasing the cost of conversion into bioethanol. Lignin could be
used as starting material for bioethanol production. However, it is a polymer very difficult to degrade
or ferment biologically (Lynd 1996), thereby decreasing overall biomass to biofuel yields, leading also
to higher waste treatment costs. Conversely, algae have been purported as alternative feedstocks for
bioethanol production (Harun et al. 2010b), due to their fast growth, efficient carbon dioxide fixation,
and potentially accumulating high amounts of carbohydrates, apart from lipids and proteins, when
compared to biomass derived from food crops such as sugarcane and maize. As microalgae can contain
high levels of starch and cellulose, having no lignin and low hemicellulose content, their saccharification
for bioethanol production appears to be a more straightforward process (Hamelinck et al. 2005; Harun et
al. 2010b). For instance, Chlamydomonas reinhardtii UTEX 90 (Choi et al. 2010) and Chlorella vulgaris
(Branyikova et al. 2011) accumulate their energy reserves in form of 55 to 60% (w/w) starch, which
can be easily hydrolysed into glucose via enzymatic and/or chemical methods. Starch contents of other
microalgae ranging between 20 to 50% (w/w) have also been reported (Matsumoto et al. 2003; Rodjaroen
et al. 2007; Rojan et al. 2011; Chun et al. 2013).
Bioethanol production from microalgae such as Chlorococcum sp. (Harun et al. 2010b) and
Chlamydomonas perigranulata was fermented to produce bioethanol, butanediol, acetic acid, and
CO 2 (Hon 2006), which supports the suitability of microalgae as promising substrates for bioethanol
production. It was found that hydrogen and carbon recovery from that fermentation was about 139 and
105%, respectively.
There are several methods of extracting sugars from biomass by using chemical and enzymatic
saccharification (Daroch et al. 2013). Chemical saccharification of biomass is typically carried out using
dilute and concentrated sulphuric acids and alkaline solutions (Van de Vyver et al. 2010). Enzymatic
saccharification (i.e., hydrolysis) methods, involving the use of cellulases, amylases, and glucoamylases,
are widely employed to hydrolyse microalgal biomass to sucrose (Rui et al. 2012; Chun et al. 2013).
Sucrose can be hydrolysed into glucose and fructose by invertase and then converted to ethanol using
zymase. Both enzymes are produced by yeasts during fermentation for bioethanol production. Cellulosic
biomass can be converted into any type of fuel including ethanol, gasoline, diesel, and jet fuel (Huber
and Dale 2009). This has resulted in considerable attention towards the application of biomethane
fermentation of microalgae to produce value-added by-products such as biogas (Daroch et al. 2013).
Fermentation of the microalgal biomass can be carried out by bacteria, yeast, and filamentous fungi,
resulting in by-products such as CO 2 and water. CO 2 produced in fermentation processes can be re-used
as carbon sources for microalgae cultivation, as well as reducing the greenhouse gases emissions. After
bioethanol production by fermentation, spent biomass is also useful for anaerobic digestion process for
production of biogas (Ueda et al. 1996; Bush and Hall 2006), making all organic matter useful for biofuel
production (Jasvinder and Gu 2010; Harun et al. 2010b). As an example, production of bioethanol,
biobutanol, and biohydrogen from pre-treated microalgae biomass by anaerobic fermentation with
and Wu 2003). Isopropanol showed virtually no negative effect on lipase activity and also the fatty acid
isopropyl esters improved the cold weather performance of the biofuel (Lee et al. 1995). Producing ethyl
esters rather than methyl esters is of considerable interest, as the extra carbon atom increases the heat
content and cetane number (Encinar et al. 2002).
Bioethanol production
Bioethanol is a biofuel that is obtained from starch-based (corn, wheat, barley) and sugar-based (sugar
beets, sugarcane) first generation feedstocks and has partially replaced petrol in some parts of the world,
for instance, Brazil and United States (Bai et al. 2008). Although bioethanol is easily produced via
fermentation, the use of food crops for its production impacts on food security and agricultural land
availability (Sun and Cheng 2002). Second generation feedstock such as lignocellulosic (i.e., agriculture
waste, forest residues, and others sources) biomass contains a complex mixture of carbohydrate
polymers from plant cell walls such as cellulose, hemicellulose, and lignin. In order to produce sugars,
lignocellulosic biomass must be pre-treated with acids or enzymes in order to breakdown these polymers
into smaller sized compounds, thus increasing the cost of conversion into bioethanol. Lignin could be
used as starting material for bioethanol production. However, it is a polymer very difficult to degrade
or ferment biologically (Lynd 1996), thereby decreasing overall biomass to biofuel yields, leading also
to higher waste treatment costs. Conversely, algae have been purported as alternative feedstocks for
bioethanol production (Harun et al. 2010b), due to their fast growth, efficient carbon dioxide fixation,
and potentially accumulating high amounts of carbohydrates, apart from lipids and proteins, when
compared to biomass derived from food crops such as sugarcane and maize. As microalgae can contain
high levels of starch and cellulose, having no lignin and low hemicellulose content, their saccharification
for bioethanol production appears to be a more straightforward process (Hamelinck et al. 2005; Harun et
al. 2010b). For instance, Chlamydomonas reinhardtii UTEX 90 (Choi et al. 2010) and Chlorella vulgaris
(Branyikova et al. 2011) accumulate their energy reserves in form of 55 to 60% (w/w) starch, which
can be easily hydrolysed into glucose via enzymatic and/or chemical methods. Starch contents of other
microalgae ranging between 20 to 50% (w/w) have also been reported (Matsumoto et al. 2003; Rodjaroen
et al. 2007; Rojan et al. 2011; Chun et al. 2013).
Bioethanol production from microalgae such as Chlorococcum sp. (Harun et al. 2010b) and
Chlamydomonas perigranulata was fermented to produce bioethanol, butanediol, acetic acid, and
CO 2 (Hon 2006), which supports the suitability of microalgae as promising substrates for bioethanol
production. It was found that hydrogen and carbon recovery from that fermentation was about 139 and
105%, respectively.
There are several methods of extracting sugars from biomass by using chemical and enzymatic
saccharification (Daroch et al. 2013). Chemical saccharification of biomass is typically carried out using
dilute and concentrated sulphuric acids and alkaline solutions (Van de Vyver et al. 2010). Enzymatic
saccharification (i.e., hydrolysis) methods, involving the use of cellulases, amylases, and glucoamylases,
are widely employed to hydrolyse microalgal biomass to sucrose (Rui et al. 2012; Chun et al. 2013).
Sucrose can be hydrolysed into glucose and fructose by invertase and then converted to ethanol using
zymase. Both enzymes are produced by yeasts during fermentation for bioethanol production. Cellulosic
biomass can be converted into any type of fuel including ethanol, gasoline, diesel, and jet fuel (Huber
and Dale 2009). This has resulted in considerable attention towards the application of biomethane
fermentation of microalgae to produce value-added by-products such as biogas (Daroch et al. 2013).
Fermentation of the microalgal biomass can be carried out by bacteria, yeast, and filamentous fungi,
resulting in by-products such as CO 2 and water. CO 2 produced in fermentation processes can be re-used
as carbon sources for microalgae cultivation, as well as reducing the greenhouse gases emissions. After
bioethanol production by fermentation, spent biomass is also useful for anaerobic digestion process for
production of biogas (Ueda et al. 1996; Bush and Hall 2006), making all organic matter useful for biofuel
production (Jasvinder and Gu 2010; Harun et al. 2010b). As an example, production of bioethanol,
biobutanol, and biohydrogen from pre-treated microalgae biomass by anaerobic fermentation with