cocktails are used for better saccharification and engineered microorganisms are
used so as to improve the yield. Various organisms (bacteria, yeast, mould) have
been investigated for ethanol production, i.e. Zymomonasmobilis; Corynebacterium
glutamicum; Pichia stipites; Clostridium thermocellum; Clostridium
phytofermentans; Saccharomyces cerevisiae and Escherichia coli. Bioethanol,
mainly produced by Saccharomyces cerevisiae, was thought to have the capacity
to be used as an alternative fuel because it limits the emission of toxic gases. One of
the disadvantages is its inability to use pentose sugars, resulting in less productivity.
However, Escherichia coli and Clostridia are the alternatives for ethanol production
as these organisms can use pentose as well as hexose sugars. Clostridium has the
capability to produce 1-butanol during the butyrate and butanol-acetone anaerobic
route that proceed at the same time of ethanol production.
Algae, which are regarded as a promising substrate for production of biodiesel,
can also be used for bioethanol production as it contains sufficient sugar and protein
components. The conversion of biomass into ethanol requires a group of microorganisms that produce the enzyme cellulase to hydrolyse the lignocellulosic biomass.
In an effort to produce large amounts of energy, enzyme-producing microorganisms
are immobilized on a stable surface for saccharification and fermentation of sugar to
convert into ethanol with high levels of efficiency and in a cost-effective manner. In
the case of brown algae, the absence of lignin results in comparatively easy saccharification which is a great advantage. However, the presence of alginate is one of
the limitations that can be controlled by the utilization of alginate by using the
metabolically engineered microorganisms (Dave et al. 2019).
Hydrolysis of lignocellulosic biomass can be achieved by hydrolytic enzymes so
that microbes can utilize them for ethanol or butanol production, but their high cost
causes hindrance in mass production of biofuels. However, some species of microorganisms namely Neurospora, Monilia, Paecilomyces, Fusarium, etc. have been
reported recently for their capability of direct fermentation of cellulose into ethanol
by the process of simultaneous saccharification and fermentation (SSF). Thus, an
alternative approach could be the consolidated bioprocessing (CBP) as it offers
cellulase enzyme production and cellulose saccharification along with fermentation
by microbes in one sole step thereby eliminating enzymes production and purification steps.
Consolidated bioprocessing can be achieved by adopting two different strategies.
One strategy is to use genetic techniques to increase the biofuel yield by cloning the
cellulase coding sequences into those microorganisms that are incapable of utilizing
cellulose directly. Microorganisms, thus, are genetically engineered to breakdown
cellulose by creating new cellulase production systems, resulting in improved
production and activity of enzymes and reduction in the cost. Another strategy is
native cellulolytic strategy in which microorganisms are used for the efficient
hydrolysis of cellulose, but with a limitation of low biofuel productivity. The process
of fermentation is advanced and improved by the engagement of several mesophilic
and thermophilic microorganisms, but compared to former ones, thermophilic
microorganism present greater possibility in consolidated bioprocessing by achieving direct production of biofuel from lignocellulosic biomass with numerous
48
N. Jaiswal et al.
used so as to improve the yield. Various organisms (bacteria, yeast, mould) have
been investigated for ethanol production, i.e. Zymomonasmobilis; Corynebacterium
glutamicum; Pichia stipites; Clostridium thermocellum; Clostridium
phytofermentans; Saccharomyces cerevisiae and Escherichia coli. Bioethanol,
mainly produced by Saccharomyces cerevisiae, was thought to have the capacity
to be used as an alternative fuel because it limits the emission of toxic gases. One of
the disadvantages is its inability to use pentose sugars, resulting in less productivity.
However, Escherichia coli and Clostridia are the alternatives for ethanol production
as these organisms can use pentose as well as hexose sugars. Clostridium has the
capability to produce 1-butanol during the butyrate and butanol-acetone anaerobic
route that proceed at the same time of ethanol production.
Algae, which are regarded as a promising substrate for production of biodiesel,
can also be used for bioethanol production as it contains sufficient sugar and protein
components. The conversion of biomass into ethanol requires a group of microorganisms that produce the enzyme cellulase to hydrolyse the lignocellulosic biomass.
In an effort to produce large amounts of energy, enzyme-producing microorganisms
are immobilized on a stable surface for saccharification and fermentation of sugar to
convert into ethanol with high levels of efficiency and in a cost-effective manner. In
the case of brown algae, the absence of lignin results in comparatively easy saccharification which is a great advantage. However, the presence of alginate is one of
the limitations that can be controlled by the utilization of alginate by using the
metabolically engineered microorganisms (Dave et al. 2019).
Hydrolysis of lignocellulosic biomass can be achieved by hydrolytic enzymes so
that microbes can utilize them for ethanol or butanol production, but their high cost
causes hindrance in mass production of biofuels. However, some species of microorganisms namely Neurospora, Monilia, Paecilomyces, Fusarium, etc. have been
reported recently for their capability of direct fermentation of cellulose into ethanol
by the process of simultaneous saccharification and fermentation (SSF). Thus, an
alternative approach could be the consolidated bioprocessing (CBP) as it offers
cellulase enzyme production and cellulose saccharification along with fermentation
by microbes in one sole step thereby eliminating enzymes production and purification steps.
Consolidated bioprocessing can be achieved by adopting two different strategies.
One strategy is to use genetic techniques to increase the biofuel yield by cloning the
cellulase coding sequences into those microorganisms that are incapable of utilizing
cellulose directly. Microorganisms, thus, are genetically engineered to breakdown
cellulose by creating new cellulase production systems, resulting in improved
production and activity of enzymes and reduction in the cost. Another strategy is
native cellulolytic strategy in which microorganisms are used for the efficient
hydrolysis of cellulose, but with a limitation of low biofuel productivity. The process
of fermentation is advanced and improved by the engagement of several mesophilic
and thermophilic microorganisms, but compared to former ones, thermophilic
microorganism present greater possibility in consolidated bioprocessing by achieving direct production of biofuel from lignocellulosic biomass with numerous
48
N. Jaiswal et al.