only an excellent organism able to degrade polysaccharide like cellulose but also an
excellent candidate for production of biohydrogen from renewable biomass (de
Vries 2003). Thermophilic conditions are favorable for degradation of plant biomass
and formation of hydrogen biologically. It is also a thermodynamic consideration
that elevated temperature is best suited for the formation of hydrogen (Kengen et al.
2009). Because of this reason, (hyper)thermophiles can yield higher hydrogen,
which reaches the theoretical limit of 4 mol H 2 per mol of hexose over mesophiles
(Verhaart et al. 2010). The theoretical yield involves the pure catabolic constituent of
glucose transformation (Bielenet al. 2013). The bacterium C. saccharolyticus has an
optimum growth temperature of 70
C. This organism is a bank of varieties of endoand exo-glycoside hydrolases, which aids in degradation and growth on a diversity
of biomass substrates that contains cellulose and hemicellulose. These endo- and
exo-glycoside hydrolases (GH) have potential to hydrolyze the glycosidic linkages
of huge varieties of macromolecules like β-glucans (starch, pullulan as well as
cellulose), xylan, and hetero-polysaccharides (hemicelluloses and pectin) (Albertson
et al. 1997). This feature makes C. saccharolyticus different from Clostridium
species that utilize cellulosome-like structures for the disintegration of recalcitrant
polysaccharides of plants. C. saccharolyticus prefers substrates in broad range, due
to the presence of great variety of transport system found in the genome. The
efficacious breakdown of recalcitrant plant polysaccharides into mono-, di-, or
oligo-saccharides is also feasible by the virtue of the fact that few of the glycosidases
have multidomains like glycoside hydrolase domains as well as carbon-binding
modules. This organism can co-utilize hexoses as well as pentoses without showing
any symptoms of carbon catabolite repression. This characteristic makes C.
saccharolyticus suitable for any consolidated bioprocess (Willquist et al. 2010). In
addition, there are various enzymes of Caldicellulosiruptor species, like β-glucosidase (BglA), β-xylosidase, β-1,4-xylanase, and a type I pullulanase that have been
successfully cloned and characterized (Schofield and Daniel 1993).
Caldicellulosiruptor species and Thermoanaerobacter species are known to degrade
crystalline cellulose with the aid of their free-acting primary cellulases. Overall,
economic feasibility of biohydrogen production from microbe including
thermophiles needs substantial improvements in consumption rates of substrate
along with hydrogen yield (Bielen et al. 2013).
3.8
Conclusion
In the current scenario, the microbial-mediated fabrication of second-generation
biofuels using lignocelluloses is much more advantageous over first-generation
biofuels. Extensive investigations have been carried out in the area of biofuel by
focusing on microorganisms that depict growth in mesophilic temperature range, i.e.,
25–37
C. However, such mesophilic microbes have no ability to exploit
lignocelluloses straightforwardly. In this context, thermophiles have been considered as promising microbes toward biofuels production as they not only break down
the lignocelluloses effectively (by consolidated bioprocessing) and reduce the
3 Role of Thermophiles in Production of Aviation Biofuels: Fueling the Future
77
excellent candidate for production of biohydrogen from renewable biomass (de
Vries 2003). Thermophilic conditions are favorable for degradation of plant biomass
and formation of hydrogen biologically. It is also a thermodynamic consideration
that elevated temperature is best suited for the formation of hydrogen (Kengen et al.
2009). Because of this reason, (hyper)thermophiles can yield higher hydrogen,
which reaches the theoretical limit of 4 mol H 2 per mol of hexose over mesophiles
(Verhaart et al. 2010). The theoretical yield involves the pure catabolic constituent of
glucose transformation (Bielenet al. 2013). The bacterium C. saccharolyticus has an
optimum growth temperature of 70
C. This organism is a bank of varieties of endoand exo-glycoside hydrolases, which aids in degradation and growth on a diversity
of biomass substrates that contains cellulose and hemicellulose. These endo- and
exo-glycoside hydrolases (GH) have potential to hydrolyze the glycosidic linkages
of huge varieties of macromolecules like β-glucans (starch, pullulan as well as
cellulose), xylan, and hetero-polysaccharides (hemicelluloses and pectin) (Albertson
et al. 1997). This feature makes C. saccharolyticus different from Clostridium
species that utilize cellulosome-like structures for the disintegration of recalcitrant
polysaccharides of plants. C. saccharolyticus prefers substrates in broad range, due
to the presence of great variety of transport system found in the genome. The
efficacious breakdown of recalcitrant plant polysaccharides into mono-, di-, or
oligo-saccharides is also feasible by the virtue of the fact that few of the glycosidases
have multidomains like glycoside hydrolase domains as well as carbon-binding
modules. This organism can co-utilize hexoses as well as pentoses without showing
any symptoms of carbon catabolite repression. This characteristic makes C.
saccharolyticus suitable for any consolidated bioprocess (Willquist et al. 2010). In
addition, there are various enzymes of Caldicellulosiruptor species, like β-glucosidase (BglA), β-xylosidase, β-1,4-xylanase, and a type I pullulanase that have been
successfully cloned and characterized (Schofield and Daniel 1993).
Caldicellulosiruptor species and Thermoanaerobacter species are known to degrade
crystalline cellulose with the aid of their free-acting primary cellulases. Overall,
economic feasibility of biohydrogen production from microbe including
thermophiles needs substantial improvements in consumption rates of substrate
along with hydrogen yield (Bielen et al. 2013).
3.8
Conclusion
In the current scenario, the microbial-mediated fabrication of second-generation
biofuels using lignocelluloses is much more advantageous over first-generation
biofuels. Extensive investigations have been carried out in the area of biofuel by
focusing on microorganisms that depict growth in mesophilic temperature range, i.e.,
25–37
C. However, such mesophilic microbes have no ability to exploit
lignocelluloses straightforwardly. In this context, thermophiles have been considered as promising microbes toward biofuels production as they not only break down
the lignocelluloses effectively (by consolidated bioprocessing) and reduce the
3 Role of Thermophiles in Production of Aviation Biofuels: Fueling the Future
77