marxianus strain was modified to exhibit endoglucanase II of T. reesei and βglucosidase of Aspergillus aculeatus on the cell surface, this organism could effectively produce 4.24 g/l of ethanol by converting 10 g/l of a cellulosic β-glucan at
48
C within 12 h (Yanase et al. 2010). Fermentation is also mediated by anaerobic
bacteria that are thermo-active. Icelandic hot springs are a hub of some thermophiles
that can produce significant amount of ethanol from lignocellulolytic hydrolysates.
Combination of cellobiohydrolase (of T. reesei) and endoglucanase (of
Acidothermus cellulolyticus) enhance the yield of saccharification (Turner et al.
2007). In addition, Thermoanaerobacter ethanolicus or Thermoanaerobacterium
saccharolyticum and C. thermocellum are other important thermophiles, which
possess immense potential to produce ethanol. These thermophiles are able to
manifest this production by the virtue of their bifunctional acetaldehyde-CoA/alcohol dehydrogenase that can form bioethanol from acetyl-coA. C. saccharolyticus, T.
tengcongensis, and P. furiosus are also thermophiles that do not possess this
enzyme/ system, due to which their ethanol production capacity is low (Bielen et
al. 2013).
3.7
Role of Thermophiles and Their Enzymes in the Production
of Hydrogen Fuel
There are many reasons to consider hydrogen gas (H 2 ) as a substitute for conventional fossil fuels. One of the imperative reasons for this substitution is that during
the oxidation of hydrogen only H 2 O is released, whereas with carbon-based biofuel
it is CO 2 . Second major reason for this substitution is that H 2 fuel cells are
comparatively more energy efficient. Dark fermentation is the term given to an
anaerobic fermentation process in which renewable feedstocks are used for the
production of biohydrogen. This process is different from photofermentative hydrogen production. CO 2 would not get released during this process and the production
of biohydrogen takes place in controlled manner, thereby preventing the environmental dissemination of CO 2 .
Thermophiles like Caldicellulosiruptor saccharolyticus and Thermotoga elfii are
some of the thermophiles that have aided in the recent development of larger-scale
microorganism-based systems employed in hydrogen production that relied traditionally on chemical/catalyst procedure (Das and Veziroglu 2001; de Vrije et al.
2002). C. saccharolyticus is considered to be an exceptional contestant for H 2
production biologically as it can produce 4 mol hydrogen per mol hexose that
approaches the theoretical limit for dark fermentation. It is Gram-positive, anaerobic,
thermophilic cellulolytic bacterium that has the potential to ferment a wide-range
variety of monosaccharides, disaccharides, and polysaccharides including acetate.
There are also copious utilizations for extremophiles in the hydrogen production
through anaerobic fermentation and hydrogenases. Strains of Caldicellulosiruptor,
Thermoanaerobacterium, Pyrococcus, and Aeropyrum possess enormous caliber
(Coker 2016). Caldicellulosiruptor saccharolyticus that was isolated in mid-eighties
is an excellent source of thermoactive glycoside hydrolases due to which it is not
76
L. Bhatia et al.
48
C within 12 h (Yanase et al. 2010). Fermentation is also mediated by anaerobic
bacteria that are thermo-active. Icelandic hot springs are a hub of some thermophiles
that can produce significant amount of ethanol from lignocellulolytic hydrolysates.
Combination of cellobiohydrolase (of T. reesei) and endoglucanase (of
Acidothermus cellulolyticus) enhance the yield of saccharification (Turner et al.
2007). In addition, Thermoanaerobacter ethanolicus or Thermoanaerobacterium
saccharolyticum and C. thermocellum are other important thermophiles, which
possess immense potential to produce ethanol. These thermophiles are able to
manifest this production by the virtue of their bifunctional acetaldehyde-CoA/alcohol dehydrogenase that can form bioethanol from acetyl-coA. C. saccharolyticus, T.
tengcongensis, and P. furiosus are also thermophiles that do not possess this
enzyme/ system, due to which their ethanol production capacity is low (Bielen et
al. 2013).
3.7
Role of Thermophiles and Their Enzymes in the Production
of Hydrogen Fuel
There are many reasons to consider hydrogen gas (H 2 ) as a substitute for conventional fossil fuels. One of the imperative reasons for this substitution is that during
the oxidation of hydrogen only H 2 O is released, whereas with carbon-based biofuel
it is CO 2 . Second major reason for this substitution is that H 2 fuel cells are
comparatively more energy efficient. Dark fermentation is the term given to an
anaerobic fermentation process in which renewable feedstocks are used for the
production of biohydrogen. This process is different from photofermentative hydrogen production. CO 2 would not get released during this process and the production
of biohydrogen takes place in controlled manner, thereby preventing the environmental dissemination of CO 2 .
Thermophiles like Caldicellulosiruptor saccharolyticus and Thermotoga elfii are
some of the thermophiles that have aided in the recent development of larger-scale
microorganism-based systems employed in hydrogen production that relied traditionally on chemical/catalyst procedure (Das and Veziroglu 2001; de Vrije et al.
2002). C. saccharolyticus is considered to be an exceptional contestant for H 2
production biologically as it can produce 4 mol hydrogen per mol hexose that
approaches the theoretical limit for dark fermentation. It is Gram-positive, anaerobic,
thermophilic cellulolytic bacterium that has the potential to ferment a wide-range
variety of monosaccharides, disaccharides, and polysaccharides including acetate.
There are also copious utilizations for extremophiles in the hydrogen production
through anaerobic fermentation and hydrogenases. Strains of Caldicellulosiruptor,
Thermoanaerobacterium, Pyrococcus, and Aeropyrum possess enormous caliber
(Coker 2016). Caldicellulosiruptor saccharolyticus that was isolated in mid-eighties
is an excellent source of thermoactive glycoside hydrolases due to which it is not
76
L. Bhatia et al.