94 Introduction to Renewable Biomaterials
Thermotogales is considered particularly promising. These obligate anaerobes ferment
glucose to acetate, CO 2 and hydrogen.
C 6 H 12 O 6 +2H 2 O↔2CH 3 COOH+2CO 2 +4H 2
Hydrogen production via this route is rather atypical fermentation reaction. In major-
ity of fermentations, metabolic intermediates are reduced to regenerate NAD+(e.g.
ethanol, butanol); in this type of fermentation, they are not. Instead, hydrogen ions are
the ultimate acceptors of electrons forming hydrogen molecules [39]. The transfer of
electrons from NADH to hydrogen ion is mediated by NADH: ferredoxin oxidoreduc-
tase and the final step of the hydrogen evolution by a hydrogenase. Such reaction takes
place during glycolysis.
Glyceraldehyde-3-phosphate+NAD++Pi↔1,3-Bisphosphoglycerate
+NADH+H+NADH+H++Fdox↔Fdred+NAD+
Fdred+2H+↔Fdox+H 2
Another molecule of hydrogen results from coupling pyruvate:ferredoxin oxidoreduc-
tase with hydrogenase.
C 3 H 4 O 3 +CoA-SH+Fdox↔CoA-S-C 2 H 3 O+Fdred+CO 2 +2H+
Fdred+2H+↔Fdox+H 2
Hydrogen is a hugely important chemical feedstock and potentially useful bio-fuel
that yields only water upon its combustion. Currently, about 95% of hydrogen is pro-
duced from reformation of natural gas and finding sustainable renewable sources of this
chemical will hugely benefit other technologies of bioenergy production, especially the
thermochemical. Hydrogen production with dark fermentation has numerous advan-
tages, especially when conducted with thermophilic organisms like Thermotogales.
These bacteria exhibit very large substrate flexibility and are capable of fermenting
various hexoses, pentoses and even longer oligosaccharides or starch [39]. Additionally
their high optimal growth temperature limits microbial contamination and simplified
product recovery. Hydrogen solubility in the fermentation broth significantly drops at
elevated temperatures that facilitates gas collection and removal. One of the products
of fermentation process, acetate, is an economically valuable and recoverable product
that could be explored commercially or as a feedstock to other processes. The advances
in the development of biological hydrogen production including dark fermentation can
have significant impact on the development of sustainable sources of this important
chemical feedstock.
3.4.9.4 Biomethane
Biomethane production in an industrial process called anaerobic digestion is a process
significantly different from other types of fermentation described earlier. Biomethane
also known as (biogas) is produced by a diverse community of bacteria and archaea,
which work together to produce methane and carbon dioxide through a series metabolic
reactions. A unique feature of this process is that end products of one group of organ-
isms become substrates for another until two gaseous products are obtained – methane
