Environmental Engineering Reference
In-Depth Information
fermentations have been optimized by evolution to produce cell biomass and not
hydrogen. Thus a portion of the substrate (pyruvate) is used in both cases to produce
ATP giving a product “acetate” that is excreted. Also, in many organisms the actual
yields of hydrogen are reduced by hydrogen recycling due to the presence of one or
more uptake hydrogenases, which consume a portion of the hydrogen produced. It
is unknown to what extent hydrogen production could be increased through meta-
bolic engineering and manipulation of culture conditions.
3.1.1
Microbes Involved
Various kinds of microorganisms take part in the dark fermentative hydrogen gen-
eration.
Bacillus,
Escherichia, Enterobacter, Ruminococcus, Citrobacter
and
Clos-
tridia
are most common microbial genera capable of producing hydrogen via fer-
mentation (Das
2009
; Davila-Vazquez et al.
2008
). Many anaerobes are capable of
producing hydrogen from hexoses in acetic acid, butyric acid and acetone-butanol
ethanol fermentations.
Clostridia
(
C. butyricum, C. welchii, C. pasteurianum, C.
beijerincki
) and mixtures have been used in many studies dedicated to hydrogen
production. The hyperthermophile
Pyrcoccus furious
, an archaebacterium, is also
known to produce H
2
, organic acids and CO
2
from carbohydrates (Fiala and Stetter
1986
; Brown and Kelly
1989
; Godfroy et al.
2000
). There are other cellulotic ther-
mophiles and extreme hyperthermophilic bacteria producing hydrogen such as
An-
aerocellum, Caldicellulosiruptor, Clostridium, Dictyoglomus, Fervidobacterium,
Spirocheta, Thermotoga and Thermoanaerobacter
(Schröder et al.
1994
).
Rumen bacteria are other strict anaerobic bacteria, which are capable of H
2
pro-
duction and other products such as acetate, ethanol, formate and CO
2
from carbohy-
drates.
Ruminococcus albus
is one of the most commonly known one (Innotti et al.
1973
). Methanogens have hydrogenase which is usually involved in the oxidation of
H
2
coupled to CH production and CO
2
reduction. On the other hand, it is well known
that
Methanosarcina barkeri
, under the conditions of inhibition of CH
4
formation, is
capable of carrying out so-called water-gas shift reaction (production of H
2
and CO
2
in stoichiometric amounts from CO and H
2
O) (Bott et al.
1986
). Strict anaerobes
have no tolerance over oxygen and do not usually survive low oxygen concentra-
tions, on the other hand, facultative anaerobes are resistant to oxygen. These bac-
teria are capable of rapidly consuming oxygen and restoring anaerobic conditions.
Among these bacteria,
Enterobacter
and other members of
Enterobacteriaceae
are
able to produce H
2
and are not inhibited by high H
2
pressures, but the H
2
yield on
glucose is lower than that of
Clostridia
(Tanisho and Ishiwata
1994
). Hydrogen
production by
Escherichia coli
is mediated by the formate hydrogenlyase (
fhl
) com-
plex as shown in Fig.
11.1
.
E. coli
can perform a 'mixed-acid fermentation' in which
glucose is metabolised to ethanol and various organic acids, including formate. This
formate is further disproportionated to carbon dioxide and hydrogen by the formate
hydrogenlyase (FHL) complex. Another facultative anaerobe, genus
Citrobacter
is
considered under the family
Enterobacteriaceae
. They are gram-negative, non spore
forming, facultative anaerobic and motile bacilli employing peritrichous flagella for
locomotion and commonly utilizing citrate as their sole carbon source.
Citrobacter
