Biohydrogen Production from Anaerobic Fermentation - Biotechnology in China: Biofuels and Bioenergy - page 151

3.2 Functional Microorganisms
Lignocellulosic hydrolysates contain a mixture of hexose and pentose sugars.
In recent years, researchers have explored a number of pentose fermentation
strains and mixed cultures [ 60 - 74 ], which are shown in Table 2 . For example,
Xu et al. [ 61 ] isolated the novel mesophilic hydrogen-producing bacterium
Clostridium sp. HR-1 from cow dung compost, which can efficiently utilize xylose
as well as glucose for hydrogen production. Under optimal conditions of pH 6.5,
37 C, and yeast extract as a nitrogen source, the average hydrogen yields and
specific hydrogen production rates were 1.63 and 2.02 mol H 2 /mol sugar and 11.14
and 9.37 mmol H 2 /h/g DCW for xylose and glucose. Ren et al. [ 63 ] reported
a thermophilic xylose-glucose fermenting strain, Thermoanaerobacterium
thermosaccharolyticum W16, and its maximum cumulative hydrogen yield and
production rate from pure glucose and xylose were 2.42 mol H 2 /mol glucose and
12.9 mmol H 2 /L/h, and 2.19 mol H 2 /mol xylose and 10.7 mmol H 2 /L/h, respec-
tively. Besides, this strain metabolizes the reducing sugars in the hydrolysate
of corn stover efficiently, and also exhibits good tolerance to inhibitors in the
hydrolysate such as acetate and furfural. However, compared with hexose-
fermenting microorganisms, only a few pentose-fermenting microorganisms have
been explored. Further isolation of pentose-fermenting microorganisms is needed to
further the utilization of lignocellulosic hydrolysates.
Direct microbial conversion, which integrates cellulolytic enzyme production,
cellulose hydrolysis, and fermentation together, is considered the most attractive
strategy for converting cellulosic biomass to biofuels. It has been reported that
some pure cultures can degrade cellulose to produce hydrogen directly. Ren et al.
[ 65 ] reported that Clostridium acetobutylicum X9 fermented microcrystalline
cellulose with a hydrogen production rate of 6.4 mmol H 2 /g biomass (DCW)/h,
with a substrate degradation of 68.3%. Further research showed that the strain [ 66 ]
can ferment stream-exploded corn stover to hydrogen with a specific production
rate of 3.4 mmol/g substrate. Compared with pure cultures, mixed cultures were
more effective in degrading cellulose owing to the synergy of different microor-
ganisms. For example, Liu et al. [ 68 ] demonstrated a mixed culture of thermophilic
anaerobic bacteria Clostridium thermocellum JN4 and T. thermosaccharolyticum
GD17 could effectively decompose cellulose and produce hydrogen with a yield of
1.8 mol H 2 /mol glucose, and could utilize several kinds of natural substrates, such
as corn cob and cornstalk as feedstock for production of hydrogen. However, the
yields of hydrogen production from direct fermentation of cellulose by cellulolytic
bacteria are very low. Successful production of biohydrogen via direct cellulose
fermentation will depend upon our ability to increase the yields of hydrogen during
the fermentation process. This will require a more comprehensive understanding of
the interactions between gene product expression, end product synthesis patterns,
and the factors that regulate carbon and electron flow in cellulolytic bacteria.