maintained. The production yield of 7.5 m
3 of H 2 was reported
by a dark unit, while 4.7 m
3 was reported by a photo unit
(Zhang et al. 2018).
4.5 Direct Microbial Conversion
The direct microbial approach or CBP is an attractive
method of producing useful products from lignocellulose via
the integration of hydrolysis and fermentation. In CBP, the
particular microorganism or microbial consortia generates a
cocktail of the cellulolytic and hemicellulolytic enzymes for
the hydrolysis of pretreated lignocellulose to hexose and
pentose, followed by in situ conversion of these products to
H 2 (Ren et al. 2016; Olson et al. 2012). The integrating
hydrolysis with fermentation reduces the acquiring price of
an enzyme for separate hydrolysis. It requires less pretreatment and reduces the total cost of installation, which leads to
a reduction in the overall costs of the process and makes
CBP an attractive method for H 2 production (Nagarajan
et al. 2019). CBP of lignocellulose to H 2 can be performed
by either using a pure culture, mixed culture, or co-culture of
microorganism.
Pure Culture
A few pure microbes have been used for the direct conversion of lignocellulosic biomass to hydrogen. Ivanova et al.
have used thermophilic Caldicellulosiruptor saccharolyticus bacterium for the hydrogen production using a variety of
untreated lignocellulose biomass, such as maize leaves,
sugarcane bagasse, laserwort (silphium), sweet sorghum, and
wheat straw. The composition of biomass has a significant
influence on the efficiency of substrates utilization and the
rate of hydrogen production. The highest optimal H 2 production of 44.7 L.H 2 /kg dry biomass and 3.8 mol H 2 /mol of
glucose yield was obtained for the wheat straw (Ivanova
et al. 2009). The thermophilic cellulolytic bacterium
Clostridium thermocellum was found to be active for the
production of H 2 using pretreated and untreated biomass,
such as bagasse (Cheng and Zhu 2016; Tian et al. 2015),
hemp residues (Agbor et al. 2014), corn stalks (Cheng and
Liu 2011), and spent mushroom (Lin et al. 2017). C. thermocellum has effectively produced 23.5 mmol/L H 2 and
61.4 mL H 2 /g of corn stalk from alkali-treated sugarcane
bagasse and untreated milled corn stalks, respectively
(Cheng and Zhu 2016; Cheng and Liu 2011). Several other
microorganisms, such as Clostridial strains, including
C. acetobutylicum X9, Clostridium sp. BOH3, C. sartagoforme FZ11, and C. butyricum FS3; Thermoanaerobacterium
including T. thermosaccharolyticum M18 and T. thermosaccharolyticum DD32 have found to be useful for CBP of a
variety of lignocellulosic biomass to hydrogen (Nagarajan
et al. 2019). Recently, Thermoanaerobacterium sp. strain F6
was used for producing hydrogen from several hemicellulosic and cellulosic material (Jiang et al. 2019). The pure
culture was also used to produce H 2 from the untreated
lignocellulosic biomass, i.e., corn cob and bagasse producing 66.7 and 30.2 mmol/L from bagasse and corn cob,
respectively (Jiang et al. 2019). Studies unveiled that the
pure culture system is attractive and preferred for mechanistic research. Further, a genetic reconstruction approach
can be employed to improve the rate of hydrolysis of cellulose and H 2 yield. However, the strain isolation technique
is complex and long and cultivates a microbe in a very small
fraction that can be employed for CBP.
Mixed Culture
The mixed culture can also be used for fermentative hydrogen production. A detailed investigation of mixed culture
activity towards hydrogen production showed a positive
relationship between the number of species with the yield of
hydrogen (Nagarajan et al. 2019). The natural resources, such
as anaerobic sludge, biomass compost sites, and rumen guts,
are used to get the microbial consortia that can be employed
to H 2 production (Ren et al. 2009; Wang et al. 2010; Chu
et al. 2011; Pérez-Rangel et al. 2015; Chen et al. 2012).
Mixed microbial consortia resulting from cow ruminal fluids,
forest soil, anaerobic sludge, and native microflora of wheat
straw was used for CBP of untreated wheat straw (Pérez-Rangel et al. 2015). The native flora of the wheat straw
was found to have the highest hydrogen production due to the
presence of several fungal strains that played a central role
(Pérez-Rangel et al. 2015). Chang et al. have studied the
hydrogen production from Napier grass using rumen microbial consortia containing Ruminococcus sp., C. papyrosolvens, Desulfovibrio desulfuricans, Ethanoligenes harbinese,
C. xylanolyticum, and C. beijerinckii (Chang et al. 2010). For
each 8-day incubation period, with 1.5% wt. Napier grass and
stem, 2% cellulose, and 27% hemicellulose conversion,
along with concomitant H 2 production were achieved (Chang
et al. 2010). Recently, the mixed culture of Clostridium
acetobutylicum with microorganism from bovine ruminal
fluid has shown a synergistic effect on hydrogen production
from acid-treated agave biomass compared to the control
experiment (Morales-Martínez et al. 2020).
Co-culture
The co-culture of microbes containing two to three
microorganisms interacts synergistically to increase the
conversion of substrate and production of H 2 (Nagarajan
et al. 2019). The interaction is beneficial as it results in the
sequential utilization of substrates, which increases the
conversion and provides a potential metabolic intermediate
Bioconversion of Lignocellulosic Residues into Hydrogen
71
3 of H 2 was reported
by a dark unit, while 4.7 m
3 was reported by a photo unit
(Zhang et al. 2018).
4.5 Direct Microbial Conversion
The direct microbial approach or CBP is an attractive
method of producing useful products from lignocellulose via
the integration of hydrolysis and fermentation. In CBP, the
particular microorganism or microbial consortia generates a
cocktail of the cellulolytic and hemicellulolytic enzymes for
the hydrolysis of pretreated lignocellulose to hexose and
pentose, followed by in situ conversion of these products to
H 2 (Ren et al. 2016; Olson et al. 2012). The integrating
hydrolysis with fermentation reduces the acquiring price of
an enzyme for separate hydrolysis. It requires less pretreatment and reduces the total cost of installation, which leads to
a reduction in the overall costs of the process and makes
CBP an attractive method for H 2 production (Nagarajan
et al. 2019). CBP of lignocellulose to H 2 can be performed
by either using a pure culture, mixed culture, or co-culture of
microorganism.
Pure Culture
A few pure microbes have been used for the direct conversion of lignocellulosic biomass to hydrogen. Ivanova et al.
have used thermophilic Caldicellulosiruptor saccharolyticus bacterium for the hydrogen production using a variety of
untreated lignocellulose biomass, such as maize leaves,
sugarcane bagasse, laserwort (silphium), sweet sorghum, and
wheat straw. The composition of biomass has a significant
influence on the efficiency of substrates utilization and the
rate of hydrogen production. The highest optimal H 2 production of 44.7 L.H 2 /kg dry biomass and 3.8 mol H 2 /mol of
glucose yield was obtained for the wheat straw (Ivanova
et al. 2009). The thermophilic cellulolytic bacterium
Clostridium thermocellum was found to be active for the
production of H 2 using pretreated and untreated biomass,
such as bagasse (Cheng and Zhu 2016; Tian et al. 2015),
hemp residues (Agbor et al. 2014), corn stalks (Cheng and
Liu 2011), and spent mushroom (Lin et al. 2017). C. thermocellum has effectively produced 23.5 mmol/L H 2 and
61.4 mL H 2 /g of corn stalk from alkali-treated sugarcane
bagasse and untreated milled corn stalks, respectively
(Cheng and Zhu 2016; Cheng and Liu 2011). Several other
microorganisms, such as Clostridial strains, including
C. acetobutylicum X9, Clostridium sp. BOH3, C. sartagoforme FZ11, and C. butyricum FS3; Thermoanaerobacterium
including T. thermosaccharolyticum M18 and T. thermosaccharolyticum DD32 have found to be useful for CBP of a
variety of lignocellulosic biomass to hydrogen (Nagarajan
et al. 2019). Recently, Thermoanaerobacterium sp. strain F6
was used for producing hydrogen from several hemicellulosic and cellulosic material (Jiang et al. 2019). The pure
culture was also used to produce H 2 from the untreated
lignocellulosic biomass, i.e., corn cob and bagasse producing 66.7 and 30.2 mmol/L from bagasse and corn cob,
respectively (Jiang et al. 2019). Studies unveiled that the
pure culture system is attractive and preferred for mechanistic research. Further, a genetic reconstruction approach
can be employed to improve the rate of hydrolysis of cellulose and H 2 yield. However, the strain isolation technique
is complex and long and cultivates a microbe in a very small
fraction that can be employed for CBP.
Mixed Culture
The mixed culture can also be used for fermentative hydrogen production. A detailed investigation of mixed culture
activity towards hydrogen production showed a positive
relationship between the number of species with the yield of
hydrogen (Nagarajan et al. 2019). The natural resources, such
as anaerobic sludge, biomass compost sites, and rumen guts,
are used to get the microbial consortia that can be employed
to H 2 production (Ren et al. 2009; Wang et al. 2010; Chu
et al. 2011; Pérez-Rangel et al. 2015; Chen et al. 2012).
Mixed microbial consortia resulting from cow ruminal fluids,
forest soil, anaerobic sludge, and native microflora of wheat
straw was used for CBP of untreated wheat straw (Pérez-Rangel et al. 2015). The native flora of the wheat straw
was found to have the highest hydrogen production due to the
presence of several fungal strains that played a central role
(Pérez-Rangel et al. 2015). Chang et al. have studied the
hydrogen production from Napier grass using rumen microbial consortia containing Ruminococcus sp., C. papyrosolvens, Desulfovibrio desulfuricans, Ethanoligenes harbinese,
C. xylanolyticum, and C. beijerinckii (Chang et al. 2010). For
each 8-day incubation period, with 1.5% wt. Napier grass and
stem, 2% cellulose, and 27% hemicellulose conversion,
along with concomitant H 2 production were achieved (Chang
et al. 2010). Recently, the mixed culture of Clostridium
acetobutylicum with microorganism from bovine ruminal
fluid has shown a synergistic effect on hydrogen production
from acid-treated agave biomass compared to the control
experiment (Morales-Martínez et al. 2020).
Co-culture
The co-culture of microbes containing two to three
microorganisms interacts synergistically to increase the
conversion of substrate and production of H 2 (Nagarajan
et al. 2019). The interaction is beneficial as it results in the
sequential utilization of substrates, which increases the
conversion and provides a potential metabolic intermediate
Bioconversion of Lignocellulosic Residues into Hydrogen
71