produce H 2 from CO [95, 102]. Moreover, Zhao and coworkers investigated the
performance of Carboxydotthermus hydrogenoformans as whole-cells catalysts in a
bioprocess for conversion of CO to H 2 [120]. The bioprocess led to a maximum H 2
production of 117.7 mmol/L/h. Furthermore, two hyperthermophilic archaea,
Thermococcus sp. AM4 and Thermococcus onnurineus NA1 can also grow on CO
and produce H 2 [113, 121, 122]. The maximum H 2 production rate of
1.55 mmol/L/h with a yield of 100% was obtained with Thermococcus onnurineus
NA1 grown on CO [113]. Recently, another thermophile with ability to catalyze
CO-dependent H 2 production has been isolated, the facultative anaerobic bacterium
Parageobacillus thermoglucosidasius [123]. Surprisingly, this bacterium can grow
under 50% CO and 50% air, first consuming the O 2 and then utilizing the CO for
the production of H 2 [123].
Anaerobic bacterial communities have also been used for the conversion of CO
to H 2 [124, 125]. Recently, Sinharoy and coworkers evaluated the CO conversion
by a bacterial consortium obtained from an upflow anaerobic sludge blanket reactor.
This consortium was able to produce a maximum H 2 production of 29.9 mmol/L
with a maximum yield of 70% from an initial CO concentration of 5 mmol/L [124].
Concerning syngas, its bioconversion to hydrogen by the photosynthetic bacterium Rhodospirillum rubrum has also been reported, with a production rate of
9.6 mmol/h and a maximum CO conversion efficiency of 81% [126]. A high
potential for bioconversion of CO from steel mill waste gas to H 2 by a engineered
Thermococcus onnurineus NA1 strain was documented [121].
Over the years, numerous studies have been performed to enhance activity of
CO-dependent H 2 production through genetic engineering and process optimization. Ainala and coworkers developed a recombinant Citrobacter amalonaticus
Y19 strain where the CO sensing transcriptional activator cooA was overexpressed.
The overexpression of cooA improved the whole-cell CO-dependent H 2 production
activity 3.4 fold [119]. BioH 2 production was also improved by the overexpressing
of the CODH gene cluster in T. onnurineus NA1, leading to a H 2 production of
124 mmol/L/h, which was 3.8-fold higher than the one obtained by the wild-type
strain [121]. In 2015, the same group manipulated the transcriptional regulatory
system of T. onnurineus NA1 obtaining a modified strain with higher H 2 production
rate (5.8-fold that the wild-type). A H 2 production rate of 191.9 mmol/L/h and a
specific H 2 production rate of 249.6 mmol/g cell /h were obtained with this engineered strain [127].
One of the major bottlenecks in biological CO conversion to H 2 is the low gas–
liquid mass transfer. Therefore, several bioreactors have been designed to overcome
this issue and it was found that the hollow fibre membrane (HFM) bioreactor is very
effective in the mass transfer of CO, reaching the highest kLa for CO [120, 128]. In
fact, the HFM bioreactor is considered as one of the most promising reactor configurations for syngas fermentation [128, 129]. Another approach that has been
investigated to increase CO solubility is the incorporation of nanoparticles in
bioreactor. This strategy is very effective even when a small amount of nanoparticles is used [130, 131]. Recently, the potential of biogenic iron nanoparticles
synthesized using green-tea extract was evaluated to enhance the biohydrogen
Biological Production of Hydrogen
265
performance of Carboxydotthermus hydrogenoformans as whole-cells catalysts in a
bioprocess for conversion of CO to H 2 [120]. The bioprocess led to a maximum H 2
production of 117.7 mmol/L/h. Furthermore, two hyperthermophilic archaea,
Thermococcus sp. AM4 and Thermococcus onnurineus NA1 can also grow on CO
and produce H 2 [113, 121, 122]. The maximum H 2 production rate of
1.55 mmol/L/h with a yield of 100% was obtained with Thermococcus onnurineus
NA1 grown on CO [113]. Recently, another thermophile with ability to catalyze
CO-dependent H 2 production has been isolated, the facultative anaerobic bacterium
Parageobacillus thermoglucosidasius [123]. Surprisingly, this bacterium can grow
under 50% CO and 50% air, first consuming the O 2 and then utilizing the CO for
the production of H 2 [123].
Anaerobic bacterial communities have also been used for the conversion of CO
to H 2 [124, 125]. Recently, Sinharoy and coworkers evaluated the CO conversion
by a bacterial consortium obtained from an upflow anaerobic sludge blanket reactor.
This consortium was able to produce a maximum H 2 production of 29.9 mmol/L
with a maximum yield of 70% from an initial CO concentration of 5 mmol/L [124].
Concerning syngas, its bioconversion to hydrogen by the photosynthetic bacterium Rhodospirillum rubrum has also been reported, with a production rate of
9.6 mmol/h and a maximum CO conversion efficiency of 81% [126]. A high
potential for bioconversion of CO from steel mill waste gas to H 2 by a engineered
Thermococcus onnurineus NA1 strain was documented [121].
Over the years, numerous studies have been performed to enhance activity of
CO-dependent H 2 production through genetic engineering and process optimization. Ainala and coworkers developed a recombinant Citrobacter amalonaticus
Y19 strain where the CO sensing transcriptional activator cooA was overexpressed.
The overexpression of cooA improved the whole-cell CO-dependent H 2 production
activity 3.4 fold [119]. BioH 2 production was also improved by the overexpressing
of the CODH gene cluster in T. onnurineus NA1, leading to a H 2 production of
124 mmol/L/h, which was 3.8-fold higher than the one obtained by the wild-type
strain [121]. In 2015, the same group manipulated the transcriptional regulatory
system of T. onnurineus NA1 obtaining a modified strain with higher H 2 production
rate (5.8-fold that the wild-type). A H 2 production rate of 191.9 mmol/L/h and a
specific H 2 production rate of 249.6 mmol/g cell /h were obtained with this engineered strain [127].
One of the major bottlenecks in biological CO conversion to H 2 is the low gas–
liquid mass transfer. Therefore, several bioreactors have been designed to overcome
this issue and it was found that the hollow fibre membrane (HFM) bioreactor is very
effective in the mass transfer of CO, reaching the highest kLa for CO [120, 128]. In
fact, the HFM bioreactor is considered as one of the most promising reactor configurations for syngas fermentation [128, 129]. Another approach that has been
investigated to increase CO solubility is the incorporation of nanoparticles in
bioreactor. This strategy is very effective even when a small amount of nanoparticles is used [130, 131]. Recently, the potential of biogenic iron nanoparticles
synthesized using green-tea extract was evaluated to enhance the biohydrogen
Biological Production of Hydrogen
265