therefore is accumulated. One possibility to uncouple the pathway from ATP
production is by using ionophores. Since the energy metabolism in A. woodii is
strictly Na
+
-dependent [59, 60], including a Na
+
-dependent Rnf [61, 62] as well as a
Na
+
-dependent ATP synthase [29, 63, 64], omitting Na
+ in the medium also leads
to an increased formate production.
The development of this whole-cell system paved the road for the biotechnological storage of hydrogen. However, further experiments with respect to
long-term stability of the system, product toxicity and the search of low-cost,
environmental-friendly uncouplers need be carried out.
The HDCR cannot only catalyze the direct hydrogenation of CO 2 to formate but
also its reverse reaction and therefore is a promising candidate for a formate-based
production of biohydrogen. Biohydrogen is a sustainable, environmentally friendly
alternative toward a fuel-based production. Current approaches for biohydrogen
production are mainly based on biophotolysis or fermentation [65, 66]. A promising
alternative is to take advantage of the exceptional high activity of the HDCR of
A. woodii and to establish an efficient whole-cell system for a formate-based biohydrogen production [14, 15]. Formate, a low-cost feedstock, can be utilized by
A. woodii in high concentrations (<300 mM formate). Therefore, a two-step closed
batch system was established [67]. In a first step, cells were grown on fructose to
mid-exponential phase and then production of H 2 was initiated by addition of
formate. These growing cells reached specific H 2 production rates of 66 mmol H 2
g CDW
−1 h
−1 , one of the highest reported so far for a mesophilic organism without
genetic modifications [68, 69]. One major problem that has to be overcome is to
abolish production of the unwanted side-product, which mainly decreased the H 2
yield. Further optimization of biohydrogen production should therefore address this
problem.
3.2 Thermoanaerobacter kivui
The thermophilic acetogen T. kivui, which was isolated from sediments of Lake
Kivu (Africa), also has a HDCR and therefore, is also a promising candidate for the
biotechnological storage of hydrogen [14]. In comparison to the HDCR from
A. woodii, the enzyme from T. kivui is even superior to chemical catalysts for the
hydrogenation of CO 2 . The CO 2 hydrogenation is catalyzed with a turnover frequency of 9 556 000 h
−1 at 60 °C and 1 bar H 2 . Interestingly, even at 30 °C, the
conversion rate is 18-fold higher compared to the one from A. woodii. The HDCR
from T. kivui is highly thermostable, making it more flexible with respect to process
parameters for a biotechnological application. However, the enzyme also requires
strict anoxic conditions, thus, making applications rather difficult.
A key advantage of T. kivui over A. woodii is that it can grow on syngas/CO in
mineral medium without the additional requirement for yeast extract and vitamins
[28, 70], whereas A. woodii cannot grow on syngas at all [71, 72]. As a thermophilic acetogen, with an optimal growth temperature of 66 °C, T. kivui also
offers several advantages in a fermentation process compared to mesophilic
116
D. Litty and V. Müller
production is by using ionophores. Since the energy metabolism in A. woodii is
strictly Na
+
-dependent [59, 60], including a Na
+
-dependent Rnf [61, 62] as well as a
Na
+
-dependent ATP synthase [29, 63, 64], omitting Na
+ in the medium also leads
to an increased formate production.
The development of this whole-cell system paved the road for the biotechnological storage of hydrogen. However, further experiments with respect to
long-term stability of the system, product toxicity and the search of low-cost,
environmental-friendly uncouplers need be carried out.
The HDCR cannot only catalyze the direct hydrogenation of CO 2 to formate but
also its reverse reaction and therefore is a promising candidate for a formate-based
production of biohydrogen. Biohydrogen is a sustainable, environmentally friendly
alternative toward a fuel-based production. Current approaches for biohydrogen
production are mainly based on biophotolysis or fermentation [65, 66]. A promising
alternative is to take advantage of the exceptional high activity of the HDCR of
A. woodii and to establish an efficient whole-cell system for a formate-based biohydrogen production [14, 15]. Formate, a low-cost feedstock, can be utilized by
A. woodii in high concentrations (<300 mM formate). Therefore, a two-step closed
batch system was established [67]. In a first step, cells were grown on fructose to
mid-exponential phase and then production of H 2 was initiated by addition of
formate. These growing cells reached specific H 2 production rates of 66 mmol H 2
g CDW
−1 h
−1 , one of the highest reported so far for a mesophilic organism without
genetic modifications [68, 69]. One major problem that has to be overcome is to
abolish production of the unwanted side-product, which mainly decreased the H 2
yield. Further optimization of biohydrogen production should therefore address this
problem.
3.2 Thermoanaerobacter kivui
The thermophilic acetogen T. kivui, which was isolated from sediments of Lake
Kivu (Africa), also has a HDCR and therefore, is also a promising candidate for the
biotechnological storage of hydrogen [14]. In comparison to the HDCR from
A. woodii, the enzyme from T. kivui is even superior to chemical catalysts for the
hydrogenation of CO 2 . The CO 2 hydrogenation is catalyzed with a turnover frequency of 9 556 000 h
−1 at 60 °C and 1 bar H 2 . Interestingly, even at 30 °C, the
conversion rate is 18-fold higher compared to the one from A. woodii. The HDCR
from T. kivui is highly thermostable, making it more flexible with respect to process
parameters for a biotechnological application. However, the enzyme also requires
strict anoxic conditions, thus, making applications rather difficult.
A key advantage of T. kivui over A. woodii is that it can grow on syngas/CO in
mineral medium without the additional requirement for yeast extract and vitamins
[28, 70], whereas A. woodii cannot grow on syngas at all [71, 72]. As a thermophilic acetogen, with an optimal growth temperature of 66 °C, T. kivui also
offers several advantages in a fermentation process compared to mesophilic
116
D. Litty and V. Müller