biocatalysts: a reduced contamination risk, reduced costs for process cooling as well
as better diffusion rates of gases at higher temperatures [73]. To take advantage of
using a thermophilic acetogen as biocatalyst for hydrogen storage, our group
established a whole-cell system for T. kivui as platform for hydrogen storage from
syngas [74]. The whole-cell system for T. kivui is similar to that for A. woodii and is
based on lowering the cellular ATP content by uncouplers (see Sect. 3.1). Interestingly, bicarbonate was found to be an effective inhibitor of the ATP synthase,
thus, lowering the cellular ATP content. Addition of bicarbonate not only abolished
formation of the unwanted side-product, acetate, but also led to a dramatic increase
of the formate conversion rate. In cell suspensions experiments (closed batch), the
specific formate production rates reached 234 mmolÁ g protein
−1 h
−1 , the highest
rates reported so far. In addition, the conversion of syngas to formate as a sole end
product could be shown for the first time. The finding of bicarbonate as an
affordable, environmental-friendly inhibitor has further opened the way toward the
biotechnological storage of hydrogen. Future experiments need to determine
the long-term stability of the whole-cell system and should address upscaling of the
process.
The HDCR of T. kivui also catalyzes the conversion of formate to H 2 with
exceptional rates and is therefore also a promising alternative for a formate-based
biohydrogen production [14]. The enzyme from T. kivui catalyzed conversion from
formate to H 2 with a TOF of 9 892 000 h
−1 at 60 °C. Even though a whole-cell
system of T. kivui for the specific formate-based biohydrogen production has not
been established yet, a similar whole-cell approach as in A. woodii (see Sect. 3.1)
can be used. Due to the superior catalytic rates of the HDCR from T. kivui, it can be
assumed that the specific H 2 production rate in a whole-cell system would be in a
range of the H 2 production rate of the thermophilic archaeon Thermoccocus
onnurineus, the highest reported so far [75, 76]. The system from T. kivui may be
even more suited then the one from T. onnurineus. In contrast to T. onnurineus
where H 2 is produced by a membrane-bound hydrogenase complex [77, 78], the
HDCR is a soluble, cytoplasmatic enzyme, and therefore, the enzyme activity is
energetically independent of the membrane potential.
3.3 Clostridium autoethanogenum
Clostridium autoethanogenum is, alongside with the closely related C. ljungdahlii
and C. ragsdalei, one of the few acetogens that is currently used for the industrial
production of ethanol from exhaust gases of steel manufacturing by the company
LanzaTech [8, 79]. Characteristically, the wild-type strain of C. autoethanogenum
(and their above-mentioned close relatives) produces mainly ethanol and acetate but
also minor amounts of 2,3-butanediol under autotrophic conditions with an optimal
pH between 5 − 5.5 (Fig. 2) [80, 81]. This rather low pH favors solventogenesis,
leading to an increased yield of highly reduced products such as ethanol [82].
Ethanol production in C. autoethanogenum comprises two different routes. In a first
route, acetyl-CoA is subsequential reduced into acetaldehyde by using the
Acetogenic Bacteria for Biotechnological Applications
117
as better diffusion rates of gases at higher temperatures [73]. To take advantage of
using a thermophilic acetogen as biocatalyst for hydrogen storage, our group
established a whole-cell system for T. kivui as platform for hydrogen storage from
syngas [74]. The whole-cell system for T. kivui is similar to that for A. woodii and is
based on lowering the cellular ATP content by uncouplers (see Sect. 3.1). Interestingly, bicarbonate was found to be an effective inhibitor of the ATP synthase,
thus, lowering the cellular ATP content. Addition of bicarbonate not only abolished
formation of the unwanted side-product, acetate, but also led to a dramatic increase
of the formate conversion rate. In cell suspensions experiments (closed batch), the
specific formate production rates reached 234 mmolÁ g protein
−1 h
−1 , the highest
rates reported so far. In addition, the conversion of syngas to formate as a sole end
product could be shown for the first time. The finding of bicarbonate as an
affordable, environmental-friendly inhibitor has further opened the way toward the
biotechnological storage of hydrogen. Future experiments need to determine
the long-term stability of the whole-cell system and should address upscaling of the
process.
The HDCR of T. kivui also catalyzes the conversion of formate to H 2 with
exceptional rates and is therefore also a promising alternative for a formate-based
biohydrogen production [14]. The enzyme from T. kivui catalyzed conversion from
formate to H 2 with a TOF of 9 892 000 h
−1 at 60 °C. Even though a whole-cell
system of T. kivui for the specific formate-based biohydrogen production has not
been established yet, a similar whole-cell approach as in A. woodii (see Sect. 3.1)
can be used. Due to the superior catalytic rates of the HDCR from T. kivui, it can be
assumed that the specific H 2 production rate in a whole-cell system would be in a
range of the H 2 production rate of the thermophilic archaeon Thermoccocus
onnurineus, the highest reported so far [75, 76]. The system from T. kivui may be
even more suited then the one from T. onnurineus. In contrast to T. onnurineus
where H 2 is produced by a membrane-bound hydrogenase complex [77, 78], the
HDCR is a soluble, cytoplasmatic enzyme, and therefore, the enzyme activity is
energetically independent of the membrane potential.
3.3 Clostridium autoethanogenum
Clostridium autoethanogenum is, alongside with the closely related C. ljungdahlii
and C. ragsdalei, one of the few acetogens that is currently used for the industrial
production of ethanol from exhaust gases of steel manufacturing by the company
LanzaTech [8, 79]. Characteristically, the wild-type strain of C. autoethanogenum
(and their above-mentioned close relatives) produces mainly ethanol and acetate but
also minor amounts of 2,3-butanediol under autotrophic conditions with an optimal
pH between 5 − 5.5 (Fig. 2) [80, 81]. This rather low pH favors solventogenesis,
leading to an increased yield of highly reduced products such as ethanol [82].
Ethanol production in C. autoethanogenum comprises two different routes. In a first
route, acetyl-CoA is subsequential reduced into acetaldehyde by using the
Acetogenic Bacteria for Biotechnological Applications
117