The WLP and its energetics have been studied extensively in acetogenic bacteria
such as A. woodii, C. ljungdahlii or Moorella thermoacetica [18, 33–35]. The
information obtained is not only helpful for better understanding of the mechanism
of energy conservation itself but also is the foundation for further optimizations of
these acetogens with respect to the production of biofuels.
3 Acetogens as Biocatalysts
The group of acetogens includes over 100 different species that are present in 23
different genera; however, more than 90% of all acetogens identified to date are
only able to produce acetate as sole product [36]. Therefore, only a few are considered for the biotechnological production of biofuels and biocommodities such as
ethanol, butanol and 2,3-butanediole [34, 37]. A list of acetogens currently pursued
for biotechnological applications with their characteristics is summarized in
Table 1. Acetogens currently considered as biocatalyst for industrial production
belong mainly to the class of Clostridia including genera such as Acetobacterium
and Clostridium [34, 38]. Key features of acetogenic Clostridia are their ability to
grow on many different substrates and their high metabolic product diversity [39,
40], which makes them well suitable for the industrial production of a broad range
of biochemicals. They can utilize a wide range of C5 (Pentose) or C6 (Hexose)
sugars. However, fermentation of these sugars is not suitable for a sustainable
production of biochemicals, since it offers low yields due to formation of significant
amounts of CO 2 and the usage of crops (mostly sugarcane or wheat) as feedstocks
competes with production of food for mankind [41, 42]. Besides sugars, they can
also utilize cheap feedstocks such as glycerol and cellulose [38]. In addition, they
are also able to utilize C1 compounds such as methanol, formate or as mentioned
before, syngas (H 2 , CO 2 , CO). The natural products produced include a wide range
of valuable chemicals such as acetate, ethanol, butanol, hexanol, acetone and more
[8]. Clostridial acetogens considered for biotechnological applications include
A. woodii, Clostridium aceticum, Clostridium autoethanogenum, C. ljungdahlii and
Clostridium ragsdalei. A. woodii and C. aceticum are mainly considered for the
production of acetate due to their high production rates, whereas the other species
mentioned are mainly considered for the production of 2,3-butanediol as well as
ethanol [39]. In addition, A. woodii offers the possibility of hydrogen storage due to
its unique enzyme complex capable of directly reducing CO 2 to formate with H 2 as
electron donor [15]. The same complex is found in the thermophilic acetogen
T. kivui making it also a promising candidate for storage of hydrogen [14], a
promising energy carrier that is considered as a sustainable, emission-free
replacement for non-renewable fossil fuels [43, 44]. In addition to hydrogen storage, the enzyme complex can also be used for carbon capture and storage [13].
So far, we only briefly described the acetogens considered for biotechnological
applications. Next, we will have a more detailed look into some highlighted acetogens with respect to their biotechnological applications in the following section.
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113
such as A. woodii, C. ljungdahlii or Moorella thermoacetica [18, 33–35]. The
information obtained is not only helpful for better understanding of the mechanism
of energy conservation itself but also is the foundation for further optimizations of
these acetogens with respect to the production of biofuels.
3 Acetogens as Biocatalysts
The group of acetogens includes over 100 different species that are present in 23
different genera; however, more than 90% of all acetogens identified to date are
only able to produce acetate as sole product [36]. Therefore, only a few are considered for the biotechnological production of biofuels and biocommodities such as
ethanol, butanol and 2,3-butanediole [34, 37]. A list of acetogens currently pursued
for biotechnological applications with their characteristics is summarized in
Table 1. Acetogens currently considered as biocatalyst for industrial production
belong mainly to the class of Clostridia including genera such as Acetobacterium
and Clostridium [34, 38]. Key features of acetogenic Clostridia are their ability to
grow on many different substrates and their high metabolic product diversity [39,
40], which makes them well suitable for the industrial production of a broad range
of biochemicals. They can utilize a wide range of C5 (Pentose) or C6 (Hexose)
sugars. However, fermentation of these sugars is not suitable for a sustainable
production of biochemicals, since it offers low yields due to formation of significant
amounts of CO 2 and the usage of crops (mostly sugarcane or wheat) as feedstocks
competes with production of food for mankind [41, 42]. Besides sugars, they can
also utilize cheap feedstocks such as glycerol and cellulose [38]. In addition, they
are also able to utilize C1 compounds such as methanol, formate or as mentioned
before, syngas (H 2 , CO 2 , CO). The natural products produced include a wide range
of valuable chemicals such as acetate, ethanol, butanol, hexanol, acetone and more
[8]. Clostridial acetogens considered for biotechnological applications include
A. woodii, Clostridium aceticum, Clostridium autoethanogenum, C. ljungdahlii and
Clostridium ragsdalei. A. woodii and C. aceticum are mainly considered for the
production of acetate due to their high production rates, whereas the other species
mentioned are mainly considered for the production of 2,3-butanediol as well as
ethanol [39]. In addition, A. woodii offers the possibility of hydrogen storage due to
its unique enzyme complex capable of directly reducing CO 2 to formate with H 2 as
electron donor [15]. The same complex is found in the thermophilic acetogen
T. kivui making it also a promising candidate for storage of hydrogen [14], a
promising energy carrier that is considered as a sustainable, emission-free
replacement for non-renewable fossil fuels [43, 44]. In addition to hydrogen storage, the enzyme complex can also be used for carbon capture and storage [13].
So far, we only briefly described the acetogens considered for biotechnological
applications. Next, we will have a more detailed look into some highlighted acetogens with respect to their biotechnological applications in the following section.
Acetogenic Bacteria for Biotechnological Applications
113