1 Introduction
Acetogenic bacteria are a phylogenetically very diverse group of strictly anaerobic
bacteria ubiquitous in nature that employ the Wood–Ljungdahl pathway (WLP) to
reduce two molecules of CO 2 to one molecule of acetyl-CoA [1–3]. Acetyl-CoA is
then further reduced to mainly acetate and in some species to minor amounts of
ethanol or butyrate. Although acetate is a common product in the metabolic pathway of acetogenic bacteria, it is not formed by all representatives of this group
under every condition tested and their ability to produce acetate therefore does not
distinguish acetogens [4, 5]. In addition, other bacterial groups, such as acetic acid
bacteria or fermenting bacteria, are also able to produce acetate from organic carbon
compounds such as alcohols or sugars [6]. A key feature of acetogenic bacteria is
their ability to grow autotrophically on H 2 + CO 2 via the WLP and the CO
dehydrogenase/acetyl‐CoA synthase (CODH/ACS) as key enzyme [3, 7]. Several
acetogens can also convert a mixture of H 2 , CO 2 and CO, called synthesis gas
(syngas) [8]. Syngas is a waste gas produced from steel manufacturing or from
gasification of agricultural waste. Depending on the source, the composition of
syngas differs [9, 10]. The capability of acetogenic bacteria to convert these gases
has led to a tremendous interest to use them as biocatalysts for the production of
biofuels such as ethanol or other valuable products [11, 12]. Especially in times of
global warming, with the release of CO 2 from fossil fuels as one of the main causes,
the call to reduce the greenhouse gas CO 2 is increasing. Therefore, the conversion
of waste gases with acetogens is of special interest since it is a promising way of
producing biofuels independent of carbohydrates while helping to reduce emissions
of CO 2 .
Acetogenic bacteria can also be used as biocatalysts to utilize one-carbon
compounds (formate or methanol) as feedstocks making them also a promising
alternative in a formate- or methanol-based bioeconomy [13]. In addition, some
acetogens, such as Acetobacterium woodii and Thermoanaerobacter kivui, possess
a unique enzyme complex capable of directly reducing CO 2 to formate with H 2 as
electron donor that is superior over any chemical catalyst for a CO 2 -based hydrogen
storage [14, 15]. Therefore, acetogens are also promising candidates as potential
catalysts for hydrogen storage or production.
2 Biochemistry of the Wood–Ljungdahl Pathway
All acetogens have in common that they reduce CO 2 to acetyl-CoA via the WLP
[1, 3, 16]. Acetyl-CoA is the central intermediate for the production of various
chemical compounds such as acetate, ethanol, 2,3-butanediol and more. The
capability for autotrophic CO 2 assimilation via the WLP makes acetogens an
important alternative for the industrial production of a variety of biofuels as well as
biocommodities.
110
D. Litty and V. Müller
Acetogenic bacteria are a phylogenetically very diverse group of strictly anaerobic
bacteria ubiquitous in nature that employ the Wood–Ljungdahl pathway (WLP) to
reduce two molecules of CO 2 to one molecule of acetyl-CoA [1–3]. Acetyl-CoA is
then further reduced to mainly acetate and in some species to minor amounts of
ethanol or butyrate. Although acetate is a common product in the metabolic pathway of acetogenic bacteria, it is not formed by all representatives of this group
under every condition tested and their ability to produce acetate therefore does not
distinguish acetogens [4, 5]. In addition, other bacterial groups, such as acetic acid
bacteria or fermenting bacteria, are also able to produce acetate from organic carbon
compounds such as alcohols or sugars [6]. A key feature of acetogenic bacteria is
their ability to grow autotrophically on H 2 + CO 2 via the WLP and the CO
dehydrogenase/acetyl‐CoA synthase (CODH/ACS) as key enzyme [3, 7]. Several
acetogens can also convert a mixture of H 2 , CO 2 and CO, called synthesis gas
(syngas) [8]. Syngas is a waste gas produced from steel manufacturing or from
gasification of agricultural waste. Depending on the source, the composition of
syngas differs [9, 10]. The capability of acetogenic bacteria to convert these gases
has led to a tremendous interest to use them as biocatalysts for the production of
biofuels such as ethanol or other valuable products [11, 12]. Especially in times of
global warming, with the release of CO 2 from fossil fuels as one of the main causes,
the call to reduce the greenhouse gas CO 2 is increasing. Therefore, the conversion
of waste gases with acetogens is of special interest since it is a promising way of
producing biofuels independent of carbohydrates while helping to reduce emissions
of CO 2 .
Acetogenic bacteria can also be used as biocatalysts to utilize one-carbon
compounds (formate or methanol) as feedstocks making them also a promising
alternative in a formate- or methanol-based bioeconomy [13]. In addition, some
acetogens, such as Acetobacterium woodii and Thermoanaerobacter kivui, possess
a unique enzyme complex capable of directly reducing CO 2 to formate with H 2 as
electron donor that is superior over any chemical catalyst for a CO 2 -based hydrogen
storage [14, 15]. Therefore, acetogens are also promising candidates as potential
catalysts for hydrogen storage or production.
2 Biochemistry of the Wood–Ljungdahl Pathway
All acetogens have in common that they reduce CO 2 to acetyl-CoA via the WLP
[1, 3, 16]. Acetyl-CoA is the central intermediate for the production of various
chemical compounds such as acetate, ethanol, 2,3-butanediol and more. The
capability for autotrophic CO 2 assimilation via the WLP makes acetogens an
important alternative for the industrial production of a variety of biofuels as well as
biocommodities.
110
D. Litty and V. Müller