8
ylic acids it is possible to obtain polymers such as polyurethanes, polyesters, and
polyamides. Among the exploitable biological pathways addressing dicarboxylic
acids production, poor is the knowledge on the potential of using CO 2 as feedstock.
Indeed, the main carbon sources are represented by glycerol, glucose, and intermediates of the tricarboxylic acids cycle, like oxaloacetate, α-ketoglutarate, and succinyl-CoA (Le Yu et al. 2018). A great effort has been focused on the engineering of
natural producers of these organic acids and the realization of artificial pathways in
industrial-applied microorganisms (i.e., Escherichia coli and Corynebacterium glutamicum), which made possible the use of CO 2 as a direct or indirect substrate for
dicarboxylic acid production. Noteworthy, in order to allow a competitive bio-based
dicarboxylic acid production and to reduce the production costs, not only the development of the host microbial cell but also the optimization of the downstream processes is required (Straathof 2011).
Succinic Acid
Succinic acid captivated a lot of attention due to its role as key intermediate to produce a large range of valuable chemicals. Indeed, from succinic acid it is possible to
generate polybutylene succinate (PBS), 1,4-butanediol, polyester polyols,
γ-butyrolactone, tetrahydrofuran (THF) (Choi et al. 2015a), N-methyl pyrrolidinone, 2-pyrrolidinone, succinate salts, poly-butyrate, and polyamides, in addition to
various green solvents (Li et al. 2011) (McKinlay et al. 2007) (Zheng et al. 2009).
Large-scale industrial production has been mostly based on petrochemical sources.
Small companies invested their resources much more on the sustainability and
renewability of the productive process, orienting the R&D on a bio-based strategy
rather than chemical-based (Clomburg et al. 2017). Succinic acid could be produced
by many anaerobic microbes, such as Corynebacterium glutamicum, Actinobacillus
succinogenes, Anaerobiospirillum succiniciproducens, Escherichia coli,
Mannheimia succiniciproducens, Clostridium ljungdahlii, Saccharomyces cerevisiae, Pichia kudriavzevii, and Basfia succiniciproducens (Zou et al. 2011). Particular
attention has been given to the metabolic engineering of such microbial strains (Ahn
et al. 2016). In the frame of bio-based production, the most used feedstock to produce succinic acid is glucose, followed by xylose and glycerol (Jiang et al. 2017).
However, alternative sources are desirable owing to the elevated cost of feedstock
supply (https://www.indexmundi.com) and according to the Kyoto and, more
recently, to the Paris agreements which are heading for the reduction of carbon
footprint. Phototrophic organisms, i.e., microalgae, can be applied for succinic acid
production, which is naturally generated in the reductive TCA cycle while reducing
CO 2 . Alternatively, the green alga Chlamydomonas reinhardtii has been used as
starch accumulation system to be degraded by the succinic acid producer
Corynebacterium glutamicum ATCC 13032 (Lee et al. 2014). Genetic manipulation
has been operated on Corynebacterium glutamicum ATCC 13032 to enable it to
degrade starch and produce only succinic acid via a modified TCA pathway. A productivity of 280 mg of succinate/g of total sugars, including starch, was achieved in
A. A. Azim et al.
ylic acids it is possible to obtain polymers such as polyurethanes, polyesters, and
polyamides. Among the exploitable biological pathways addressing dicarboxylic
acids production, poor is the knowledge on the potential of using CO 2 as feedstock.
Indeed, the main carbon sources are represented by glycerol, glucose, and intermediates of the tricarboxylic acids cycle, like oxaloacetate, α-ketoglutarate, and succinyl-CoA (Le Yu et al. 2018). A great effort has been focused on the engineering of
natural producers of these organic acids and the realization of artificial pathways in
industrial-applied microorganisms (i.e., Escherichia coli and Corynebacterium glutamicum), which made possible the use of CO 2 as a direct or indirect substrate for
dicarboxylic acid production. Noteworthy, in order to allow a competitive bio-based
dicarboxylic acid production and to reduce the production costs, not only the development of the host microbial cell but also the optimization of the downstream processes is required (Straathof 2011).
Succinic Acid
Succinic acid captivated a lot of attention due to its role as key intermediate to produce a large range of valuable chemicals. Indeed, from succinic acid it is possible to
generate polybutylene succinate (PBS), 1,4-butanediol, polyester polyols,
γ-butyrolactone, tetrahydrofuran (THF) (Choi et al. 2015a), N-methyl pyrrolidinone, 2-pyrrolidinone, succinate salts, poly-butyrate, and polyamides, in addition to
various green solvents (Li et al. 2011) (McKinlay et al. 2007) (Zheng et al. 2009).
Large-scale industrial production has been mostly based on petrochemical sources.
Small companies invested their resources much more on the sustainability and
renewability of the productive process, orienting the R&D on a bio-based strategy
rather than chemical-based (Clomburg et al. 2017). Succinic acid could be produced
by many anaerobic microbes, such as Corynebacterium glutamicum, Actinobacillus
succinogenes, Anaerobiospirillum succiniciproducens, Escherichia coli,
Mannheimia succiniciproducens, Clostridium ljungdahlii, Saccharomyces cerevisiae, Pichia kudriavzevii, and Basfia succiniciproducens (Zou et al. 2011). Particular
attention has been given to the metabolic engineering of such microbial strains (Ahn
et al. 2016). In the frame of bio-based production, the most used feedstock to produce succinic acid is glucose, followed by xylose and glycerol (Jiang et al. 2017).
However, alternative sources are desirable owing to the elevated cost of feedstock
supply (https://www.indexmundi.com) and according to the Kyoto and, more
recently, to the Paris agreements which are heading for the reduction of carbon
footprint. Phototrophic organisms, i.e., microalgae, can be applied for succinic acid
production, which is naturally generated in the reductive TCA cycle while reducing
CO 2 . Alternatively, the green alga Chlamydomonas reinhardtii has been used as
starch accumulation system to be degraded by the succinic acid producer
Corynebacterium glutamicum ATCC 13032 (Lee et al. 2014). Genetic manipulation
has been operated on Corynebacterium glutamicum ATCC 13032 to enable it to
degrade starch and produce only succinic acid via a modified TCA pathway. A productivity of 280 mg of succinate/g of total sugars, including starch, was achieved in
A. A. Azim et al.