processes. A relevant example of the later in MES is the conversion of CO 2 into
formate by FDH enzyme. Chen and colleagues [110] studied the production of Poly
3-hydroxybutyrate (PHB) in a FDH-assisted MES system with genetically engineered Ralstonia eutropha, using formate production as intermediate electron
carrier for further microbial conversion.
3.2 Possible Products
Volatile fatty acids (VFAs) have been the first and the most frequently reported
organics obtained from CO 2 reduction in MES [111]. The production of acetate,
which is the main representative product of this group, is mediated by homoacetogenic microorganisms that use, as mentioned before, the Wood-Ljungdahl route
for its synthesis [87, 111]. Acetate productivities in MES can be as high
as *0.8 gÁL
−1
Ád
−1 with efficiencies near 100% in terms of current-to-acetate
conversion, and achieving product titers up to 13.5 gÁL
−1 [96, 106, 107]. Butyrate is
the second most frequently found VFA in MES [103], and was shown to be formed
from CO 2 or to be synthetized from acetate with moderate input of energy [103]. As
a result, butyrate is usually found as a by-product in acetate-producing MES [100].
Nevertheless, when butyrate is the target product, productivities can reach up to
0.16 gÁL
−1
Ád
−1 with maximum titers of 5.5 gÁL
−1 [112]. Other co-products (VFAs
and average chain fatty acids) were also found in MES systems that produce acetate
but not being directly targeted to date [113].
Short-chain alcohols (such as ethanol, butanol and glycerol) is another group of
easily marketable chemicals that can be found in the biocathodes of MES, although
with modest productivities, selectivities and titers [114–116]. MES can be directed
to the production of these organics when an excess of reducing power (as hydrogen
for instance) is found in the cathodic chamber [117]. MES also make it possible to
obtain alcohols from volatile fatty acids by using the cathode (instead of hydrogen)
as the electron donor for the reductive process [114]. This route is thermodynamically favourable and so it is expected that alcohols production in MES would
follow primarily these mechanisms.
Both, VFAs and alcohols can be used as building blocks to obtain longer chain
organic compounds through chain elongation reactions (CER) [103]. As MES are
commonly inoculated with mixed microbial cultures, CER can occur naturally
within the catholyte as observed by several authors [112, 118].
Methane is another valuable product commonly found in the biocathodes of
MES. It can be produced not only from hydrogen and/or acetate (usually present in
the cathodic environment of MES) through well-known routes that involve
hydrogenothophic and/or acetorophic methanogens [119] respectively, but also via
direct electron transfer mechanisms from the cathode [120, 121]. These processes
that ultimately comprise the reduction of CO 2 to CH 4 at a biocathode using electrical current are usually termed as electromethanogenesis [105]. Initial studies on
microbial electromethanogenesis reported production rates below 5 LCH4Ád
−1
Ám
−2
with an overall energy efficiency of *80% [121]. Improved reactor designs,
96
C. M. Cordas et al.
formate by FDH enzyme. Chen and colleagues [110] studied the production of Poly
3-hydroxybutyrate (PHB) in a FDH-assisted MES system with genetically engineered Ralstonia eutropha, using formate production as intermediate electron
carrier for further microbial conversion.
3.2 Possible Products
Volatile fatty acids (VFAs) have been the first and the most frequently reported
organics obtained from CO 2 reduction in MES [111]. The production of acetate,
which is the main representative product of this group, is mediated by homoacetogenic microorganisms that use, as mentioned before, the Wood-Ljungdahl route
for its synthesis [87, 111]. Acetate productivities in MES can be as high
as *0.8 gÁL
−1
Ád
−1 with efficiencies near 100% in terms of current-to-acetate
conversion, and achieving product titers up to 13.5 gÁL
−1 [96, 106, 107]. Butyrate is
the second most frequently found VFA in MES [103], and was shown to be formed
from CO 2 or to be synthetized from acetate with moderate input of energy [103]. As
a result, butyrate is usually found as a by-product in acetate-producing MES [100].
Nevertheless, when butyrate is the target product, productivities can reach up to
0.16 gÁL
−1
Ád
−1 with maximum titers of 5.5 gÁL
−1 [112]. Other co-products (VFAs
and average chain fatty acids) were also found in MES systems that produce acetate
but not being directly targeted to date [113].
Short-chain alcohols (such as ethanol, butanol and glycerol) is another group of
easily marketable chemicals that can be found in the biocathodes of MES, although
with modest productivities, selectivities and titers [114–116]. MES can be directed
to the production of these organics when an excess of reducing power (as hydrogen
for instance) is found in the cathodic chamber [117]. MES also make it possible to
obtain alcohols from volatile fatty acids by using the cathode (instead of hydrogen)
as the electron donor for the reductive process [114]. This route is thermodynamically favourable and so it is expected that alcohols production in MES would
follow primarily these mechanisms.
Both, VFAs and alcohols can be used as building blocks to obtain longer chain
organic compounds through chain elongation reactions (CER) [103]. As MES are
commonly inoculated with mixed microbial cultures, CER can occur naturally
within the catholyte as observed by several authors [112, 118].
Methane is another valuable product commonly found in the biocathodes of
MES. It can be produced not only from hydrogen and/or acetate (usually present in
the cathodic environment of MES) through well-known routes that involve
hydrogenothophic and/or acetorophic methanogens [119] respectively, but also via
direct electron transfer mechanisms from the cathode [120, 121]. These processes
that ultimately comprise the reduction of CO 2 to CH 4 at a biocathode using electrical current are usually termed as electromethanogenesis [105]. Initial studies on
microbial electromethanogenesis reported production rates below 5 LCH4Ád
−1
Ám
−2
with an overall energy efficiency of *80% [121]. Improved reactor designs,
96
C. M. Cordas et al.