transfer/accept electrons from solid surfaces (cathodes) to reduce CO 2 into organic
compounds [95]. The cathode is usually made of carbonaceous materials (such as
carbon felt or carbon brush) due to their high surface area, biocompatibility, conductivity and chemical stability. The interactions mechanisms between the cathode
and the electrotrophs are still under discussion, though they usually are divided as
direct (where microorganism directly receives electrons from the cathode) and
indirect (where a chemical mediator, such as methyl viologen or neutral red,
operates as an intermediary between the cathode and the microbe) [96, 97].
The anode serves as the counter-electrode and its function is to provide the
necessary protons and electrons to the cathodic reaction. As MES technology has
not yet moved beyond the lab, most of the current MES designs favour simplicity
over efficiency. As a result, dimensionally stable electrodes are frequently used to
carry out the anodic reaction. A reference electrode (which provides a well-known
and stable potential) is frequently placed on the cathodic chamber to fix/control the
cathode potential. Finally, the presence of the IEM is crucial because it prevents the
oxygen generated in the anodic reaction from reaching the cathodophilic anaerobic
microbial community on the cathode, which is quite sensitive to this gas.
Biocathodes are usually inoculated with inoculums obtained from anaerobic
sludge or sediments [98]. Although the mixed microbial communities that finally
develop on the bio-cathodic environment inevitably depends on the source of the
inoculum, they tend to get enriched in microorganisms like Sporomusa ovata and
Clostridium ljungdahlii [99]. These two microorganism belong to the group of
homoacetogenic bacteria, which means that they are capable of producing acetic
acid from CO 2 following the Wood-Ljungdahl pathway (this explains why acetic
acid is the most common end product found in MES [100]). Sulfate-reducing
bacteria and hydrogen producers are also commonly found on biocathodes, as well
as hydrogenotrophic and acetoclastic archaea in the case on methane-producing
systems [101–103].
When researchers perform experiments with pure cultures, they often resort to S.
ovata and C. ljungdahlii [99, 103–105], precisely because of their relatively high
abundance in systems inoculated with mixed cultures. To date, experiments with
pure cultures have yielded the highest titers, productivities and selectivities [103],
although they require sterility and fine control of reaction conditions all of which
makes them not the bests candidates for future real-life applications [103]. In
contrast, the use of mixed microbial consortia harvested from natural or engineered
environments demand less restrictive conditions and reduces process cost [98]. As a
result, most of the experiments available in the literature make use of mixed cultures, reporting moderate productivities and selectivity. Still, when enriched and
acclimated following suitable procedures, mixed cultures can reach comparable
results (in terms of productivity and selectivity) to those obtained with pure cultures
[106–108].
The in vivo catalytic reactions for CO 2 reduction are driven by enzymes such as
the carbonic anhydrase and other enzymes belonging to the oxidoreductase family,
that determine the end product of their electrotrophic metabolism [88, 91, 109].
These enzymes take part in multi-enzymatic cascade reactions or in single-enzyme
Carbon Dioxide Utilization—Bioelectrochemical Approaches
95
compounds [95]. The cathode is usually made of carbonaceous materials (such as
carbon felt or carbon brush) due to their high surface area, biocompatibility, conductivity and chemical stability. The interactions mechanisms between the cathode
and the electrotrophs are still under discussion, though they usually are divided as
direct (where microorganism directly receives electrons from the cathode) and
indirect (where a chemical mediator, such as methyl viologen or neutral red,
operates as an intermediary between the cathode and the microbe) [96, 97].
The anode serves as the counter-electrode and its function is to provide the
necessary protons and electrons to the cathodic reaction. As MES technology has
not yet moved beyond the lab, most of the current MES designs favour simplicity
over efficiency. As a result, dimensionally stable electrodes are frequently used to
carry out the anodic reaction. A reference electrode (which provides a well-known
and stable potential) is frequently placed on the cathodic chamber to fix/control the
cathode potential. Finally, the presence of the IEM is crucial because it prevents the
oxygen generated in the anodic reaction from reaching the cathodophilic anaerobic
microbial community on the cathode, which is quite sensitive to this gas.
Biocathodes are usually inoculated with inoculums obtained from anaerobic
sludge or sediments [98]. Although the mixed microbial communities that finally
develop on the bio-cathodic environment inevitably depends on the source of the
inoculum, they tend to get enriched in microorganisms like Sporomusa ovata and
Clostridium ljungdahlii [99]. These two microorganism belong to the group of
homoacetogenic bacteria, which means that they are capable of producing acetic
acid from CO 2 following the Wood-Ljungdahl pathway (this explains why acetic
acid is the most common end product found in MES [100]). Sulfate-reducing
bacteria and hydrogen producers are also commonly found on biocathodes, as well
as hydrogenotrophic and acetoclastic archaea in the case on methane-producing
systems [101–103].
When researchers perform experiments with pure cultures, they often resort to S.
ovata and C. ljungdahlii [99, 103–105], precisely because of their relatively high
abundance in systems inoculated with mixed cultures. To date, experiments with
pure cultures have yielded the highest titers, productivities and selectivities [103],
although they require sterility and fine control of reaction conditions all of which
makes them not the bests candidates for future real-life applications [103]. In
contrast, the use of mixed microbial consortia harvested from natural or engineered
environments demand less restrictive conditions and reduces process cost [98]. As a
result, most of the experiments available in the literature make use of mixed cultures, reporting moderate productivities and selectivity. Still, when enriched and
acclimated following suitable procedures, mixed cultures can reach comparable
results (in terms of productivity and selectivity) to those obtained with pure cultures
[106–108].
The in vivo catalytic reactions for CO 2 reduction are driven by enzymes such as
the carbonic anhydrase and other enzymes belonging to the oxidoreductase family,
that determine the end product of their electrotrophic metabolism [88, 91, 109].
These enzymes take part in multi-enzymatic cascade reactions or in single-enzyme
Carbon Dioxide Utilization—Bioelectrochemical Approaches
95