4 Conclusion and Future Perspectives
Nature mimicking through bioelectrochemical approaches seems to a promising
route to take advantage of the available atmospheric CO 2 as a source of added-value
compounds and to reduce its current levels. From simple systems using only one
enzyme, to multi enzymatic and whole cells’ BES, MES and MFC, and independently of the high efficiencies, one drawback is present, that is the cost either of the
enzymes purification, the co-factors or the materials to be used in these systems.
Several approaches have been gaining importance to overcome those limitations,
namely, the use of artificial or engineered enzymes, obtained by different methods
e.g. site-directed mutagenesis, or by designing novel “enzymes” (usually using
more simple molecular structures, hybrid metal–organic frameworks, proteins or
polypeptides as base) mimicking the catalytic centres and activities [140–143].
Some examples include, for instance, metalloporphyrins (namely with Fe) immobilized in highly conductive materials such as carbon nanotubes that were successfully tested towards CO 2 reduction in modified electrodes and fuel cells with
promising results [144]; the synthesis or isolation of iron-sulfur clusters embedded
in supramolecular “artificial” structures, such as cyclodextrins or polypeptides, for
example for nitrogenase complex iron-cluster mimicking [145] and the incorporation of Mo in the more simple rubredoxin protein, by substitution of the native Fe,
towards mimicking the FDH catalytic centre [146]. Other approaches, using
directed evolution is the activity improvement of existing enzymes, such as FDH, to
obtain more efficient variants [143]. Also, the adaptation of existing microorganisms towards novel or improved functions, aiming incorporation in MES systems,
such as methanogenic mixed cultures for the reduction of CO 2 to CH 4 [147].
MES is currently limited to the generation of mainly acetate, a low-value product
that makes it difficult the implementation of this technology to a commercial scale. Still,
the low amounts of longer chain fatty acids (such as butyric or caproic acids which are
much more interesting from a commercial point of view) that have been occasionally
found in the catholytes, indicates that MES has the capability for chain elongation. As
advanced by LaBelle et al. [148], producing higher amounts of these (and other)
organics would demand the development of means to control the regulation of the
relevant genes in the electrosynthetic microbiome. An example of this is the work
reported in Wu et al. [149] where the authors describe an electroactive succinateproducing cell factory engineered in E. coli, that harnessed the electrons arriving at the
cathode to reduce fumarate. Overall, it is widely admitted that synthetic biology can
help to make MES a commercially sound technology by allowing producing new
organics of interest for industry [148, 150]. However, to realize its full potential, it is
also recognized that synthetic biology must be supported by high-throughput electrochemical screening technologies, searchable databases of electroactive microorganisms
and suitable data analysis techniques and engineering tools [150].
Acknowledgements This work was supported by the Associate Laboratory for Green Chemistry—
LAQV which is financed by national funds from FCT/MCTES (UIDB/50006/2020). Adrián Escapa
thanks Ente Regional de la Energía de Castilla y Leon (project ref: EREN_2019_L3_ULE).
Carbon Dioxide Utilization—Bioelectrochemical Approaches
99
Nature mimicking through bioelectrochemical approaches seems to a promising
route to take advantage of the available atmospheric CO 2 as a source of added-value
compounds and to reduce its current levels. From simple systems using only one
enzyme, to multi enzymatic and whole cells’ BES, MES and MFC, and independently of the high efficiencies, one drawback is present, that is the cost either of the
enzymes purification, the co-factors or the materials to be used in these systems.
Several approaches have been gaining importance to overcome those limitations,
namely, the use of artificial or engineered enzymes, obtained by different methods
e.g. site-directed mutagenesis, or by designing novel “enzymes” (usually using
more simple molecular structures, hybrid metal–organic frameworks, proteins or
polypeptides as base) mimicking the catalytic centres and activities [140–143].
Some examples include, for instance, metalloporphyrins (namely with Fe) immobilized in highly conductive materials such as carbon nanotubes that were successfully tested towards CO 2 reduction in modified electrodes and fuel cells with
promising results [144]; the synthesis or isolation of iron-sulfur clusters embedded
in supramolecular “artificial” structures, such as cyclodextrins or polypeptides, for
example for nitrogenase complex iron-cluster mimicking [145] and the incorporation of Mo in the more simple rubredoxin protein, by substitution of the native Fe,
towards mimicking the FDH catalytic centre [146]. Other approaches, using
directed evolution is the activity improvement of existing enzymes, such as FDH, to
obtain more efficient variants [143]. Also, the adaptation of existing microorganisms towards novel or improved functions, aiming incorporation in MES systems,
such as methanogenic mixed cultures for the reduction of CO 2 to CH 4 [147].
MES is currently limited to the generation of mainly acetate, a low-value product
that makes it difficult the implementation of this technology to a commercial scale. Still,
the low amounts of longer chain fatty acids (such as butyric or caproic acids which are
much more interesting from a commercial point of view) that have been occasionally
found in the catholytes, indicates that MES has the capability for chain elongation. As
advanced by LaBelle et al. [148], producing higher amounts of these (and other)
organics would demand the development of means to control the regulation of the
relevant genes in the electrosynthetic microbiome. An example of this is the work
reported in Wu et al. [149] where the authors describe an electroactive succinateproducing cell factory engineered in E. coli, that harnessed the electrons arriving at the
cathode to reduce fumarate. Overall, it is widely admitted that synthetic biology can
help to make MES a commercially sound technology by allowing producing new
organics of interest for industry [148, 150]. However, to realize its full potential, it is
also recognized that synthetic biology must be supported by high-throughput electrochemical screening technologies, searchable databases of electroactive microorganisms
and suitable data analysis techniques and engineering tools [150].
Acknowledgements This work was supported by the Associate Laboratory for Green Chemistry—
LAQV which is financed by national funds from FCT/MCTES (UIDB/50006/2020). Adrián Escapa
thanks Ente Regional de la Energía de Castilla y Leon (project ref: EREN_2019_L3_ULE).
Carbon Dioxide Utilization—Bioelectrochemical Approaches
99