5 Conclusion
MFCs have been intensively investigated over the past decades with tremendous
advances. Despite all the improvements made to increment MFC power output, the
low EET rate from electroactive microorganisms to the electrode surfaces remains a
bottleneck that prevents the practical application of BES [173–175]. At this time,
means to improve electroactivity are being explored, on the biochemical, genetic
and technological fronts [121, 176–178].
EET has been observed and studied in phylogenetically diverse microorganisms
[16], indicating that this microbial trait is widespread in nature. Despite the
microbial diversity, only a few species have emerged as model organisms for the
study of EET (e.g., Geobacter sulfurreducens, Shewanella oneidensis and Thermincola spp.). Of all the EET model organisms, S. oneidensis MR-1 is the most
extensively studied and presently has the best characterized molecular mechanism
of EET [23, 32, 179]. The extensive study of Shewanella versus other electroactive
bacteria has to do with a number of combined factors such as robust growth under
oxygen conditions, existence of sequenced genome, straightforward genetic
manipulation, and robust strategies for overexpression of the relevant multiheme
cytochromes which makes these bacteria ideal to work with. It is well known that
Geobacter species are the dominant members in acetate fed BES electrode biofilms
and that Thermincola spp. are able to grow at higher temperature, and both produce
higher current densities compared to Shewanella [16, 176, 180–182], making them
more attractive candidates for BES applications. Despite these positive aspects,
several difficulties mainly involving growth and genetic manipulation have rendered these bacteria more challenging to study and fully characterize their EET
pathways, with numerous gaps in the understanding of their molecular mechanisms
of EET.
Using as model organism bacteria such as S. oneidensis MR-1, our understanding on how EET occurs has increased greatly. Here, MHCs continuously
revealed themselves as key players, creating an efficient redox network that stretches from the cytoplasmic membrane, across the periplasmic space and through the
outer membrane, transferring electrons directly or indirectly to their insoluble
acceptors [29, 183]. The detailed functional characterization of the MHCs from
microorganisms capable of EET will ultimately lead to a more rational design and
optimized biotechnological applications which use these organisms. This optimization can be biological or technological, using different approaches such as
molecular biology to tune the reduction potentials of hemes found in the MHC
involved in the electron transfer pathway [121, 162, 184, 185], manipulation of
electron mediator synthesis pathways [186] reprogramming gene regulatory circuits
to enhance electron transfer pathways [187] or even surface enhancement of
electrodes for improved cellular contact [188]. Either way, all stand to benefit from
the full characterization of these complex electron transfer pathways.
Bacterial Power: An Alternative Energy Source
233
MFCs have been intensively investigated over the past decades with tremendous
advances. Despite all the improvements made to increment MFC power output, the
low EET rate from electroactive microorganisms to the electrode surfaces remains a
bottleneck that prevents the practical application of BES [173–175]. At this time,
means to improve electroactivity are being explored, on the biochemical, genetic
and technological fronts [121, 176–178].
EET has been observed and studied in phylogenetically diverse microorganisms
[16], indicating that this microbial trait is widespread in nature. Despite the
microbial diversity, only a few species have emerged as model organisms for the
study of EET (e.g., Geobacter sulfurreducens, Shewanella oneidensis and Thermincola spp.). Of all the EET model organisms, S. oneidensis MR-1 is the most
extensively studied and presently has the best characterized molecular mechanism
of EET [23, 32, 179]. The extensive study of Shewanella versus other electroactive
bacteria has to do with a number of combined factors such as robust growth under
oxygen conditions, existence of sequenced genome, straightforward genetic
manipulation, and robust strategies for overexpression of the relevant multiheme
cytochromes which makes these bacteria ideal to work with. It is well known that
Geobacter species are the dominant members in acetate fed BES electrode biofilms
and that Thermincola spp. are able to grow at higher temperature, and both produce
higher current densities compared to Shewanella [16, 176, 180–182], making them
more attractive candidates for BES applications. Despite these positive aspects,
several difficulties mainly involving growth and genetic manipulation have rendered these bacteria more challenging to study and fully characterize their EET
pathways, with numerous gaps in the understanding of their molecular mechanisms
of EET.
Using as model organism bacteria such as S. oneidensis MR-1, our understanding on how EET occurs has increased greatly. Here, MHCs continuously
revealed themselves as key players, creating an efficient redox network that stretches from the cytoplasmic membrane, across the periplasmic space and through the
outer membrane, transferring electrons directly or indirectly to their insoluble
acceptors [29, 183]. The detailed functional characterization of the MHCs from
microorganisms capable of EET will ultimately lead to a more rational design and
optimized biotechnological applications which use these organisms. This optimization can be biological or technological, using different approaches such as
molecular biology to tune the reduction potentials of hemes found in the MHC
involved in the electron transfer pathway [121, 162, 184, 185], manipulation of
electron mediator synthesis pathways [186] reprogramming gene regulatory circuits
to enhance electron transfer pathways [187] or even surface enhancement of
electrodes for improved cellular contact [188]. Either way, all stand to benefit from
the full characterization of these complex electron transfer pathways.
Bacterial Power: An Alternative Energy Source
233