3 Electroactive Microorganisms
In Nature, there is a great diversity of microorganisms that can be used in MFCs,
with more than 100 different electroactive species presently identified [16, 64].
Most of these electroactive microorganisms are Gram negative bacteria, with more
than half belonging to the Proteobacteria phylum. Also, the majority of these
electroactive species are: (i) mesophilic; (ii) have low tolerance to high salinity;
(iii) possess motility; (iv) have biofilm formation capabilities; and (v) exhibit anodic
EET activity, with most species performing EET via mediated electron transfer and
only a small percentage capable of performing direct EET [64]. As MFC research
advances, we expect that significantly more electroactive microorganisms will be
discovered, especially those that can exist and thrive in more extreme environments
[65].
Presently, it is well known that mixed microbial cultures colonizing anodes in
MFCs produce greater current densities with higher columbic efficiency, compared
with experiments using pure cultures [66]. Presently, the highest current densities
obtained are from microbial mixed cultures that are dominated by d-Proteobacteria
of the Geobacter genus [16]. The reason for this is the fact that mixed cultures have
a higher flexibility towards external factors due to symbiotic effects. This allows a
greater diversity regarding metabolic pathways, as well as the combination of
different electron transfer mechanisms that permit a complete oxidation of the
organic substrates existent in the MFC reactor [67, 68].
Though mixed microbial cultures produce more current density, MFCs operating
with pure cultures of electroactive bacteria are preferred for the detailed investigation of EET mechanisms as they allow a better characterization of the pathways
than in mixed cultures. Understanding the processes by which electroactive
organisms transfer electrons to an electrode, as well as microbial-electrode interactions will allow the enhancement of EET and ultimately benefit operational
performance of the MFCs and enable their future practical applications. The Gram
negative proteobacterium S. oneidensis MR-1 has been used as a model organism to
understand EET [23, 32]. The ability to grow Shewanella robustly under oxygen
conditions, the large quantity of sequenced genomes, and their easy genetic
manipulation makes these bacteria ideal to work with, both in the laboratory and in
BES applications.
3.1 Shewanella: A Model Organism
Organisms currently assigned to the genus Shewanella have been recognized for
nearly 90 years, having first been isolated from the surface of rotten butter in 1931
[69]. Over the subsequent decades, these bacteria received little attention, with the
exception of the name, that was frequently changed. In 1985, based on 5S rRNA
sequence data a reclassification was proposed [70]. At this time the new genus
Shewanella was created, to honor Dr. James Shewan for his contributions in the
Bacterial Power: An Alternative Energy Source
221
In Nature, there is a great diversity of microorganisms that can be used in MFCs,
with more than 100 different electroactive species presently identified [16, 64].
Most of these electroactive microorganisms are Gram negative bacteria, with more
than half belonging to the Proteobacteria phylum. Also, the majority of these
electroactive species are: (i) mesophilic; (ii) have low tolerance to high salinity;
(iii) possess motility; (iv) have biofilm formation capabilities; and (v) exhibit anodic
EET activity, with most species performing EET via mediated electron transfer and
only a small percentage capable of performing direct EET [64]. As MFC research
advances, we expect that significantly more electroactive microorganisms will be
discovered, especially those that can exist and thrive in more extreme environments
[65].
Presently, it is well known that mixed microbial cultures colonizing anodes in
MFCs produce greater current densities with higher columbic efficiency, compared
with experiments using pure cultures [66]. Presently, the highest current densities
obtained are from microbial mixed cultures that are dominated by d-Proteobacteria
of the Geobacter genus [16]. The reason for this is the fact that mixed cultures have
a higher flexibility towards external factors due to symbiotic effects. This allows a
greater diversity regarding metabolic pathways, as well as the combination of
different electron transfer mechanisms that permit a complete oxidation of the
organic substrates existent in the MFC reactor [67, 68].
Though mixed microbial cultures produce more current density, MFCs operating
with pure cultures of electroactive bacteria are preferred for the detailed investigation of EET mechanisms as they allow a better characterization of the pathways
than in mixed cultures. Understanding the processes by which electroactive
organisms transfer electrons to an electrode, as well as microbial-electrode interactions will allow the enhancement of EET and ultimately benefit operational
performance of the MFCs and enable their future practical applications. The Gram
negative proteobacterium S. oneidensis MR-1 has been used as a model organism to
understand EET [23, 32]. The ability to grow Shewanella robustly under oxygen
conditions, the large quantity of sequenced genomes, and their easy genetic
manipulation makes these bacteria ideal to work with, both in the laboratory and in
BES applications.
3.1 Shewanella: A Model Organism
Organisms currently assigned to the genus Shewanella have been recognized for
nearly 90 years, having first been isolated from the surface of rotten butter in 1931
[69]. Over the subsequent decades, these bacteria received little attention, with the
exception of the name, that was frequently changed. In 1985, based on 5S rRNA
sequence data a reclassification was proposed [70]. At this time the new genus
Shewanella was created, to honor Dr. James Shewan for his contributions in the
Bacterial Power: An Alternative Energy Source
221