Electron shuttles are available in the media (e.g., humic acids) or can be
endogenously produced (e.g. flavins) by microorganisms. The possible involvement
of endogenous electron shuttles in reduction of poorly soluble metal minerals by
Shewanella was first proposed by Newman and Kolter [52]. Later, Lies et al.
demonstrated that iron oxide entrapped within nanoporous glass beads could be
reduced by S. oneidensis MR-1, confirming the participation of electron shuttles in
the dissimilatory iron respiration of this bacterium [53]. The ability of flavins to
enhance iron reduction was first examined by Myers and Myers [54], showing that
addition of flavins to the growth medium increased ferric reductase activity in S.
oneidensis MR-1. Since then it was confirmed by numerous researchers that
members of the Shewanella genus are capable of secreting flavins, such as riboflavin, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) [55–
57]. Also, S. oneidensis MR-1 can accumulate these flavins to high concentrations
in solution (250–500 nM) to be used as electron shuttles for EET to the electrodes
[55].These high levels of flavins enhance the electron transfer efficiency by several
fold and therefore are cost effective since the ATP used on flavin production and
secretion is negligible when compared with the resulting energetic advantage.
Although, electron shuttling seems to be the primary mechanism of EET, outer
membrane MHCs still play a key role in mediated EET and are responsible for at
least 95% of the reduction of extracellular flavins at physiological relevant rates
[58]. Indeed, the Shewanella Mtr complex plays an essential role in flavins’
reduction, with the outer-membrane MHC MtrC accounting for approximately 50%
of the activity observed [59]. Kinetic results showed that direct contact between the
outer-multiheme cytochromes (e.g. OmcA and MtrC) and insoluble iron substrates
or MFC anodes could not account for the rates of electron transfer observed when
using whole cells assays [60, 61], with this gap in electron transfer rates resolved
with the addition of flavins. This demonstrated that outer-membrane cytochromes
are not the only elements responsible for the EET at relevant kinetic rates and that
direct and mediated electron transfer occur in tandem in S. oneidensis MR-1 [60].
Indeed, it has been shown that mediated EET, and not direct EET, is the primary
mechanism of EET employed by S. oneidensis, accounting for approximately 75%
of its EET capacity [57].
With reduction potentials of −219 mV (FMN and FAD) and −208 mV (riboflavin) [30], flavins have the capability to act as efficient extracellular redox
mediators for the reduction of metal oxides at neutral pH (redox couple
ferrihydrite/Fe(II) has a reduction potential ranging from −100 to +100 mV [62]).
Thus, Shewanella species that can secrete and utilize flavins as electron shuttles
have an advantage in environments that contain poorly soluble metal oxides but
lack exogenous redox mediators, such as humic acids. Another advantage of flavin
secretion by Shewanella is their potential application in the construction of MFCs
without addition of costly exogenous redox mediators [63]. Furthermore, flavin
secretion by Shewanella may also support mediated EET by other microbial species
present in the BES and thereby increase the efficiency of current generation in
mixed cultures [56].
220
B. M. Fonseca et al.
endogenously produced (e.g. flavins) by microorganisms. The possible involvement
of endogenous electron shuttles in reduction of poorly soluble metal minerals by
Shewanella was first proposed by Newman and Kolter [52]. Later, Lies et al.
demonstrated that iron oxide entrapped within nanoporous glass beads could be
reduced by S. oneidensis MR-1, confirming the participation of electron shuttles in
the dissimilatory iron respiration of this bacterium [53]. The ability of flavins to
enhance iron reduction was first examined by Myers and Myers [54], showing that
addition of flavins to the growth medium increased ferric reductase activity in S.
oneidensis MR-1. Since then it was confirmed by numerous researchers that
members of the Shewanella genus are capable of secreting flavins, such as riboflavin, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) [55–
57]. Also, S. oneidensis MR-1 can accumulate these flavins to high concentrations
in solution (250–500 nM) to be used as electron shuttles for EET to the electrodes
[55].These high levels of flavins enhance the electron transfer efficiency by several
fold and therefore are cost effective since the ATP used on flavin production and
secretion is negligible when compared with the resulting energetic advantage.
Although, electron shuttling seems to be the primary mechanism of EET, outer
membrane MHCs still play a key role in mediated EET and are responsible for at
least 95% of the reduction of extracellular flavins at physiological relevant rates
[58]. Indeed, the Shewanella Mtr complex plays an essential role in flavins’
reduction, with the outer-membrane MHC MtrC accounting for approximately 50%
of the activity observed [59]. Kinetic results showed that direct contact between the
outer-multiheme cytochromes (e.g. OmcA and MtrC) and insoluble iron substrates
or MFC anodes could not account for the rates of electron transfer observed when
using whole cells assays [60, 61], with this gap in electron transfer rates resolved
with the addition of flavins. This demonstrated that outer-membrane cytochromes
are not the only elements responsible for the EET at relevant kinetic rates and that
direct and mediated electron transfer occur in tandem in S. oneidensis MR-1 [60].
Indeed, it has been shown that mediated EET, and not direct EET, is the primary
mechanism of EET employed by S. oneidensis, accounting for approximately 75%
of its EET capacity [57].
With reduction potentials of −219 mV (FMN and FAD) and −208 mV (riboflavin) [30], flavins have the capability to act as efficient extracellular redox
mediators for the reduction of metal oxides at neutral pH (redox couple
ferrihydrite/Fe(II) has a reduction potential ranging from −100 to +100 mV [62]).
Thus, Shewanella species that can secrete and utilize flavins as electron shuttles
have an advantage in environments that contain poorly soluble metal oxides but
lack exogenous redox mediators, such as humic acids. Another advantage of flavin
secretion by Shewanella is their potential application in the construction of MFCs
without addition of costly exogenous redox mediators [63]. Furthermore, flavin
secretion by Shewanella may also support mediated EET by other microbial species
present in the BES and thereby increase the efficiency of current generation in
mixed cultures [56].
220
B. M. Fonseca et al.