Page 40 - BIOME 2022 | Issue 1

RENEWABLES | FEATURE
A relies on reversible and water-soluble electron
I shuttling molecules to fetch metabolic electrons
II
III
B extracellular milieu. This metabolic capability bears
Figure 2. (A) a schematic representation
of electron shuttling mechanism models
currently available and (B) a scanning electron characterized in three main bacterial species;
micrograph showing electrochemically active
bacteria attaching onto electrode surfaces
using conductive appendages known as
bacterial “nanowires”.
40 CES BIOME ISSUE 01 | 2022
between the anode and the bacterium (Figure
2A, III). These natural electron acceptors could
be naturally made compounds by the bacterial
population such as flavonoid molecules, humic
acids, phenazine molecules and natural quinone
molecules. Well-known examples of microbes to
use naturally produce electron shuttling molecules
are Shewanella oneidensis utilizing quinone
and flavonoid type of mediator molecules and
Pseudomonas aeruginosa making use of phenazine
type mediators to shuttle electrons to the MFC
anode. Artificial mediator molecules such as
quinine, phenoxazine, phenozine, phenothiazine
and thionine could also be exogenously added to
the anode electrolyte in order to drive extracellular
electron transfer by the bacterial populations within
microbial fuel cell anodes.
What bacteria can conduct extracellular
electron transfer reactions in MFCs?
Not all microorganisms possess the ability to
transfer their metabolic electrons outside their
cellular membranes into entities located in the
the formal technical term “exo-electrogenesis”
in scientific literature. Although it is not limited to
certain species of bacteria, this ability is only limited
to microorganisms including a limited number of
species of cyanobacteria, yeasts and very rarely,
some species of algae. Exo-electrogenesis has
hitherto not been observed and reported in
plants. The current general scientific consensus
is that higher plants and animals always transport
their terminal electron acceptors (such as oxygen)
inside their cellular membranes and that they are
incapable of conducting exo-electrogenesis.
Naturally occurring exo-electrogenic molecular
mechanisms were first identified and scientifically
Shewanella oneidensis, Geobacter sulfurreducens
and Geobacter metallireducens. In their natural
habitats, these bacterial species such as
Shewanella oneidensis, Geobacter sulfurreducens
and Geobacter metallireducens utilize insoluble
terminal electron acceptors such as Manganese
oxides and Iron oxides contained within anaerobic
marine and lake sediment environments. These
insoluble terminal electron acceptors are