Which Geobacteraceae prove to be prevalent
in such samples depends on the specific environment
being tested. For example, if electrodes are
placed in marine sediments, Desulfuromonas
species predominate, whereas if the electrodes
are placed in freshwater sediments, Geobacter
species predominate. Although Geobacter and
Desulfuromonas species have similar physiologies,
Desulfuromonas prefer marine salinity,
while Geobacter favor freshwater.
A hallmark of Geobacteraceae is their ability
to transfer electrons onto extracellular electron
acceptors. For example, Geobacter and Desulfuromonas
species support growth by coupling
the oxidation of organic compounds to the reduction
of Fe(III) or Mn(IV) oxides. Furthermore,
these microorganisms can transfer electrons
to other metals and to the quinone
moieties of humic substances, which are so large
that they must be reduced outside bacterial
cells. Reducing Fe(III) oxides is an
important means for degrading organic
matter in aquatic sediments, submerged
soils, and subsurface environments. Molecular
analyses of such environments reveal
that, in general, Geobacteraceae are
the predominant Fe(III)-reducing microorganisms
in zones in which Fe(III) reduction
is important.
Holmes and Bond found that
Geobacteraceae can also use electrodes
as extracellular electron acceptors.
Both Desulfuromonas and Geobacter
species can grow by oxidizing organic
compounds to carbon dioxide, with
electrodes serving as the sole electron
acceptor. Moreover, more than 95% of
the electrons derived from oxidizing
such organic matter can be recovered as
electricity. In sediment fuel cells,
Geobacteraceae oxidize organic compounds
but, instead of transferring electrons
to Fe(III) or Mn(IV), their natural
electron acceptors, they transfer electrons
onto electrodes (Fig. 1). The electrons
flow through the electrical circuit
to the cathode, where they react with
oxygen to form water