Generation of High Current Densities by Pure Cultures of Anode-Respiring
Geoalkalibacter
spp. under Alkaline and Saline Conditions in Microbial Electrochemical Cells
Authors: Badalamenti Jonathan P. , Krajmalnik-Brown Rosa , Torres César I. 
Published: May 2013

Anode-respiring bacteria (ARB) generate electric current in microbial electrochemical cells (MXCs) by channeling electrons from the oxidation of organic substrates to an electrode. Production of high current densities by monocultures in MXCs has resulted almost exclusively from the activity of
Geobacter sulfurreducens
, a neutrophilic freshwater Fe(III)-reducing bacterium and the highest-current-producing member documented for the
Geobacteraceae
family of the
Deltaproteobacteria
. Here we report high current densities generated by haloalkaliphilic
Geoalkalibacter
spp., thus broadening the capability for high anode respiration rates by including other genera within the
Geobacteraceae
. In this study, acetate-fed pure cultures of two related
Geoalkalibacter
spp. produced current densities of 5.0 to 8.3 and 2.4 to 3.3 A m
−2
under alkaline (pH 9.3) and saline (1.7% NaCl) conditions, respectively. Chronoamperometric studies of halophilic
Glk. subterraneus
DSM 23483 and alkaliphilic
Glk. ferrihydriticus
DSM 17813 suggested that cells performed long-range electron transfer through electrode-attached biofilms and not through soluble electron shuttles.
Glk. ferrihydriticus
also oxidized ethanol directly to produce current, with maximum current densities of 5.7 to 7.1 A m
−2
and coulombic efficiencies of 84 to 95%. Cyclic voltammetry (CV) elicited a sigmoidal response with characteristic onset, midpoint, and saturation potentials, while CV performed in the absence of an electron donor suggested the involvement of redox molecules in the biofilm that were limited by diffusion. These results matched those previously reported for actively respiring
Gb. sulfurreducens
biofilms producing similar current densities (~5 to 9 A m
−2
).

IMPORTANCE
This study establishes the highest current densities ever achieved by pure cultures of anode-respiring bacteria (ARB) under alkaline and saline conditions in microbial electrochemical cells (MXCs) and provides the first electrochemical characterization of the genus
Geoalkalibacter
. Production of high current densities among the
Geobacteraceae
is no longer exclusive to
Geobacter sulfurreducens
, suggesting greater versatility for this family in fundamental and applied microbial electrochemical cell (MXC) research than previously considered. Additionally, this work raises the possibility that different members of the
Geobacteraceae
have conserved molecular mechanisms governing respiratory extracellular electron transfer to electrodes. Thus, the capacity for high current generation may exist in other uncultivated members of this family. Advancement of MXC technology for practical uses must rely on an expanded suite of ARB capable of using different electron donors and producing high current densities under various conditions.
Geoalkalibacter
spp. can potentially broaden the practical capabilities of MXCs to include energy generation and waste treatment under expanded ranges of salinity and pH.