structure. The Marinobacter biocathode was composed of large
colonization patches, while the Bacillus and Pseudoalteromonas
biocathodes showed only small micro-colonies or single cells
scattered on the electrode surface. The biofilm coverage ratio and
the biofilm structure affected the current density provided by the
biocathodes.
The catalytic effect was fairly efficient compared to the low
coverage ratios measured. A previous study carried out on natural
marine biofilms developed on stainless steel showed that biofilm
coverages around 1e5% were sufficient to detect, at open circuit,
the potential ennoblement due to oxygen reduction catalysis
(Mattila et al., 1997). The monospecies biocathodes formed here
confirmed that low colonization ratios could induce a significant
catalytic effect.
The scattered micro-colonies or single cells suggest that electrode colonization was mainly due to the initial adhesion of the
cells from the cell culture rather than the development of a biofilm
on the electrode surface during the polarization time. Electrode
immersion in the cell culture was fundamental to obtaining current
with pure strains extracted from a seawater biofilm. The poor
reproducibility of the current densities observed between the
replicates that were performed in strictly identical conditions
(Table 1) was probably due to the poor control of this first adhesion
step. Conversely, the failure of the polarization experiments in
previous studies (Erable et al., 2010; Parot et al., 2011), which did
not involve the preliminary adhesion from a concentrated cell
culture, can now be attributed to the difficulty in colonizing the
electrode surface from the diluted inoculum.
The set of 20 independent experiments reported in Table 1 show
clear general trends. Firstly, the biocathodes formed at 0.20 V/SCE
gave lower currents when switched to 0.30 V/SCE (average current density 7.2 mA m2) than the biocathodes formed directly
at 0.30 V/SCE (average current density 20.5 mA m2 in the same
medium). The biocathodes formed at 0.30 V/SCE were more
efficient than those formed at 0.20 V/SCE. The polarization potential used during the biocathode formation is consequently an
important parameter that directly impacts its electrochemical
capability.
Secondly, the results show that artificial seawater gave lower
current densities than natural seawater (on average, 6.4 mA m2 vs.
20.5 mA m2) but, to our knowledge, this is the first time that
microbial strains have revealed the ability to catalyse O2 reduction
in synthetic seawater. Although current densities are still modest,
the possibility of designing monospecies biocathodes in synthetic
seawater is a considerable advance in the development of O2-
reducing biocathodes operating at high salinities, mainly because it
offers a well-controlled system in which fundamental