Electroactivity of the isolated strains
The electroactivity of the isolates was checked with stainless
steel cathodes polarized at 0.20 V/SCE in filtered natural seawater.
All the isolates produced stable current densities in the
0.5e2 mA m2 range (Table 1). A previous study of 19 strains isolated from a similar seawater O2-reducing biocathode reported
similar current densities of 0.4 and 1 mA m2 but for only two
isolates (Acinetobacter johsonii and Winogradskyella poriferorum),
while the others did not give any current (Erable et al., 2010). Here,
the isolation procedure led to only 4 strains but each revealed its
ability to catalyse O2-reduction. After 14 days, the polarization was
changed to 0.30 V/SCE and the current density increased immediately by a factor of 4e10. A maximum of 15 mA m2 was produced
by the Pseudoalteromonas sp.
Similar experiments were reproduced in filtered natural
seawater and in synthetic seawater, but starting the polarization
directly at 0.30 V/SCE. Each strain produced significant current
densities of up to 40 mA m2 (Table 1) which decreased drastically
when stirring was stopped. For instance, during the 3rd assay in
sterile natural seawater, the reduction current densities above
30 mA m2 (in absolute value) displayed by the three strains
decreased to around 10 mA m2 in the absence of stirring. The
current was limited by oxygen transfer to the electrode surface,
which was significantly slower in a quiescent solution. In contrast,
aeration of the reactors by air bubbling significantly increased the
current densities as shown, for instance, on cyclic voltammetries
(Fig. 2), which confirmed that the current value was controlled by
oxygen transfer. Surprisingly, the current started as soon as the
polarization was established (Fig. 1) while, more generally, the
current would be expected to start from zero and increase gradually
with time. Here, in contrast, the current was maximum since the
beginning of polarization (e.g. Bacillus, Marinobacter and Pseudoalteromonas sp. in Fig. 1) or represented a considerable percentage of the maximum (e.g. around 60% of the maximum for
Roseobacter sp. in Fig. 1).
The main difference in the present protocol with respect to the
previous study was the preliminary immersion of the clean electrode in the bacterial suspension before transferring it into the
electrochemical reactor. This step was not present in the previous
studies. It seems probable that, thanks to this preliminary step,
microbial cells were present on the electrode surface from the
beginning of the electrochemical experiments. This supplementary
step explained why the strains were able to produce current from
the beginning of polarization. The initial adhesion of the bacterial
cells to the electrode surface is consequently a crucial step in the
formation of monospecies biocathodes. The small increase in current during 14 days of polarization (Fig. 1) indicated that the current was due to the initial adhesion of the cells during the
preliminary immersion of the clean electrode in the bacterial suspension and not to gradual microbial colonization of the surface
during the electrochemical experimentation.
At the end of the experiments, the microbial colonization of the
electrode surfaces was always low, with biofilm coverage ratios
from 7 to 25 %, as determined by epifluorescent microscopy for the
biocathodes formed in synthetic seawater (1st experiment in
Table 1, Fig. 3). There was no straightforward correlation between
the biofilm coverage ratio and the current density provided by the
biocathodes. Nevertheless, the lowest current density, which was
observed with the Roseobacter sp. (0.5 mA m2) in this experiment,
corresponded to the lowest coverage ratio (7%). The other three
biocathodes provided current densities from 3 to 35 mA m2 with
biofilm coverage ranging from 15 to 25 %. Moreover, in this group,
the highest current density (35 mA m2) was provided by the
Marinobacter biocathode, which showed a different biofilm
Electroactivity of the isolated strainsThe electroactivity of the isolates was checked with stainlesssteel cathodes polarized at 0.20 V/SCE in filtered natural seawater.All the isolates produced stable current densities in the0.5e2 mA m2 range (Table 1). A previous study of 19 strains isolated from a similar seawater O2-reducing biocathode reportedsimilar current densities of 0.4 and 1 mA m2 but for only twoisolates (Acinetobacter johsonii and Winogradskyella poriferorum),while the others did not give any current (Erable et al., 2010). Here,the isolation procedure led to only 4 strains but each revealed itsability to catalyse O2-reduction. After 14 days, the polarization waschanged to 0.30 V/SCE and the current density increased immediately by a factor of 4e10. A maximum of 15 mA m2 was producedby the Pseudoalteromonas sp.Similar experiments were reproduced in filtered naturalseawater and in synthetic seawater, but starting the polarizationdirectly at 0.30 V/SCE. Each strain produced significant currentdensities of up to 40 mA m2 (Table 1) which decreased drasticallywhen stirring was stopped. For instance, during the 3rd assay insterile natural seawater, the reduction current densities above30 mA m2 (in absolute value) displayed by the three strainsdecreased to around 10 mA m2 in the absence of stirring. Thecurrent was limited by oxygen transfer to the electrode surface,which was significantly slower in a quiescent solution. In contrast,aeration of the reactors by air bubbling significantly increased thecurrent densities as shown, for instance, on cyclic voltammetries(Fig. 2), which confirmed that the current value was controlled byoxygen transfer. Surprisingly, the current started as soon as thepolarization was established (Fig. 1) while, more generally, thecurrent would be expected to start from zero and increase graduallywith time. Here, in contrast, the current was maximum since thebeginning of polarization (e.g. Bacillus, Marinobacter and Pseudoalteromonas sp. in Fig. 1) or represented a considerable percentage of the maximum (e.g. around 60% of the maximum forRoseobacter sp. in Fig. 1).The main difference in the present protocol with respect to theprevious study was the preliminary immersion of the clean electrode in the bacterial suspension before transferring it into theelectrochemical reactor. This step was not present in the previousstudies. It seems probable that, thanks to this preliminary step,microbial cells were present on the electrode surface from thebeginning of the electrochemical experiments. This supplementarystep explained why the strains were able to produce current fromthe beginning of polarization. The initial adhesion of the bacterialcells to the electrode surface is consequently a crucial step in theformation of monospecies biocathodes. The small increase in current during 14 days of polarization (Fig. 1) indicated that the current was due to the initial adhesion of the cells during thepreliminary immersion of the clean electrode in the bacterial suspension and not to gradual microbial colonization of the surfaceduring the electrochemical experimentation.At the end of the experiments, the microbial colonization of theelectrode surfaces was always low, with biofilm coverage ratiosfrom 7 to 25 %, as determined by epifluorescent microscopy for thebiocathodes formed in synthetic seawater (1st experiment inTable 1, Fig. 3). There was no straightforward correlation betweenthe biofilm coverage ratio and the current density provided by thebiocathodes. Nevertheless, the lowest current density, which wasobserved with the Roseobacter sp. (0.5 mA m2) in this experiment,corresponded to the lowest coverage ratio (7%). The other threebiocathodes provided current densities from 3 to 35 mA m2 withbiofilm coverage ranging from 15 to 25 %. Moreover, in this group,the highest current density (35 mA m2) was provided by theMarinobacter biocathode, which showed a different biofilm
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