Local Acidification Limits the Current Production and Biofilm Formation of Shewanella oneidensis MR-1 With Electrospun Anodes

Abstract

The anodic current production of Shewanella oneidensis MR-1 is typically lower compared to other electroactive bacteria. The main reason for the low current densities is the poor biofilm growth on most anode materials. We demonstrate that the high current production of Shewanella oneidensis MR-1 with electrospun anodes exhibits a similar threshold current density as dense Geobacter spp biofilms. The threshold current density is a result of local acidification in the biofilm. Increasing buffer concentration from 10 to 40 mM results in a 1.8-fold increase of the current density [(590 ± 25) μA cm^-2] while biofilm growth stimulation by riboflavin has little effect on the current production. The current production of a reference material below the threshold did not respond to the increased buffer concentration but could be enhanced by supplemented riboflavin that stimulated the biofilm growth. Our results suggest that the current production with S. oneidensis is limited (1) by the biofilm growth on the anode that can be enhanced by the choice of the electrode material, and (2) by the proton transport through the biofilm and the associated local acidification.

Keywords: Shewanella oneidensis MR-1; biofilm; electrospinning; local acidification; mass transport.

Figure 1

Effect of the buffer capacity on the current production. The initial buffer concentration of 10 mM was increased on day 6 to 20 mM (+10 mM) and on day 8 to 40 mM (+20 mM). Note the stable pH during the experiment. The shaded area corresponds to the sample standard deviation of three anodes. The numerical values of the current densities on day 6, 8, and 10 can be found in Supplementary Table 4.

Figure 2

Effect of phosphate buffer concentration on the current production. The current production of ES300 shows a linear response to the buffer concentration. The y-axis intercept of 239 μA cm^-2 corresponds to the threshold current density of the MR-1/ES300 composite. The threshold current density and the slope of the linear response is slightly higher than for dense mixed community biofilms reported by Torres et al. (2008). See Discussion for a details. The numerical values can be found in Supplementary Table 4.

Figure 3

Effect of riboflavin (500 and 1,000 nM) on the current production. (A) Current production of ES300 and (B) C-Tex 13. The shaded area corresponds to the sample standard deviation of three anodes. The numerical values of the maximum and final current densities can be found in Supplementary Table 4.

Figure 4

Current production with inoculum spikes on day 6 and 8. The current density is evaluated before each inoculum spike on day 6 and 8, and at the end of the experiment on day 10. The shaded area corresponds to the sample standard deviation of three anodes. The numerical values can be found in Supplementary Table 4.

Figure 5

The effect of the medium components on the current production. The effect size is measured as fold change of the current production. The asterisks indicate significance levels: p < 0.01 (^**), p < .05 (^*), p ≥ 0.05 (ns). The significance levels of the spike experiments were calculated against the control without spikes (Supplementary Figure 5) and the effect of riboflavin against the current production without riboflavin (Figures 3A,B). The numerical values can be found in Supplementary Table 4.

Figure 6

Dry weight equivalents of the biofilm on the anodes and planktonic cells. Addition of flavin enhances biofilm formation on C-Tex 13. PBS addition improves mainly the biofilm formation on ES300. The numbers at the column tops indicate the total growth yield in the reactor (total dry weight equivalent at the end of the experiment divided by the dry weight equivalent of the inoculum). Note the different scale of the inoculation experiment, depicted on the right. The dry weight of the biofilms attached to the anodes (n = 3) and the dry weight equivalent of planktonic cells were evaluated at the end of the experiment on day 6 (supplemented RF, IM) or 10 (spike experiments). The numerical values can be found in Supplementary Table 4.

Figure 7

Current and dry weight for all experimental conditions. ES300 enables a higher current production per biomass dry weight as compared to C-Tex 13. The value in brackets was excluded from the linear regression (C-Tex 13 with IM). The reasoning is discussed in the text.