Improvement of the anodic bioelectrocatalytic activity of mixed culture biofilms by a simple consecutive electrochemical selection procedure

Abstract

In this paper we demonstrate that the anodic, bioelectrocatalytic performance of wastewater inoculum based, mixed culture microbial biofilms can be considerably improved by using a consecutive, purely electrochemical selection and biofilm acclimatization procedure. The procedure may represent an alternative to a repetitive mechanical biofilm removal, re-suspension and electrochemically facilitated biofilm formation. By using the proposed technique, the bioelectrocatalytic current density was increased from the primary to the secondary biofilm from 250 microAcm(-2) to about 500 microAcm(-2); and the power density of respective microbial fuel cells could be increased from 686 mWm(-2) to 1487 mWm(-2). The electrochemical characterization of the biofilms reveals a strong similarity to Geobacter sulfurreducens biofilms, which may indicate a dominating role of this bacterium in the biofilms.

Fig. 1

(a) Schematic drawing of CdSe/ZnS quantum dots encapsulated by amphiphilic alginate surfactant; (b) particle size distribution of AA-QDs by dynamic light scattering. After sonication, the distribution was narrowed down to the range between 18.1 and 28.7 nm with a mean diameter of 23.1 nm.

Fig. 2

(a) Photoluminescence and UV–vis spectra of AA-QDs. UV–vis absorption spectrum of AA-QDs was detected in the range of 300–700 nm. The photoluminescence of AA-QDs was measured in the range of 400–700 nm with an excitation wavelength of 390 nm. The photoluminescence with the emission wavelength peaked at 534 nm indicating bright green fluorescence of AA-QDs in aqueous solution shown in the inset photograph; (b) TEM image of the QD–virus complexes formed in the cationic polybrene solution. The light-color particles with the size around 40–50 nm are dengue virus (indicated by arrow). The dark-color particles tagged on the viral particles (indicated by arrowhead) are AA-QDs (about 10–15 nm). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Fig. 3

Photomicrographical images of BHK-21 cells exposed separately to AA-QDs with and without polybrene for various periods of time. (a)–(c) Images of BHK-21 cells incubated with QDs without polybrene for 30, 60, and 120 min, respectively. QD was not detected intracellularly under fluorescent confocal microscopy until 120 min post-incubation; (d)–(f) images of BHK-21 cells incubated with QDs in the presence of 10 μg/mL polybrene. QD was not detected intracellularly under fluorescent confocal microscopy until 60 min post-incubation. (g)–(i) Images of BHK-21 cells incubated with QDs in the presence of 50 μg/mL polybrene. QD was clearly detected intracellularly under fluorescent confocal microscopy after 30 min post-incubation. The normalized fluorescence intensities per selected cell area of (b)–(i) based on the fluorescence intensity per selected cell area of (a) are 1.02, 1.06, 0.94, 2.55, 3.65, 1.70, 3.93, and 5.20, respectively. Scale bar = 25 μm.

Fig. 4

Photomicrographical images of BHK-21 cells exposed separately to QD–virus complexes of different MOIs with 10 μg/mL polybrene for various periods of time. (a)–(c) Images of BHK-21 cells incubated with QD–virus complexes of MOI being 0.1 for 30, 60, and 120 min, respectively; (d)–(f) images of BHK-21 cells incubated with QD–virus complexes of MOI being 0.5 for 30, 60, and 120 min, respectively; (g) six sequential z-section images of selected infected cells were taken by a confocal microscope, which revealing the QD–virus complexes were indeed internalized into BHK-21 cells rather than absorbed on cell surface. The normalized fluorescence intensities per selected cell area of (b)–(f) based on the fluorescence intensity per selected cell area of (a) are 1.00, 1.33, 1.19, 2.56, and 3.54, respectively. Scale bar = 25 μm.

Fig. 5

Photomicrographical images of BHK-21 cells exposed to QD–virus complexes with 50 μg/mL polybrene at 30, 60, and 120 min, respectively. The normalized fluorescence intensities per selected cell area of (b) and (c) based on the fluorescence intensity per selected cell area of (a) are 1.23 and 2.73, respectively. MOI = 0.5 and scale bar = 25 μm.

Fig. 6

Photomicrographical images of BHK-21 cells treated with and without allophycocyanin for one hour, then exposed to QD–virus complexes with 50 μg/mL polybrene for various periods of time. (a)–(d) Images of BHK-21 cells incubated with QD–virus complexes for 30 min, after cells pre-treated with allophycocyanin of 0, 6.25, 31.25, and 125 μg/mL, respectively; (e)–(h) images of BHK-21 cells incubated with QD–virus complexes for 60 min, after cells pre-treated with allophycocyanin of 0, 6.25, 31.25, and 125 μg/mL, respectively. The normalized fluorescence intensities per selected cell area of (b)–(h) based on the fluorescence intensity per selected cell area of (a) are 0.72, 0.66, 0.54, 2.92, 1.02, 0.66, and 0.52, respectively. MOI = 0.5 and scale bar = 25 μm.