Growth and current production of mixed culture anodic biofilms remain unaffected by sub-microscale surface roughness.

Bioelectrochemical systems couple electricity demand/supply to the metabolic redox reactions of microorganisms. Generally, electrodes act not only as electron acceptors/donors, but also as physical support for an electroactive biofilm. The microorganism-electrode interface can be modified by changing the chemical and/or topographical features of the electrode surface. Thus far, studies have reported conflicting results on the impact of the electrode surface roughness on the growth and current production of biofilms. Here, the surface roughness of the glassy carbon electrodes was successfully modified at the sub-microscale using micro electrodischarge machining, while preserving the surface chemistry of the parent glassy carbon. All microbial electrodes showed similar startup time, maximum current density, charge transport ability across the biofilm and biomass production. Interestingly, an increase in the average surface cavity depth was observed for the biofilm top layer as a function of the electrode surface roughness (from 7 µm to 16 µm for a surface roughness of 5 nm to 682 nm, respectively). These results indicated that the surface roughness at a sub-microscale does not significantly impact the attachment or current production of mixed culture anodic biofilms on glassy carbon. Likely earlier observations were associated with changes in surface chemistry, rather than surface topography.

Microbial characterization of anode-respiring bacteria within biofilms developed from cultures previously enriched in dissimilatory metal-reducing bacteria.

This work evaluated the use of a culture enriched in DMRB as a strategy to enrich ARB on anodes. DMRB were enriched with Fe(III) as final electron acceptor and then transferred to a potentiostatically-controlled system with an anode as sole final electron acceptor. Three successive iron-enrichment cultures were carried out. The first step of enrichment revealed a successful selection of the high current-producing ARB Geoalkalibacter subterraneus. After few successive enrichment steps, the microbial community analysis in electroactive biofilms showed a significant divergence with an impact on the biofilm electroactivity. Enrichment of ARB in electroactive biofilms through the pre-selection of DMRB should therefore be carefully considered.

Specific and efficient electrochemical selection of Geoalkalibacter subterraneus and Desulfuromonas acetoxidans in high current-producing biofilms.

Two different saline sediments were used to inoculate potentiostatically controlled reactors (a type of microbial bioelectrochemical system, BES) operated in saline conditions (35 gNaCl l(-1)). Reactors were fed with acetate or a mixture of acetate and butyrate at two pH values: 7.0 or 5.5. Electroactive biofilm formation lag-phase, maximum current density production and coulombic efficiency were used to evaluate the overall performance of reactors. High current densities up to 8.5 A m(-2) were obtained using well-defined planar graphite electrodes. Additionally, biofilm microbial communities were characterized by CE-SSCP and 454 pyrosequencing. As a result of this procedure, two anode-respiring bacteria (ARB) always dominated the anodic biofilms: Geoalkalibacter subterraneus and/or Desulfuromonas acetoxidans. This suggests that a strong electrochemically driven selection process imposed by the applied potential occurs in the BES system. Moreover, the emergence of Glk. subterraneus in anodic biofilms significantly contributes to broaden the spectrum of high current producing microorganisms electrochemically isolated from environmental samples.

High current density via direct electron transfer by the halophilic anode respiring bacterium Geoalkalibacter subterraneus.

In this study the characterization of Geoalkalibacter subterraneus is presented, which is a novel halophilic anode respiring bacterium (ARB) previously selected and identified in a potentiostatically controlled bioelectrochemical system (BES) inoculated with sediments from a salt plant. Pure culture electroactive biofilms of Glk. subterraneus were grown during chronoamperometric batch experiments at a graphite electrode poised at +200 mV (vs. SCE) with 10 mM acetate as the electron donor. These biofilms exhibited the highest current density (4.68 ± 0.54 A m(-2)) reported on a planar material with a pure culture under saline conditions (3.5% NaCl). To investigate possible anodic electron transfer (ET) mechanisms, cyclic voltammetry (CV) of mature visible apparent reddish biofilms was performed under bioelectrocatalytic substrate consumption (turnover) and in the absence of the substrate (non-turnover). CV evidenced a well defined typical sigmoidal shape and a pair of clear redox couples under turnover and non-turnover conditions, respectively. Moreover, the calculation of their formal potentials indicated the presence of a common ET mechanism present in both CV conditions between -427.6 ± 0.5 (Ef,2) and -364.8 ± 4.5 mV (Ef,3). Confocal laser scanning microscopy inspection showed a biofilm structure composed of several layers of metabolically active bacteria that spread all over the electrode material within a biofilm up to 76 ± 7 µm thick. Such high value compared to the thickness values normally reported in the literature for pure culture electroactive bacteria justifies further investigations. Taken together, these results suggest that Glk. subterraneus performs a direct ET mechanism in contact with the electrode material. Furthermore, direct current generation from saline wastewater significantly expands the application of BESs.

Fate of cadmium in activated sludge after changing its physico-chemical properties by thermal treatment.

The effect of thermal treatment of activated sludge on cadmium uptake was investigated in respect with potential modifications of floc surface properties. Soluble fraction biochemical composition and floc size measurements evidenced floc destructuration and the release of (in)organic ligands in solution. Characterization of sludge samples by potentiometric titrations and IR spectra showed the transfer of functional groups from particulate to soluble fraction as well as the higher availability of phosphate groups originating from cell membrane phospholipids after thermal treatment. Batch biosorption tests demonstrated that cadmium uptake was highly affected by sludge modifications due to thermal treatment. For temperatures below 95 degrees C, floc size decrease allowed a better availability of binding sites, resulting in a higher sorption capacity. At temperatures above 95 degrees C, the effect of released soluble ligands and of the lower total number of surface functional groups limited cadmium uptake. Uptake mechanisms were also affected by sludge thermal treatment as surface complexation involving ion exchange tends to become predominant over precipitation.