CONICET | Buscador de Institutos y Recursos Humanos

Cathodic bacteria can use an electrode (cathode) as inexhaustible electron donor. They are applied in diverse processes that include wastewater treatment and biosynthesis of value added products from CO2. Unfortunately, as the surface of cathodes is negatively charged, there is a repulsive force between the electrode and the negatively charged cells that hinders adhesion to the electrode. This results in low biomass formation and a limited performance of the biocathode.Alternately, a strategy known as polarity reversion is used to avoid the mentioned electrostatic repulsion by polarizing the electrode at a positive potential during the adhesion phase. Afterwards, the potential is switched to bacterial negative working potential of the process of interest. In this way, biocathode performance is enhanced due to an increase of the biomass on the electrode surface. This strategy is mainly applied empirically, and fundamental studies elucidating the impact of electrode surface charge on initial bacterial adhesion, biofilm development and current generation have not yet been extensively explored.The aim of this work was to analyze the effects of electrode charge on electroactive bacteria adhesion and its consequences on cathodic biofilm development. The experimental setup consisted on thin film electrochemical cell with a semi-transparent cathode that can be mounted on the stage of an optical microscope to follow bacteria attachment and biofilm development in situ and in vivo. The adhesion of Thiobacillus denitrificans, a model cathodic bacteria, was assessed at two potentials exhibiting negative and positive surface charges on the electrode (-300 and 400 mV vs SHE), quantifying the evolution in time of irreversible adhered bacteria. After the adhesion phase, bacteria were grown at a typical potential applied on cathodes (-300 mV vs SHE) allowing biofilm formation. Cathodic biofilm current generation and bacteria coverage were measured through images analysis performed with ImageJ software and a custom IA (deep learning model) developed in our lab. The number of adhered bacteria was four times greater with positive surface charge on electrode, consistent with the DLVO model for bacterial adhesion. Biofilm development also depended on the potential applied on the adherence phase. The biofilm uniformly covered the electrode surface after two days of continuous growth with the inversion of polarity. This was reflected on a progressive increment of current density, reaching a value (-17.6 uA/cm2) near the maximum reported in bibliography for T. denitrificans. Instead, when the electrode was polarized at a negative potential during the adhesion stage, current density remained much lower (-3 uA/cm2) and cathodic bacteria grew forming dispersed clusters. Future work will be aimed at analyzing the effect of other variables on the adhesion phase such as ionic strength, surface roughness or the chemical composition of the electrode.

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