Biofilm formation has been studied to understand modifications in electrode interface to create new monitoring devices related with different fields as medical, environmental and industrial. In biomining industry, specifically in bioleaching processes, Acidithiobacillus thiooxidans is an acidophilic bacterium that enhances bioleaching as it biooxidizes sulfur which normally passivates ores surfaces and diminishes the bioleaching efficiency. In this work, cells of Acidithiobacillus thiooxidans colonized a graphite surface during seven days in acidic culture medium contained elemental sulfur as nutrient, resulting in bioelectrode. In order to monitor the biofilm formation as bioelectrode, microscopic and electrochemical characterization was performed. Confocal laser scanning microscopy images show biofilms formation since first day of graphite immersion in medium, exopolymeric substances (EPS) becomes hydrophobic after 3 days of immersion which is related with lipidic exopolymers, while on day 5 there is an important change in EPS composition resulting in a higher proportion of exopolysaccharides than lipidic substances. This shift on the EPS composition was related to the analysis of EIS spectra obtained for bioelectrodes: An increase of electrochemical resistance of biofilm at the initial 3 immersion days, meanwhile the presence exopolysaccharides decreased such resistance. Cyclic voltammetry characterization of bioelectrodes using culture medium as electrolyte, shows an irreversible reduction process since day 1, which increases in longer exposure times, even when the characterization was performed with agitation. This suggests the reduction process is associated to the adsorbed S⁰ on graphite bioelectrode. The cathodic peak is near to the calculated reduction potential in acid media (0.1 to 0.2) V vs. NHE. It is known that the sulfur oxidation in acidithiobacilli as A. thiooxidans and A. caldus, initiated via S₈ reduction after its activation (protonation) by the GSH and sulfonate (-SH) groups found in an outer-membrane protein [1-4].

M. Méndez-Tovar acknowledges to Universidad Autónoma Metropolitana-Iztapalapa and CONACYT for PhD grant 212534.

References:

[1] Mangold S., Valdés J., Holmes D.S., Front Microbiol, 2011,2,1

[2] Bobadilla R.A., Cortés M.P., Padilla L., Biotechnol. Bioeng., 2013,110, 2242

[3] Yin, H.,Zhang, X., Li X.,BMC Microbiol. 2014,14,179

[4] López‐Cázares I.,Patrón‐Soberano O.A.,García‐ Meza J.V.,Minerals, 2017,7,156