RESPIRING OR NOT RESPIRING… ELECTRODES. QUESTIONS ABOUT ELECTRON CONDUCTION AND CHARGE ACCUMULATION IN ELECTRICITY-PRODUCING BACTERIA

SCHROTT GERMÁN D.; BONANNI P. SEBASTIAN; ROBUSCHI LUCIANA; BUSALMEN J.P.

Mar del Plata

Congreso; VIII Congreso argentino de microbiología general; 2012

SAMIGE

Microbes that can transfer electrons to extracellular electron acceptors such as Fe(III) oxides, are important in organic matter degradation and mineral cycling in soil and sediments. Notably, some of these bacteria are able to couple complete oxidization of organic matter to carbon dioxide to extracellular electron transport to a polarized electrode. This is exploited in Microbial fuel cells (MFCs) to obtain electric energy directly from bacterial metabolism. Geobacter sulfurreducens is a model organism in the study of extracellular electron transfer to electrodes. It is a strict anaerobe that can grow on polarized electrodes forming biofilms of more than 300 µm thick. Indeed, it is genetically tractable and the whole genome is sequenced. Aiming at enhancing MFCs efficiency, a main goal is to understand the mechanism of electron transport along the biofilm thickness and the interactions between bacteria and the electrode. Although it is currently accepted that for making a direct contact with the electrode G. sulfurreducens makes use of external cytochromes, the mechanism of electron transport from central metabolic pathways to the electrode of cells on the top layers of the biofilm is still a matter of intense discussion. External cytochromes and (putatively) conductive pili give support to alternative models of electron transport, but a consensus has not been achieved yet. In addition, external cytochromes have been called to participate in charge storage in the lack of electron acceptor, a process that can be monitored electrochemically in electricity-producing biofilms through chrono-potentiometric and chrono-coulometric measurements. In this work transient potential and current profiles have been analyzed all over the growth of biofilms on polarized electrodes to gain information on the role of external cytochromes in charge storage and electron conduction. Modeling work on discharge current data demonstrates that accumulated charge increases along biofilm growth. Chronopotentiometric profiles evidence at least three storing sites with different redox potential. Results support the participation of external cytochromes in electron conduction and charge storage.