Stepping stones in the electron transport from cells to electrodes in Geobacter sulfurreducens biofilms.

Geobacter sulfurreducens bacteria grow on biofilms and have the particular ability of using polarized electrodes as the final electron acceptor of their respiratory chain. In these biofilms, electrons are transported through distances of more than 50 µm before reaching the electrode. The way in which electrons are transported across the biofilm matrix through such large distances remains under intense discussion. None of the two mechanisms proposed for explaining the process, electron hopping through outer membrane cytochromes and metallic like conduction through conductive PilA filaments, can account for all the experimental evidence collected so far. Aiming at providing new elements for understanding the basis for electron transport, in this perspective article we present a modelled structure of Geobacter pilus. Its analysis in combination with already existing experimental evidence gives support to the proposal of the "stepping stone" mechanism, in which the combined action of pili and cytochromes allows long range electron transport through the biofilm.

A long way to the electrode: how do Geobacter cells transport their electrons?

The mechanism of electron transport in Geobacter sulfurreducens biofilms is a topic under intense study and debate. Although some proteins were found to be essential for current production, the specific role that each one plays in electron transport to the electrode remains to be elucidated and a consensus on the mechanism of electron transport has not been reached. In the present paper, to understand the state of the art in the topic, electron transport from inside of the cell to the electrode in Geobacter sulfurreducens biofilms is analysed, reviewing genetic studies, biofilm conductivity assays and electrochemical and spectro-electrochemical experiments. Furthermore, crucial data still required to achieve a deeper understanding are highlighted.

Evaluation of potato-processing wastewater treatment in a microbial fuel cell.

Wastewaters from potato-processing industries have been traditionally treated by a sequence of steps that include the production of methane as the anaerobic one. This work explores the feasibility of replacing or supplementing methanogenesis with the emerging technology of microbial fuel cells (MFCs). Electricity producing biofilms have been enriched from a real anaerobic sludge, and the conversion of potato-processing wastewater into electricity has been studied. When tested as a single treatment step, MFCs were able to process the wastewater with high COD removal but with low energetic conversion efficiency. On the other hand, as a complimentary step for methanogenesis, they improved conversion efficiency and significantly reduced the organic matter load of the final effluent. These results point at the combination of the energetic yield of methanogenesis and the improved COD removal of the electricity producing treatment as the implementation choice.