Abstract
Genus Shewanella is a gram-negative microorganism which has a unique property of utilizing solid state metal-oxide as a terminal electron acceptor. The extracellular electron transfer (ET) occurs via cytochromes located in the outer membrane (outer membrane-cytochrome, OMC). Due to the unique property, Shewanella has attracted a lot of attentions from the view point of biogeochemical cycle and microbial fuel cell. In this review, our recent results on the extracellular ET will be introduced with Shewanella loihica PV-4. We used various electrochemical techniques as well as spectroscopic ones to investigate the ET between the microbes and the (solid) electrode. Based on the various experimental results, we show evidences of the existence of direct ET path from OMC to electrode. We also show that the addition of water soluble Mn Porphyrines, semiconducting iron oxide nano colloids, etc., greatly enhances the ET efficiency. The extracellular ET process can be applied to microbial fuel cell. The power output of thin fuel cell, so far we achieved, is ca. 2 W/l, which is almost equal to one of the most efficient bio-fuel cells using isolated protein catalysts.
