Investigations of electron transport and storage mechanisms in microbial biofilms

Abstract

Electron transport is a fundamental mechanism in a variety of biological systems such as photosynthesis and aerobic respiration. However, the transport has long been considered to occur only over short distances (\u3c 1 μm), primary by metalloproteins. Recently, the conduction of electrons over large distances (\u3e 10 μm) along networks of microbial pilin filaments known as microbial nanowires has been invoked to explain a wide range of important redox phenomena that influence carbon and mineral cycling in soils and sediments, bioremediation, corrosion, interspecies electron transfer and anaerobic conversion of organic wastes to methane or electricity. However, there has never been any direct experimental demonstration of this long-distance electron transport. In fact, previous measurements of microbial biofilms have noted just the opposite: that biofilms act as insulators, not conductors. In this thesis, we reconcile these confounding observations with the demonstration that biofilms of several species of commonly studied microorganisms do function as insulators, whereas biofilms of Geobacter sulfurreducens , common in soils and sediments can form a conductive matrix, with a conductivity comparable to synthetic conductive polymers. We show that biofilms are capable of conducting electrons over 1.25 cm, many thousands of times the size of a cell. Biofilm conductivity was found to be proportional to the abundance of pilin filaments and the conductivity of sheared pilins was comparable to biofilms. We also found that biofilm conductivity regulates fuel cell current density. We demonstrate that electron transport in the biofilms does not occur via localized charge carriers known as cytochromes, as almost universally predicted, but rather through delocalized electronic states. Moreover, we report a quantum mechanical interference phenomenon of weak localization in pilin nanowires. Additionally, we demonstrate that cytochromes can be used to store electrons with capacitance comparable to commercial supercapacitors. Furthermore, the degree of conductivity and capacitance within the films can be tuned via changes in gene expression or gate bias. This study demonstrates that pilin-associated long-distance electron transport through a microbial matrix is feasible, establishes approaches that could be used for evaluating the possibility of electron flow through natural microbial communities, and demonstrates the potential for developing novel bioelectronic materials