While a recent report from the National Academies of the Sciences concluded that conservation is the short-term key to many energy issues, work continues on alternative energy production techniques like wind, solar, biomass, and fuel cells. For mobile applications, fuel cells have quickly become the technological leader because they offer high energy density (relative to other green technologies), low weight, and generally high mechanical durability. In this month's Biosensors and Bioelectronics, a research team from University of Massachusetts Amherst describes their work on microbial fuel cells enhanced by directed evolution.

A wide array of fuel cell technologies exist, but most fall into two catagories: solid oxide fuel cells (SOFCs) or polymer electrolyte membrane fuel cells (PEMFCs). SOFCs conduct O2- across ceramic membranes and produce high current densities with little degradation over time. Unfortunately, the ionic conduction mechanism requires high operating temperatures—usually several hundred degrees centigrade. PEMFCs conduct either protons or hydroxyls, but suffer from low current densities and significant degredation over time. While these systems show substantial promise, there is no clear leader for most mobile applications and there is room in several niche markets for other types of fuel cells.

To produce ATP, bacteria generate charge gradients when metabolizing nutrients, and these could theoretically be harvested. In the early 1990's, proof-of-concept microbial fuel cells (MFCs) were developed using bacterial electrolytes. These systems fed simple sugars to bacteria in anaerobic conditions and with an applied field. Electrons produced during digestion move to the anode while protons diffuse to the cathode, where they can recombine with any electrons that have conducted across a load circuit.

In order to produce electricity, it is imperative that MFC's bacteria conduct electrons to the cathode. The researchers at UMass Amherst realized that there has never been any natural selective pressure that would enhance electronic conduction in bacteria, so they used directed evolution to produce highly conducting bacteria.

G. sulfurreducens bacteria were cultured on a graphite electrode under a 400 mV applied bias. The goal was to force the bacteria to adapt to conditions inside the MFC with the hope that they would evolve greater functionality in the process. Several colonies were isolated after five months in the MFC environment and re-cultured under normal conditions. When placed in an MFC cell, the specially cultured bacteria grew much more rapidly—current saturated after 50 hours as opposed to 400 hours—and they provided twice the current density of normally cultured bacteria.

Analysis of the enhanced bacteria showed that there were two primary adaptions. First, pili, fine, thread-like structures that connect neighboring cells, dramatically increased in the new bacteria. These structures are thought to be responsible for electronic conduction in bacterial films. Also, unlike their precursors, the enhanced bacteria all had flagella that allowed both motility and enhanced attachment to anode surfaces. It is unclear which adaptation is primarily responsible for the enhanced performance.

MFCs must show significant improvement (orders of magnitude) in order to match SOFCs and PEMFCs; simply doubling performance will not make them viable for most applications. However, by harnessing evolution, it may be possible to rapidly accelerate development of reasonably performing systems. As a member of the ceramic research community with vested interests in SOFC development, I am not quite ready to salute my new bacterial overlords, but this is a fascinating area of research that has real potential in many niche applications.

Biosensors and Bioelectronics DOI:10.1016/j.bios.2009.05.004