Future shock from the microbe electric

Abstract

How can the future not look bright when you are dealing with a microbial process that can power a light bulb? The study of microbial fuel cells and, more generally, microbe–electrode interactions is rapidly amping up, not only in power production, but also in the number of investigators and areas of study. The most intense focus has been on wastewater treatment and this is likely to continue for some time. It was probably safe to say 5 years ago that any compound that microorganisms can degrade could be converted to electricity in a microbial fuel cell, but if there was ever was any doubt, this point has been proven over and over again in a plethora of recent studies. It is clear from this work that a major limitation in converting complex wastes to electricity is the initial microbial attack on the larger, difficult to access molecules, just as it is in any other treatment option. It may well be that the intensive focus on the degradation of complex organic matter in other bioenegy fields will soon make a contribution here. However, there are other issues specific to microbial fuel cell technology. At present the rate that even simple organic compounds can be converted to electricity is much too slow for practical wastewater treatment. For example, columbic efficiency (i.e. the percentage of electrons available in the organic substrate that are recovered as current) is often diminished by methane production, indicating that even relatively slow-growing methanogens are competing with the current-producing microorganisms. This is despite the fact that electron transfer to oxygen, the ultimate electron acceptor in microbial fuel cells, is much more thermodynamically favourable than methane production. Some contend that the limitations to current production in waste treatment can be solved with improved engineering of microbial fuel cell design and that there is little need to focus on the microbiology of microbial fuel cells for waste treatment because as better fuel cell designs are developed, the appropriate microorganisms will naturally colonize the systems and produce more power. That may be, but it also seems likely that, going forward, the mechanisms for microbe–electrode interactions will become better understood and this could significantly inform optimal microbial fuel cell design. Furthermore, it is likely that we will find that it is possible to greatly increase the current-producing capabilities of microorganisms. This is because there has been no previous evolutionary pressure for microorganisms to optimally produce current. Many of the microorganisms that function best in microbial fuel cells are dissimilatory Fe(III)-reducing microorganisms, which have evolved to specialize in extracellular electron transfer to insoluble, extracellular electron acceptors. However, microorganisms reducing Fe(III) in sedimentary environments are typically in direct contact with the Fe(III). In contrast, when microorganisms are producing high current densities in microbial fuel cells, only a small fraction of the microorganisms in the anode biofilm are in direct contact with the anode surface. Most must transfer electrons over substantial distances through the biofilm. It is not clear that there has ever been substantial selective pressure on microorganisms for such long-range electron transfer. Thus, there should be ample room for improvement. Another unnatural request that we make on microorganisms when they are asked to generate high current densities is the requirement to metabolize organic compounds very rapidly. The natural habitat of most of the microorganisms that have been shown to be most effective in current production is the subsurface or aquatic sediments. These are rather low-energy environments in which there has probably not been much selective pressure for rapid growth and metabolism. Other challenges to anode-reducing microorganisms include the necessity to tolerate the low pH that can develop within the anode biofi