Imagine the future of energy. The future might look like a new power plant on the edge of town—an inconspicuous bioreactor that takes in yard waste and locally-grown crops like corn and wood chips and churns out electricity to area homes and businesses. Or the future may take the form of a stylish-looking car that refills its tank at hydrogen stations. Maybe the future of energy looks like a device on the roof of your own home – a small appliance, connected to the household electric system, that uses sunlight and water to produce the electricity that warms your home, cooks your food, powers your television and washes your clothes.
All these futuristic energy technologies may become reality some day, thanks to the work of the smallest living creatures on earth: microorganisms. "Microbial energy conversion" is the shorthand term for technologies like these. In microbial energy technologies, microorganisms make fuels out of raw organic materials, thereby converting the chemical energy in the biomass into chemical energy in the form of ethanol or hydrogen, for example. In addition, microbes can convert solar energy to hydrogen. Those fuels are then burned to make electrical energy or, in the case of internal combustion engines, kinetic energy to power a car. Another technology that falls under the heading of microbial energy conversion is the microbial fuel cell, a bioreactor in which bacteria transform the chemical energy in biomass directly into electrical energy.
The world faces a potentially crippling energy crisis in the next 30 to 50 years. Global populations are climbing, driving an ever-increasing demand for energy to power manufacturing, transportation, heat, and other needs. "World energy consumption is projected to increase by 71 percent from 2003 to 2030" (Energy Information Administration/International Energy Outlook 2006). Petroleum, the foundation of the current transportation system, peaked in production in the U.S. in the mid 1980s, and world production is projected to peak in the next 25 to 50 years. Moreover, the burning of fossil fuels and the resulting release of carbon dioxide and combustion pollutants has brought about global climate change, the effects of which we are only beginning to understand. The means of preventing the twin catastrophes of energy scarcity and environmental ruin is not clear, but one part of the solution may lie in microbial energy conversion.
The American Academy of Microbiology convened a colloquium March 10-12, 2006, in San Francisco, Calif., to discuss the production of energy fuels by microbial conversions. The status of research into various microbial energy technologies, the advantages and disadvantages of each of these approaches, research needs in the field and education and training issues were examined, with the goal of identifying routes for producing biofuels that would both decrease the need for fossil fuels and reduce greenhouse gas emissions. Colloquium participants made a number of recommendations for moving forward with research and education in this field.
Microorganisms may be used to generate a number of fuels, including ethanol, hydrogen, methane, lipids and butanol, among which ethanol production is the most mature technology. As a liquid, ethanol is relatively easy to store and it can fit into the existing fuel infrastructure. However, ethanol adsorbs water readily and cannot be shipped through common-carrier pipelines, which inevitably contain water. Processes that generate higher molecular weight alcohols, such as butanol, can be produced with similar technologies and are more compatible with existing infrastructure. Currently, the production of ethanol from the most abundant forms of biomass, namely cellulose and lignocellulose, is comparatively difficult and expensive. Future research in alcohol production needs to focus on increasing the productivity and yield of processes that make alcohol from biomass and processes that generate alternative alcohols.
Hydrogen, a potent energy carrier, may be produced in any of a variety of ways. Oxygenic photosynthesis in cyanobacteria and other microbes can be harnessed to make hydrogen from water in a promising technology that may meet energy needs in the long term. The resources needed for this process, water and sunlight, are in practically unlimited supply, but the efficiency of the process is low. We need new photobioreactor designs to help increase efficiency of light capture and hydrogen removal in these systems. Hydrogen also may be produced using the cellular machinery of nitrogen fixation, through fermentation of biomass, through iron metabolism in photosynthesis, by metabolizing carbon storage compounds, or by microbial mats.
Methane is another useful microbially produced fuel. Methanogenesis is a relatively simple, predictable microbial process, and methane fits into the infrastructure in place of natural gas. Although methane production is a well-explored area, much of the current data have derived from operations optimized for waste disposal, not for methane generation. Research in methane production should focus on optimizing the productivity of methanogenic microbial communities and bioreactor design.
The study of microbial fuel cells is in its infancy, and yield and current density are low in today’s systems, but the potential to make great leaps of progress in yield and performance is great. Research in microbial fuel cells should focus, in part, on the discovery and development of novel bacteria capable of transferring electrons from biomass substrates to an electrode.
Overarching research needs in the field include bioprospecting, the search for novel microorganisms and genes that can aid in energy conversion. Research is also needed to explore the dynamics of microbial communities, enzymology, the biology of non-growing cells, modeling, genomics, nanotechnology, new microbiological techniques and bioreactor engineering.
Specific education and training in microbial energy conversion technologies are needed to provide the foundation for insights that will allow major increases in biofuel generation. We need our brightest and best minds to meet the challenges of this multidisciplinary effort.
Merry Buckley, Judy Wall. 2006. Microbial energy conversion.
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