Microbes harvest electricity from waste organic matter

Derek R. Lovley, University of Massachusetts

Editor’s note: Lovley recently received an $8.9 million, three-year grant from the Department of Energy (DOE) for this research, as part of the “Genomes to Life” program. Genomes to Life “plans to take advantage of solutions that nature has already devised to help solve problems in energy production, environmental cleanup and carbon cycling,” according to DOE. Lovely’s award is part of a total $103 million package to 16 universities and research hospitals and four private research institutions.

Every day plants store vast quantities of energy by fixing carbon dioxide into organic matter. Recent studies in our laboratory have demonstrated a strategy for converting this energy into electricity. We have discovered microorganisms that can oxidize organic matter to carbon dioxide and transfer the electrons derived from this oxidation to electrodes.

History and hypothesis

This discovery originated from a project conducted in collaboration with Dr. Lenny Tender of the Naval Research Laboratories. Dr. Tender and colleagues found that electricity was produced when a graphite electrode was placed in black marine mud and connected to a graphite electrode in the overlying water. It was initially hypothesized that the electricity resulted from the oxidation of reduced chemicals naturally present in the mud, such as sulfide and ferrous iron. However, we suspected that there might be a microbiological explanation.

We examined the microorganisms on the surface of electrodes placed in the mud by scraping the microorganisms off the electrodes with a razor blade and then sequencing portions of the microbes’ DNA. We found that the surface of electrodes had been specifically colonized by microorganisms known as geobacters.

Geobacters are microorganisms that grow in mud and soil by using iron in the same manner that humans use oxygen. When you or I have a cheeseburger for lunch we get energy from this by using the oxygen that we breathe to oxidize the proteins, fats, and sugars in the meat, cheese and bun to carbon dioxide. This is an electrical reaction in which electrons are transferred from the organic compounds to oxygen. Since there is little or no oxygen in mud, geobacters oxidize their food to carbon dioxide with iron oxide minerals. These iron oxide minerals, which are often abundant in mud and soils, are chemically equivalent to the rust found on unprotected iron exposed to elements. Geobacters do not dine on cheeseburgers, but rather, gain their energy from other forms of organic matter derived primarily from dead plant material that settles into the mud. Geobacters oxidize the organic compounds and transfer electrons onto the iron oxides.

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Although geobacters naturally use iron oxide minerals, if a graphite electrode is placed in the mud they will use the electrode in the same the way that they would otherwise use the iron. In this manner electrons are stripped from organic compounds and transferred to the electrode in the mud. Electrons flow from this electrode to the electrode in the overlying water where oxygen is present and here the electrons combine with oxygen to form water.

The potential impact

Thus, this novel microbial process can be used to harvest electricity from organic matter that otherwise would have little practical value. At the present time the amount of power generated by these sediment batteries is small, only enough to power electrical equipment with very low power requirements, such as a hand-held calculator. However, even at these low levels, sediment batteries may be useful for powering monitoring devices in remote locations, such as the bottom of the ocean, where it is difficult and expensive to return to replace expended batteries.

With further optimization, the ability of geobacters to generate electricity from waste organic matter may have many additional applications. For example, sewage represents a significant quantity of waste organic matter that to must be processed prior to release into the environment. Typically, this is performed in sewage treatment plants in which microorganisms oxidize the waste organic matter by transferring electrons to oxygen. The energy released from the oxidation of the organic matter is not recovered. If, instead of using oxygen, electrodes could be placed in the sewage treatment system, then it should be possible to harvest energy from this waste organic matter in the form of electricity.

Also, the microbial conversion of corn into ethanol is a well-known process for transforming biomass into a usable energy form. It seems likely that it should be possible to engineer a geobacter-electrode system which could convert, not only corn, but also a wide diversity of waste or readily renewable organic materials into electricity. Electricity production may be more beneficial than producing ethanol, because electricity is easier to deliver to the point of use and has a wider range of applications than ethanol.

Many small-scale applications are also viable. For example, it should be possible to feed geobacters contained in a small, at-home system, the grass clippings from your lawn to generate power to charge batteries for an electric lawn mower that can power your mowing next weekend. Instead of sending food through your garbage disposal, why not feed it to a geobacter-electrode systems to charge up batteries for your MP3 player?

In order to realize these dreams, we need to better understand how geobacters transfer electrons to electrodes. Toward this goal, we are intensively studying the workings of geobacters with grants provided by the Department of Energy through the Microbial Genome and Genomes-to-Life programs, as well as with grants from the Office of Naval Research and the Defense Advanced Research Project Agency of the Department of Defense. At the present time we only have some guesses at how this process works. But thanks to these funding agencies we now have the complete genome sequence of several geobacter species and are investigating which genes are necessary for electricity production. With reasonable progress it is hoped that soon we will understand not only how geobacters make electricity, but also how we can genetically improve their electricity production capabilities in order to make large-scale harvesting of electricity from organic matter a reality.

Lovley is a distinguished university professor and the head of the microbiology department at the University of Massachusetts, Amherst. Details on his research are available at www.geobacter.org. He can be reached at dlovley@microbio.umass.edu or 413-545-9651.

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