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Ken Nealson: Mysteries of a Metal-Munching Microbe

Ken Nealson unlocks the secret chemistry of a remarkable bug.

shewanella

Shewanella has an unusual approach to cellular respiration

Drop a nail in a bucket of water and you’ll see rust—a.k.a. corrosion—within a few days. It’s the result of a chemical reaction between the iron in the nail and dissolved oxygen in the water.

Corrosion of metals was long considered to be firmly in the domain of chemistry. Until, that is, Ken Nealson, Ph.D., and colleagues dug into the muck at the bottom of Oneida Lake near Syracuse, NY and pulled out microbes with the amazing ability to breathe metals.

For nearly a quarter-century, Nealson has worked to uncover how bacteria in the Shewanella family extract energy from metals like iron, manganese and uranium. In Shewanella, Nealson has found inspiration for bacteria-powered batteries, new approaches to waste management and insights about life on other planets.

Nealson is Wrigley Chair in Environmental Studies and Professor of Earth Sciences and Biological Sciences at the University of Southern California and Director of the Microbial and Environmental Genomics Group at the J. Craig Venter Institute. A pioneer in the field of geobiology, Nealson studies how living organisms interact with the planet’s mineral and metal chemistry.

Let Them Eat Iron

Whether you’re a person devouring a pizza or a microbe burrowing through mud, eating and breathing are ultimately all about electron flow. The flow of electrons from a source (electron donor) to a destination (electron acceptor) is the basis for all metabolism. Metabolism, also known as cellular respiration, is the chemical process by which cells capture energy and use it to survive, grow and reproduce.

The cells of animals, plants and most other familiar life forms use oxygen as their electron acceptor during respiration. Shewanella’s special talent comes from its ability to use iron, manganese, chromium or uranium as its electron acceptor.

Scientists used to think it was impossible for organisms to extract energy from metals because, as solids, metals cannot be accessed by the enzymatic machinery inside bacterial cells. Nealson discovered Shewanella has found a resourceful way around this problem: Instead of bringing pieces of metal inside the cell, Shewanella bacteria deploy respiratory enzymes outside their cellular membranes. By essentially wearing their lungs on the outside, the bacteria can simply latch on to a chunk of metal and begin harvesting energy from it.

Nealson describes Shewanella—which he affectionately calls “shewie”—as “a remarkable bug that is capable of doing really remarkable environmental chemistry.” Its highly unusual approach to cellular respiration has offered critical insights for the budding field of electromicrobiology—the study of how microbes transfer electrons to different surfaces.

Bugs to Do Our Bidding?

Nealson believes Shewanella and other bacteria have a lot to teach us about harvesting energy from unconventional sources. In his electromicrobiology research, Nealson explores how we can use these organisms’ unique abilities to improve technologies for producing energy, cleaning up waste and fighting corrosion.

We have thousands of species, maybe millions of species of microbes… They had to have invented wonderful machines to accomplish all the things that we want to do.” – Kenneth Nealson

For example, Nealson and his colleagues have developed microbial fuel cells that essentially stretch out microbes’ metabolism and put it to work for humans. These devices take advantage of the electron flow from the bacteria by coupling it to an anode, which the bacteria use as an electron acceptor as they would use iron or manganese in nature.

Because the energy source in these fuel cells is organic carbon, the fuel cells can potentially make energy from industrial or municipal waste—transforming that waste into energy and clean water. Microbial fuel cells are still too inefficient to be viable at large scales, but Nealson is collaborating with biologists, chemists and engineers to refine these systems for greater efficiency.

Nealson and his colleagues have done some genetic detective work to more effectively harness Shewanella’s unique powers. By sequencing the genomes of about 20 different Shewanella strains, they were able to pinpoint the three key genes behind the three key enzymes that allow Shewanella to work its magic. Inserting these genes into other bacteria, such as E. coli, gives those bacteria the ability to harvest energy from metals like iron and manganese, as well.

Nealson’s work on Shewanella also provides fundamental insights about the chemical, geological and physical signatures of life. These insights could help scientists look at samples from Mars or other places and know whether they contain evidence of extraterrestrial organisms.