How does a microbe know how to share electrons with an inanimate object? A wide variety of microbes can send electrons into, or accept electrons from, conducting materials. Witness the fuel cells that rely on different types of bacteria to exchange electrons with graphite electrodes.
But investigators have wondered how that ability arose. Most organisms internally generate energy by coupling the addition of electrons to one molecule with their removal from another. But some microbes find themselves in circumstances where they must cooperate to generate the energy for life, swapping molecules or electrons with other species. Do these microbes enhance their energy management, and thus their ability to grow, by shuttling electrons back and forth to one another through conductive materials in their environment? Research published in Proceedings of the National Academy of Sciences on June 4 suggests the answer is yes; some bacteria do indeed build electricity-conducting grids in the wild.
"Microbes use conductive minerals as electric wires for transferring electrons between each other," says microbiologist Kazuya Watanabe of the Tokyo University of Pharmacy and Life Sciences, part of the team that performed the research. This marks the first time anyone has provided "solid evidence" that different species transfer electrons to each other in that way, he adds.
The researchers tested a variety of solutions containing the soil bacteria Geobacter sulfurreducens and Thiobacillus denitrificans, which thrive by eating acetate (an organic compound that makes vinegar sour) and nitrate (a negatively charged molecule of biologically available nitrogen and oxygen), respectively, when they can find a spare electron or two. When the scientists placed either of these microbes alone into a solution containing the two compounds, nothing happened. Nor did the situation improve when both types of microbes were put together into this solution of their favorite foods, suggesting the organisms lacked the ability to directly transfer electrons between them.
But when the scientists added magnetite, an electricity-conducting iron-based mineral famous as the lodestone that provided ancient magnets, the bacteria busily got to work eating, cooperating merrily by shuttling electrons back and forth via the magnetite grains. And, although the same effect could be had by adding the rusted red iron mineral hematite, which is a poor conductor, the resulting microbial growth was much smaller and slower (and nonexistent when nonconductive aluminum minerals were tried).
In fact, the only thing slowing the microbes down in the presence of magnetite was their own ability to grow. "We think [such electron swapping] must be quite common in soil, sediments and ores," Watanabe says.
When the scientists examined the cells in the growing communities closely, they found nanoparticles of magnetite on the surface of cells and, in some cases, grains of the iron mineral connecting microbial pairs. They observed, in other words, a basic, biological electric grid and one that, because of its size, offers very little resistance to the flow of electrons.
Prior to this research, microbial ecologist Lars Peter Nielsen of Aarhus University in Denmark and his colleagues had shown that microbes working in the oxygen-free muck at the bottom of Denmark's Aarhus Bay exchanged electrons over relatively large distances of centimeters, although how the bacteria managed the trick remained unknown. One hypothesis was that they built nanoscale wires for the task, or released molecules from cell to cell. But using electrical conductors already in the environment would require much less of an energy and material investment than building a biological structure like a nanowire or molecule, and that is what microbes may do based on the new findings.
Now, Nielsen, who was not involved in the new study, is speculating about how microbes take best advantage of natural conductors around them. "With electric conductors around, it seems very attractive to exchange goods into a common and more liquid currency -- electrons," he argues. "To get the full benefit of these mini electrodes, I imagine that the microbes may have developed structures to control the location of them and not just rely on coincidental contact," he adds.
That idea remains to be proved. In the meantime the new discovery suggests that microbes like G. sulfurreducens and T. denitrificans may build electric grids wherever they find themselves. After all, magnetite and other conducting minerals abound on Earth, and such metal-based grids, by allowing the long-distance transfer of electrons, would foster microbial growth. Humans may benefit from bacterial grid-building as well. Understanding how the microbes construct their grids may help us to build a better fuel cell to put that potential to work for us.