Deep on the ocean floor, colonies of bacteria appear to have connected themselves via microscopic power grids that would be the envy of any small town. Much remains unknown about the process, but if confirmed the findings could revolutionize scientists' understanding of how the world's smallest ecosystems operate.
Oxygen-breathing bacteria that live on the ocean bottom have a problem. Those sitting atop the sediment have ready access to oxygen in the water but not to the precious mineral nutrients that lie out of reach a centimeter or so below the ground. Meanwhile, those microbes that live in the sediment can access the nutrients, but they lack oxygen. How do both groups survive?
Microbial ecologist Lars Peter Nielsen of Aarhus University in Denmark figured the surface and subsurface bacteria were somehow exchanging oxygen and nutrients with one another. To find out how, he and colleagues scooped up some mud from the bottom of the 20-meter-deep ocean in Aarhus Bay and other waters near the university and plopped it into a beaker in their lab.
Then the researchers did something they knew would make the bacteria unhappy: They started removing the oxygen from the water. If the bacteria were swapping materials, as Nielsen had suspected, those living below the surface of the mud would have gradually noticed that their oxygen supply was being cut off; they would have registered chemical changes in the sediment that could be detected by sensors. But instead, Nielsen and colleagues witnessed something far more rapid. Almost as soon as the researchers began removing the oxygen, the subsurface bacteria stopped consuming hydrogen sulfide in the mud. More important, this metabolic shutdown was a sign that the buried bacteria almost instantly realized something in the environment far above them had changed. The researchers also detected very rapid pH changes in the water in the beaker.
These responses occurred too quickly for any sort of chemical exchange or molecular process such as osmosis, says Nielsen. The most plausible option, his team reports in the 25 February issue of Nature, is that the bacteria are somehow communicating electrically by transmitting electrons back and forth. How exactly they do this is unclear, but Nielsen suspects the organisms may all be connected to each other via a microscopic electric grid, possibly made from tiny grains of metal, such as iron and manganese, in the sediment.
If the wiring idea turns out to be true, it essentially would turn the bacterial community into a cross-sediment power grid—one that would span some 20 kilometers if scaled up for humans. Instead of receiving oxygen from the surface and turning it into energy—something the researchers say is not possible given the thickness of the sediment depth observed—the buried bacteria would simply receive energy in the form of electrons from the grid. In response, the subsurface bacteria could survive while buried and send nutrients back up to their comrades on the surface via chemical migration.
Still, Nielsen says, much remains unknown about how such a grid would work. "What are the wires made of?" he asks. "How do they connect to cells and one another, and how are they built?"
Geobiologist Kenneth Nealson of the University of Southern California in Los Angeles agrees that new findings "fall into the category of 'must have an explanation other than chemistry.' The excitement now lies in coming up with the mechanism[s] responsible for electron movement."