While it is hard for most of us to imagine life without oxygen, bacteria have been finding other ways to breathe for billions of years. A particularly successful group of anaerobic (non-oxygen breathing) bacteria is the Geobacter. From acid mine drainage sites to iron-rich rocks buried meters beneath the Earth and rusting ship wreckage on the ocean floor, Geobacter specialize in environments inhospitable for most life. How do they do so? For one, Geobacter don’t need oxygen. They can “breathe” using a number of other elements, including iron, sulfur and uranium. Recently, another startling feature of Geobacter was discovered that may shed light on their success as iron-breathers. They are electrically conductive.
Before we get to electrical conductivity, a bit of background on iron breathing, or “iron reducing” in technical lingo. All life on earth requires energy. On a molecular level, all life acquires energy in much the same way: stripping electrons from one substance (usually, but not always, carbon) and transferring said electrons to another substance- an electron acceptor. Oxygen is the preferred electron acceptor among multicellular organisms because of its high electron affinity. This just means you get more “bang for your buck” using oxygen to strip electrons off your food than using, say, iron or sulfur. But oxygen is not found everywhere, and many microbes have become adapted to using other electron acceptors in lieu of oxygen. In theory, this makes sense. In practice, iron is a bit of a head-scratcher. In its oxidized (i.e., electron-depleted) form, iron is a heavy, insoluble metal that cannot easily cross cell membranes. For decades, scientists have assumed that iron-reducers like Geobacter have some adaptation that allows them to use iron outside of their cells for respiration.
This is where electricity comes in. Like many bacteria, Geobacter has long, filamentous appendages called pili extending from its body. Pili allow bacteria to sense their environment, similarly to whiskers or antennae. Sometimes, bacteria use pili to directly interact with their environment, releasing chemical compounds or exchanging genetic information with other bacteria. It turns out Geobacter’s pili are highly electrically conductive- as conductive as synthetic organic metals. This discovery has led scientists to hypothesize Geobacter’s pili serve as “electrical wires” that conduct electrons from inside the cell to iron in the environment.
Genetic studies have provided substantial evidence to support the “nanowire” hypothesis. The pilA gene encoding pili proteins is more highly expressed when Geobacter is grown with insoluble iron than soluble iron. That is, there is a direct relationship between pili production and the presence of iron that cannot cross cell membranes. To obtain direct evidence that pili are involved in iron respiration, scientists have created “knockout” strains of Geobacter that lack the pilA gene. Sure enough, pilA knockouts cannot respire insoluble iron, but they still grow using soluble iron that can diffuse across their cell membranes. Experimental evidence from culture studies also supports the link between electrical conductivity and pili. When grown in environments where electrical conductivity may provide an advantage, such as on graphite electrodes, Geobacter produce more pili.
A microbe with metallic conductivity is more than just a curious oddity of nature. Geobacter’s conductive pili raise exciting prospects for engineers in the emerging field of bioelectronics, who envision creating nano-powergrids out of “microbial wires”. Geobacter grown on electrodes may one day serve as a cheap energy source- if we can find a way to harness that energy.