Figure 1. EM image of Geobacter nanowires. (Malvankar, 2015 Fig. S1). Source
What if instead of inhaling oxygen and exhaling carbon dioxide, humans exhaled lightening? Well, okay... Maybe it would be more like exhaling static electricity. It sounds like a comic book ability, but there are microbes that do just that.
Respiration by humans (and other multicellular organisms) occurs on two levels. First, the physical inhalation of oxygen, which is transported to individual cells where it then participates in the final step of cellular respiration. After sugars are degraded to create energy, the individual electrons that result are transferred to oxygen and hydrogen molecules to generate water.
Using oxygen as the final electron acceptor in respiration is the most energetically efficient option but cells that live and grow anaerobically (like many bacteria) are forced to find other options. Most anaerobic bacteria use soluble molecules like sulfate or nitrate that can be easily imported and then exported after electron transfer. Other bacterial species, however, get a bit more creative with respiration.
Shewanella oneidensis is a bacterial species known as a dissimilatory metal-reducing bacteria (DMRB) since it uses poorly soluble, metal-containing minerals as electron acceptors. Minerals used by DMRB commonly include the metals iron, manganese, cobalt or even uranium. But, it being difficult to import such insoluble compounds, S. oneidensis must somehow transfer electrons extracellularly, thus “exhaling” electricity.
A 2006 research study observed that when S. oneidensis was grown anaerobically, the bacteria produced pilus-like structures on the cell surface capable of conducting electricity. The nanowires lost their ability to conduct electrons in the absence of two c-type cytochromes or a secretion system component responsible for positioning c-type chromosomes correctly at the cell surface. These cytochromes act as extensions of the electron transport chain, shuttling electrons out of the cell and down the nanowire.
There is some dispute as to what is designated as a nanowire. Geobacter species also produce nanowires (Fig. 1) that are structurally and functionally distinct from Shewanella nanowires. But DMRB aren’t the only types of bacteria to conduct electricity along outer membrane projections. In fact, the same research group found a cyanobacterium, a strain of Synechocystis, and a thermophilic fermenter, Pelotomaculum thermopropionicum, that produces nanowire-like structures. The researchers had a brilliant idea: they co-cultured P. thermopropionicum with Methanothermobacter thermautotropicus and found nanowires connecting them! The authors hypothesized that the function of nanowires may extend beyond mineral electron dumps to symbiotic electron-transfers, even occurring within single-species biofilms.
For just as there are some bacteria that “exhale” electrons, there are others that “inhale” them. This isn’t quite as dramatic as nanowires, but just as fascinating. In 2015, researchers figured out how some bacteria (in this case Methanococcus maripaludis and Sporomusa sphaeroides), strip electrons from metals, like iron, in their environments.
After noticing that electron stripping rates increased during later growth phases, the researchers hypothesized that a small, secreted molecule or protein was responsible. By studying the electron-stripping properties of spent media, they identified the culprits—two enzymes. These enzymes adhere to the metal surface, pulling off electrons and pairing them with either a proton to generate hydrogen, or carbon dioxide and a proton to generate formate. The resulting products are soluble and taken up by the bacteria so quickly that researchers can’t even measure the creation of hydrogen or formate in the media unless they remove the bacterial cells.
The act of inhaling or exhaling electrons has serious consequences, and applications, for the human world. Each year, billions of dollars are lost to rusting infrastructures caused, in part, by stripping of electrons. But, there are also billions of dollars to be made by fixing carbon dioxide into industrial chemicals like acetate or with the creation of efficient microbial fuel cells—essentially, bacterial batteries. By pairing the right bacteria (or bacterial enzymes) with the right conditions, it’s possible to create safer, cheaper, and renewable batteries.
They’ve proved us wrong about Halloween legends and X-men like superpowers, what will microbes come up with next?
- Seeing is believing: novel imaging techniques help clarify microbial nanowire structure and function.
- Microbial nanowires for bioenergy applications.
- Microbial fuel cell as new technology for bioelectricity generation: A review