Electroactive Bacteria: A “Mind-Blowing” Case of Intelligent Design
Editor’s note: We are delighted to welcome Otangelo Grasso, a graduate of the Summer Seminar on Intelligent Design, as a contributor.
As journalist Carl Zimmer reported not long ago in the New York Times, “Wired Bacteria Form Nature’s Power Grid: ‘We Have an Electric Planet.’” Electroactive bacteria were running current through “wires” long before humans discovered electricity. Now that is worthy of note and analysis. How did they learn this very sophisticated trick?
In a tweet, Zimmer did not hide his amazement, admitting that that the “discovery that microbes build electric wires all over the world is mind-blowing.” Though an outspoken advocate of unguided Darwinian evolution, Zimmer in his article did not explain how bacteria might have gotten that ability by evolutionary means.
Zimmer isn’t the only one to confess his astonishment. Electroactive bacteria were unknown until 1993, when Derek Lovley at the University of Massachusetts at Amherst discovered and described Geobacter metallireducens. As Lovley told New Scientist in 2010:
They grow biological wires to share energy in the form of electrons…. I think it’s probably one of the most surprising things I’ve seen working in microbiology. [Emphasis added.]
How We Breathe
Some background on respiration may be helpful here. For advanced multicellular organisms, oxygen is essential to life. During aerobic respiration, it is the final acceptor of electrons in the electron transport chain. In anaerobic (non-oxygen breathing) respiration, on the other hand, as in some bacteria, a variety of acceptors other than oxygen exist. Such bacteria thus can survive without oxygen, which is good thing for them. Some bacteria grow in places where there is no oxygen, or too little oxygen for respiration, or where other chemicals that will do the job are more abundant. Indeed oxygen is poisonous to many bacteria. One group of anaerobic bacteria are electroactive. Living meters below the Earth’s surface, and even on the ocean floor, these bacteria are adapted to live in environments inhospitable to most other life forms.
Geobacter bacteria “breathe” using elements such as iron, sulfur, and uranium. They employ microbial nanowires that conduct electricity (as flowing electrons). Geobacter nanowires are filaments called “pili,” according to Wikipedia. Pili (plural of pilus) is Latin for “hairs.” These “hairs” are thin rod-like appendages, about 1/100,000 the width of a human hair.
Bacteria may have dozens of pili on their surfaces. Bacteria use pili for various functions, including adhesion to surfaces, DNA transfer, locomotion, and gliding. In the most fascinating case, that of electroactive bacteria, they make electrical connections with minerals.
According to another 2010 article in New Scientist:
Some researchers believe that bacteria in ocean sediments are connected by a network of microbial nanowires. These fine protein filaments could shuttle electrons back and forth, allowing communities of bacteria to act as one super-organism. Now Lars Peter Nielsen of Aarhus University in Denmark and his team have found tantalising evidence to support this controversial theory. “The discovery has been almost magic,” says Nielsen. “It goes against everything we have learned so far. Microorganisms can live in electric symbiosis across great distances. Our understanding of what their life is like, what they can and can’t do — these are all things we have to think of in a different way now.”
Specialized pili of the bacterium Geobacter sulfurreducens conduct electrons from inside the cell to the iron external to the cell. The metal functions as the terminal electron acceptor for respiration. This, again, is in contrast to humans (and most animals, fungi, and plants) where the terminal electron acceptor is oxygen. In our case, during respiration, electrons are removed from oxidized fuels, such as hydrocarbons, or glucose, inside cells. Oxidation entails the loss of electrons. These electrons are then combined with oxygen, from the air you breathe. The oxygen is reduced to water, since reduction is the gain of electrons. Without a terminal electron acceptor, the flow of electrons stops. This means respiration stops, along with the supply of energy from fuels.
If the terminal electron acceptor is solid, like iron, then it cannot be easily imported into the cell. The solution is to leave it outside the cell and to send the electrons to it. The specialized pili conduct electrons from the respiratory system — that is, the electron transport system required to make ATP, the energy currency in the cell — to the final electron acceptor. Nanowires are among the smallest known electrical wires. And remember, they were doing their job long before humans discovered electricity.
A Marvel of Microtechnology
The architecture involved in nanowires is an ultracomplex, microtechnological marvel. Earlier this year, researchers at the University of Virginia made a significant advance in unraveling nanowire structure. Nanowires, it turns out, have a core of precisely stacked, ordered, and spaced metal-containing hemes (the active part of hemoglobin in red blood cells). These line up to create a continuous path along which electrons travel:
“The technology [to understand nanowires] didn’t exist until about five years ago, when advances in cryo-electron microscopy allowed high resolution,” said [Edward H.] Egelman, of UVA’s Department of Biochemistry and Molecular Genetics. “We have one of these instruments here at UVA, and, therefore, the ability to actually understand at the atomic level the structure of these filaments.” According to the report of his research, “Scientists had believed Geobacter sulfurreducens conducted electricity through common, hair-like appendages called pili. Instead, a researcher at the School of Medicine and his collaborators have determined that the bacteria transmit electricity through immaculately ordered fibers made of an entirely different protein. These proteins surround a core of metal-containing molecules, much like an electric cord contains metal wires. This ‘nanowire,’ however, is 100,000 times smaller than the width of a human hair.”
So Geobacter used highly specialized pili, rather than ordinary pili, to conduct electricity.
The UVA scientists published their results in the journal Cell. The technical details give a sense of the complexity involved:
G. sulfurreducens nanowires are assembled by micrometer-long polymerization of the hexaheme cytochrome OmcS, with hemes packed within 3.5–6 Å [ 1 Å = 10−10 m] of each other. The inter-subunit interfaces show unique structural elements such as inter-subunit parallel-stacked hemes and axial coordination of heme by histidines from neighboring subunits. Wild-type OmcS filaments show 100-fold greater conductivity than other filaments from a DomcS strain, highlighting the importance of OmcS to conductivity in these nanowires. This structure explains the remarkable capacity of soil bacteria to transport electrons to remote electron acceptors for respiration and energy sharing.
Nature’s Ingenious Solutions
Facing daunting technical problems, nature comes up with solutions that are in most cases far more advanced than those in equivalent devices made by man. For example, the journal Environmental Science, published by the Royal Society of Chemistry, reports that some microbes can link with each other to form longer, living electrical cables that allow them to penetrate even deeper into oxygen-free areas. As researchers came to appreciate such ingenious innovations, biomimetics has become a growing field of scientific investigation. Nanowires, among the other wonders of biology, have much to teach us. As Derek Lovley has explained:
Microbial nanowires are a revolutionary electronic material with substantial advantages over man-made materials. Chemically synthesizing nanowires in the lab requires toxic chemicals, high temperatures and/or expensive metals. The energy requirements are enormous. By contrast, natural microbial nanowires can be mass-produced at room temperature from inexpensive renewable feedstocks in bioreactors with much lower energy inputs. And the final product is free of toxic components.
How electrons in this context are transported across long distances was unknown until the 1990s, after many physiological, biochemical, and electrochemical experiments. For electron transfer to work, the architecture of G. sulfurreducens pili must be precisely arranged. As in other living systems, this order is only functional once it is fully set up. How unguided evolutionary mechanisms could produce such a system remains very unclear. One of the few papers to address the origin of electron-conductive pili makes the following claim:
The results suggest that e-pili of Geobacter sulfurreducens and Geobacter metallireducens, and presumably close relatives, are a relatively recent evolutionary development.
But considerable nanotechnology is required to assemble these marvelous wires. The claim above is not accompanied by any detailed or convincing explanation of how this “evolutionary development” was accomplished.
Somehow, G. sulfurreducens “know” how to assemble molecules in their pili in an exact sequential and functional order. The steps involved require the assistance of many elements, including assembly chaperones. Whether these amazing pili can be explained by evolution, without recourse to intelligent design, is of course the key question. The irreducible precision of their construction, though, strongly suggests design. The wires require their several parts to be arranged in just the right way, ordered and stacked in the right sequence, to be able to conduct electrons. Several experiments have demonstrated that if this arrangement of the filaments is not exactly right, electron transfer is not possible.
So, where did nanowires come from? How did they arise? That is a question worth putting to Carl Zimmer.
Image: Nanowires, by Asw-hamburg [CC BY-SA 4.0], via Wikimedia Commons.