Recently, Carl Zimmer wrote an article on 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 learned the trick. It caught my attention, and I began my own investigation.
On Twitter, Zimmer wrote:
The discovery that microbes build electric wires all over the world is mind-blowing, even for the scientists uncovering the planetary grid.
Interestingly, even being an outspoken advocate of evolution, Zimmer did not mention how they got that ability by evolutionary means in his article.
Electroactive bacteria were unknown until Derek Lovley at UMass Amherst discovered Geobacter metallireducens and described them in 1993 1 He wrote:
“ 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.”2
Oxygen is life-essential for advanced multicellular organisms. It is the final acceptor of electrons in the electron transport chain during aerobic respiration, but a variety of acceptors other than oxygen exist in anaerobic respiration.
Bacteria use different ways to breathe and can survive without the need of oxygen. One group of anaerobic (non-oxygen breathing) bacteria are electroactive. Living meters below the Earths surface, and even on the ocean floor, these bacteria are adapted to live in environments inhospitable for most other life forms. How do they do so? For example, Geobacter doesn’t need oxygen. They “breathe” using other elements, like iron, sulfur and uranium. They use microbial nanowires that conduct electricity (as flowing electrons). Nanowires are rotein filaments called Pili ('Pilus' is Latin for 'hair', plural pili) which are thin rod-like appendages that some bacteria have. These make electrical connections with minerals. Their size is 1/100,000 the width of a human hair.
Pili are truly fascinating. Bacteria possess no pili, one pilus, a few pili, or they can be clothed in hundreds of pili, giving them a hairy appearance. They are used by bacteria for various functions, like adhesion to surfaces, DNA transfer, locomotion, gliding, but the most fascinating is that they are used as electron nanowires, to which we will give a closer look.
An article in 3 reported:
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.”
The 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 in respiration. In humans (and most animals, fungi and plants) the terminal electron acceptor is oxygen. Electrons are removed from oxidised fuels (oxidation is loss of electrons), such as hydrocarbons, like glucose, inside cells during respiration. These electrons are then combined with oxygen (from the air you breathe in) to make water (the oxygen is reduced to water since reduction is gain of electrons). Without a terminal electron acceptor, the flow of electrons stops and respiration stops and the supply of energy from fuels also stops. Some bacteria grow in places where there is no 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.
If the terminal electron acceptor is solid like iron, then it cannot be easily imported into the inside of the cell. The solution is to leave it outside the cell and to send the electrons to it. The pili conduct electrons from the respiratory system (from the electron transport system which is required to make ATP, the energy currency in the Cell) onto the final electron acceptor. This makes nanowires among the smallest known electrical wires and they were doing their job long before man discovered electricity!
Pili are capable of transferring electrons to extracellular electron acceptors in a process termed extracellular respiration (Lovley, 2008) 4
In 2017 an article in Euronews reported:
"Some microbes grow little electricity-conducting wires, which act as a kind of snorkel, allowing the bacteria to penetrate deeper into sediment, where they can use this electrochemical process to survive where there is no oxygen. These so-called electroactive bacteria can purify water up to ten times faster than conventional methods. That’s why, instead of gravel, the experimental wetland is filled with conductive material. It acts as a physical support, and at the same time accelerates the metabolic processes that purify water."
“Microbial nanowires” play an important role in the global geochemical cycling of metals, minerals, and carbon in the environment, bioremediation of contaminants. 5
In April 2019, the University of Virginia noticed a significant advance in unravelling the nanowire structure. 6 The nanowires have a core of precisely stacked, ordered and spaced metal-containing heme's (similarly used in blood cell's) , called hexaheme cytochrome OmcS. They line up to create a continuous path along which electrons travel. They wrote:
“The technology [to understand nanowires] didn’t exist until about five years ago, when advances in cryo-electron microscopy allowed high resolution,” said 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. 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.
The science paper publishing the results summarizes:
Here, we report a 3.7 A° resolution cryoelectron microscopy structure, which surprisingly reveals that, rather than PilA, G. sulfurreducens nanowires are assembled by micrometer-long polymerization of the hexaheme cytochrome OmcS, with hemes packed within 3.5–6A° 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.
A following article published in 19 June 2019 in Nature Communications, noticed:
Cryo-EM reveals the structural basis of long-range electron transport in a cytochrome-based bacterial nanowire. Both e-pili and multi-heme outer-surface cytochromes have been implicated as being important for Geobacter’s long-range extracellular electron exchange with other cells and soil minerals
The Royal Society of Chemistry journal reported that scientist even discovered microbes that have recently been shown to link with each other together to form longer, living electrical cables that allow them to penetrate even deeper into oxygen-free areas.
Solutions for various technical problems are often discovered in nature, and they are in most cases far more advanced and sophisticated than equivalent devices made by man. For that reason, biomimetics has become a growing field of scientific investigation and is applied in a variety of fields. As Lovley explains::
"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."
The interest in e-pili is interdisciplinary, there is potential to use e-pili for bioenergy, bioremediation, sensing technologies, bioelectronics for wearable sensors or therapeutics 7
The way in which electrons are transported across the biofilm matrix through such large distances has remained unknown and under intense discussion until recently. A lot of experimental work, following physiological, biochemical and electrochemical approaches, was carried out in order to determine how the electrons are transported from the cells to the electrode through such large distances. 8
The pili architecture is an ultracomplex, microtechnological marvel. E-pili conductivity has been attributed to the truncated PilA monomer, which permits tight packing of aromatic amino acids to form a conductive path along the length of e-pili. 9
Below, an illustration of the extracellular reduction of Fe(III) oxide minerals in G. sulfurreducens
Components of the GS T4P and their role in the dynamic protrusion and retraction of the pilus fibre during the reduction of Fe(III) oxides to magnetite (grey legend) and soluble Fe(II) (not shown). The textboxes at the bottom of the illustration indicate the sequential steps of prepilin (prePilA) processing by the PilD signal peptidase in the inner membrane (IM), the polymerization of the mature pilin (PilA) and pilus protrusion across the outer membrane (OM), electron discharge, and pilus depolymerization to recycle the pilins in the inner membrane for a new round of polymerization. The corresponding extracellular steps of metal binding, reduction, and release of the reduced mineral are shown on top (italicized and in bold). 10
Assembly of G. sulfurreducens Pili
The architecture of G. sulfurreducens Pili must be precise and optimally arranged, otherwise, electron transfer cannot occur. How evolutionary unguided mechanisms could explain the origin of such a precise order, which is only functional once fully set up, has not been explained. One of the few papers that addressed the origin of electron conductive pili, in its conclusive remarks, made following claim:
The results suggest that e-pili of Geobacter sulfurreducens and Geobacter metallireducens, and presumably close relatives, are a relatively recent evolutionary development. 11
without going into detail how they got to that conclusion. There is considerable nanotechnology required to assemble these marvellous wires of bacterial nano-technology. G. sulfurreducens have the " know how", to assemble the Pili in a precise, correct sequential functional order. But to do so, assembly Chaperones are essential.
Pilus biogenesis at the outer membrane of Gram-negative bacteria
Geobacter cells are connected to the electrode by an extracellular matrix composed of pilA protein, polysaccharides and several c-type cytochromes. All these proteins were found to be necessary for an efficient electron transport from the cells to the electrode. The pilA protein has two isoforms with different specific functions. The short isoform, bounded in the intracellular fraction, influences the secretion of several outer membrane c-type cytochromes to the extracellular space and stabilizes the long isoform. The long isoform is required for secretion of PilA outside of the cell and is essential for biofilm formation on certain surfaces. When the gene encoding for both isoforms is suppressed, yielding a mutant, the respiration of iron oxides and the production of current in G. sulfurreducens biofilms are greatly inhibited. 5
Spc (small pilin chaperones) specifically associate with PilA proteins and are required for PilA stability suggests that Spc serves as a PilA chaperone. Chaperones assist in the correct folding of bacterial pilus. Spc does not share sequence or structural similarities with known chaperone proteins. These considerations suggest that Spc represents a previously unrecognized class of chaperones. 12
Of course, when it comes to explaining the origin of Pili, the question is, if they are the product of evolution. Evidence shows, that not only the wires "per sé" require several parts, just right, ordered and stacked in the right sequence, to be able to conduct electrons. Several experiments have demonstrated that if the filaments are not " just right" electron transfer is not conducted. But the assembly also requires essential assembly chaperones, which are crucial in the assembly process. Furthermore, Cyanobacteria do have electron transport pili as well and belong to the oldest known life forms on earth. The evidence leads to the inference that a masterful intelligent designer, most probably came up with this nanotechnology far before man invented electric wires !!
This site has a nice video on the subject:
Many More Bacteria Have Electrically Conducting Filaments
Last edited by Admin on Tue Jul 23, 2019 8:28 am; edited 28 times in total