How did Extracellular electron Transport evolve?

How did Extracellular electron Transport evolve?

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According to an on-line article at “Some microbes can eat and breath electricity”. This refers partly to the fact that Shewanella use extracellular electron transfer. Does such a different metabolic process imply a separate origin for this type of life, or, if not, how could it have evolved?

Very interesting how journalists can make something catchy out of scientific evidence. Kudos to that!

supposedly "eat" electricity

Bacteria don't eat electricity. Electricity in it's most basic form is the flow of charge (in this case ions) from a higher gradient (concentration) to a lower gradient (concentration).

So these bacteria, they utilise this flow of charge within their biological processes allowing them to survive and propagate.

if these exotic bacteria really originate from a separate genesis or just an example of how diverse life on Earth can get if given the chance?

These bacteria are not the only ones utilising electrochemical gradients to power their biological processes. Eukaryotic cells contain an organelle called mitochondria. I will not go into details, you can read that here, but eukaryotic life arose as a result of an endosymbiosis event where two organisms developed a symbiotic relationship, and the mitochondria was born, when organism A engulfed organism B. Mitochondria, also called the powerhouse of the cell, contains it's own DNA, which is very dissimilar from the nuclear DNA, which leads many people to believe that originally they came from an integration event between two different organisms. Mitochondria functions, by ways of the ETC to provide energy to the cell.

That is the dominant role for mitochondria, and it also processes an electrochemical gradient. So no, the answer is no these exotic bacteria did not originate from a separate genesis, but rather an example of how diverse life on earth can get. Believe me there are more weirder things nature has come up with, that bacteria processing heavy metal ions in their biological processes.

As stated by David, I may have created a correlation where there was none. My apologies for that. To point you the right way though,

Shewanella belongs to a class of bacteria called Dissimilatory metal-reducing bacteria, which are characterized by their ability to couple metal reduction with metabolism, if you take a look at the link you will find that Shewanella also contain a very high concentration of sigma-factors; factors which are associated with providing transcriptional robustness under environmental stress.

Coming to their origin, which is the topic of discussion here. I would point you towards MRSA here, the analogous question here is was Methicillin resistant staphylococcus aureus born out of nowhere? The answer to this question is no, this relates to the overuse of antibiotics. Here is a link from the NIH which will give you a brief history of the origin of MRSA.

Principally, this same concept can be generalized here, if you put a population (in this case Shewanella) under stress (heavy metal ions), a large number of the population will die off, but a smaller subset will develop a resistance towards the stress.

Please note, that this line of thought contains two possibilities, the first being that individuals developed the resistance with the aid of mutations in their DNA allowing them a survival advantage over the population, a popular example is the presence of the Delta32 mutation in the CCR5 gene which makes a subset of humans resistant to HIV(AIDS) infection. Did this happen in response to the HIV outbreak the world faced in the late 20th century? No, this most probably happened much earlier (In humans a mutation must move to the germline to propagate across generations) and only came to light after a significant amount of resources had been invested into HIV research. Meaning, that the mutant was already present during the diagnosis, but the selective advantages came to light after the stress response had been faced. Therefore MRSA could have developed from an existing subset of Staph since antibiotic treatments began in the 1940s (please take into consideration that bacterial replication is faster than human, therefore time flows faster and there is no concept of germline and somatic mutations in bacterial fission). But also, it is entirely possible that MRSA existed beforehand, and that continued treatment with antibiotics resulted in the emergence of the same subset.

So no, the answer is no these exotic bacteria did not originate from a separate genesis, but rather an example of how diverse life on earth can get. Believe me there are more weirder things nature has come up with, that bacteria processing heavy metal ions in their biological processes.

The presence of the sigma factor already tells you that they share the same genesis as all other existing microorganisms on earth, and as to how they developed to gain a survival advantage in environments containing high concentration of heavy metal ions, that relates to the stress response of a population outlined above.

Addendum: The shoddy article that provoked this question concerns two completely different bacteria (although you would be forgiven for not knowing)

The article concerns mainly Shewanella, which is one of a number of well-known bacteria that can use metal ions as terminal electron acceptors for the electron transport chain: i.e. they donate electrons to the metal.

However the authors have also done work on completely different - and also well-known - bacteria called chemlithotrophs that obtain electrons, which they can use as a source of reducing power (analogous to electrons from light energy).


The question of whether Shewanella has a separate origin is easily settled by the phylogenetic similarity to other bacteria based on its DNA and its repertoire of proteins, including those involved in energy generation and electron transfer.

So, No, Shewanella oneidensis MR1 is just one of several bacteria that use final electron-acceptors other than oxygen in their respiration.

However, there seems to be a misunderstanding about the role of the extracellular electron transport in the provision of energy for the organism - people seem rather shocked by the electric current - so let me clarify the situation.

The initial source of 'energy' is reduction of carbon metabolites

The metabolism of Shewanella has been reviewed by Fredrickson et al. Like other bacteria it can be grown on carbon compounds which give rise to intermediates that are oxidized by NAD+ - e.g. in the tricarboxylic acid cycle - resulting in NADH. (I'm ignoring FAD/FADH2 for simplicity.) This involves the transfer of two electrons (the second is to H+) - as we remember from chemistry, oxidation is the removal of electrons. It is the NADH that can be thought to embody the 'energy', in so far as the reaction of its reoxidation by an appropriate electron acceptor can result in a large decrease in Gibbs free energy (see, e.g. Berg et al. 18.2).

The electron transport chain allows the free energy changes to produce ATP

Just as with aerobic bacteria such as Escherichia coli, all the potential free energy of oxidation of NADH is not liberated in one step, but in a series of steps, with smaller free energy changes. Some of these result in the extrusion of hydrogen ions into the intermembrane space, producing an electrochemical gradient which drives the production of ATP via the trans-membrane ATP synthase in oxidative phosphorylation. (It's still oxidative phosphorylation even though oxygen is not involved.) The components of the electron transport chain differ partially from the standard one presented in text books for bacteria with a different final electron acceptor (see e.g. review by Haddock and Jones). However it is important to realize that this is where the ATP is generated in Shewanella, before the electrons get to the cell surface and have any involvement with the extracellular space.

The role of the extracellular metal oxide is as final electron acceptor

For the sequence events from the reoxidation of NADH to be completed, the two electrons have finally to be transferred to some external oxidizing agent. For E.coli it is molecular oxygen with cytochrome oxidase catalysing its conversion to water. Other bacteria use different electron acceptors, some use sulphate (which becomes reduced to sulphite) for example, although the free energy changes of these reactions will not be the same as with oxygen. In the case of Shewanella the final electron acceptor is usually Mn(IV), which is reduced to Mn(III), or Fe(III), which is reduced to Fe(II), although other metal electron-acceptors are known (see e.g. Hartshorn et al. 2009).

What about the electricity?

What's clearly different about Shewanella is that the solid metal oxide electron acceptor is extracellular, and therefore not in contact with the multi-cytochrome electron donors in the bacterial outer membrane, unlike the diffusable oxygen or sulphate ions. Furthermore, no enzyme is involved. Hence, rather than the electrons being transfered in a reaction at the active site of an enzyme, they flow through the extracellular milleu as an electric current. The electrochemistry of this may be intriguing, but what is clear is that this is not providing any energy to the bacterium, which is certainly not 'eating' - or in any other way utilizing - electricity.

And evolution?

Shewanella is certainly different, and had to evolve new electron-transport components and transporters to take these components to the outer membrane (see diagram, below). But bacteria with other electron acceptors had to evolve similarly new redox components (Alberts et al. discuss this in The Molecular Biology of the Cell). Whenever bacteria have taken over a new (and often unpromising) niche, new enzymes or protein have evolved to allow them to do so. And electricity certainly isn't foreign to Nature, nor is the interaction of bacteria with their external environment.

Multi-haem cytochromes in the outer membrane of Shewanella and reduction of extracellular Fe(III): from Fredrickson et al. 2008.

Chemlithotrophs: something completely different

The original article actually refers to two types of bacteria. The first is Shewanella, which is well studied and for which the quotation below is entirely in concord with my answer, above

Shewanella consumes electrons from carbohydrates, but it sheds them in an unusual way: “It swims up to the metal oxide and respires it.” Nealson says.

However, the title of the article is based on the following teaser:

One of Nealson's graduate students, Annette Rowe, has found six new bacterial strains dredged from the ocean floor that don't need a source of carbon at all, reports Powell. They can live off of electricity alone.

Now it is clear nonsense that anything can live - in the sense of grow - without a source of carbon (and other elements) to supply the structural components that make up the organism, but it turns out that it has been known for a long time that certain micro-organisms can oxidize metals - i.e. in contrast to Shewanella obtain electrons from metals, rather than donate them to metal ions. These are known as chemlithotrophs and often obtain their carbon from carbon dioxide, and usually use molecular oxygen as the final electron acceptor. The enzymic details of this process obviously differ between chemlithotrophs, and the down-voting of this question makes me reluctant to invest the time into researching and then presenting details of the metabolism.

The latest paper that I can find from Rowe and Nealson, published in Frontiers in Microbiology 2015, makes it clear that they are studying lithotrophs, and what is new is that they are replacing the metals by electrodes as electron donors, partly in order to find new lithotrophs which can utilize different redox potentials. Very different from the smoke and mirrors of the Smithsonian article!

Watch the video: Generating the Proton Gradient: The Electron Transport Chain Cellular Energetics #2 (September 2022).


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