Have we found alien life? This interview did the rounds via Facebook recently. There is only one answer.
We have found bacteria that can run their electron transport chain using an electrical connection to an external electron acceptor. In more ordinary bacteria electrons travel down the ETC to complex IV where, under oxidative metabolism conditions, they are passed to oxygen as the terminal acceptor, still within the cell.
Bacteria are the most sophisticated metabolists on Earth. To stick a series of cytochromes together to form a wire, from the end of an electron transport chain to an extracellular acceptor of electrons, is no big deal. Once a bacterium is using such a wire (which will, undoubtedly, be using quantum tunnelling effects much as the FeS clusters of complex I do) to access extra cellular terminal acceptors it is absolutely no problem to make that terminal acceptor anything with an appropriate redox potential.
Under these conditions a cathode electrode will allow the metabolism of NADH from the cytosol, via complex I and the CoQ couple down the ETC and out along the cytochrome wire. A cathode electrode, whatever it is made of, is an electron acceptor. That's the definition.
So growing bugs on a cathode is utterly unremarkable. NADH comes from "food". Electrons from NADH drive the ETC to make ATP and are dumped out-of-cell on to the cathode.
Where things get slightly interesting is that you can also apply a negative voltage to the end of the wire, making it an anode, a net supplier of electrons. Certain types of bacteria can function under these conditions without any carbon input. From the article:
“A lot of organisms that can put electrons onto an electrode can also do the opposite and take electrons from one”—though not at the same time—Rowe says. That ability to reverse course surprises me, and Rowe, too. “I’d think it would be really hard on the organisms. You’re basically stealing energy from them. But they do okay.”
This is incorrect, it lacks perception.
It is perfectly possible to run the ETC in reverse. Bacteria do this on occasions, for reasons best known to themselves, especially if their normal metabolic substrates cannot generate NADH directly. Usually ATP is used to drive ATP synthase in reverse to maintain the membrane potential. Electrons flow in reverse to reduce NAD+ to NADH which can then be used for anabolism. All that Shewanella spp need to do is to use the negative voltage of an external electrode to facilitate reverse electron flow and they can be generating NADH for anabolism without any carbon source. About a third of a volt does the trick, from the interview.
Whether this process would allow the fixation of atmospheric carbon dioxide to actually allow growth is irrelevant. What matters is that there is absolutely nothing about these bacteria living on "pure electricity" which suggests anything other than a clever piece of wiring added on to the end of a fairly normal ETC.
This is what Shewanella oneidensis looks like with its complex I and CoQ couple:
A clever bacterium? Yes, with very busy periplasm. But anyone who thinks that any bacteria are simple is stupid. What if you think that these particular ones are alien life forms?
"Kenneth Nealson is looking awfully sane for a man who’s basically just told me that he has a colony of aliens incubating in his laboratory".
It is only an alien life form if aliens developed the same electron transport chain as any Earthly organism which uses oxidative phosphorylation to chemical acceptors.
How much does Nealson know?