Bacterium Meets Photocathode
Headlines promote a breakthrough in “artificial photosynthesis.” It’s exciting technology, but–artificial? So what are those bacteria doing twined around the photocathodes?
The Berkeley scientists’ article in Nano Letters is titled “Nanowire–Bacteria Hybrids for Unassisted Solar Carbon Dioxide Fixation to Value-Added Chemicals.” What they seem to have made is:
(1) A silicon-nanowire array (the “photoanode”) splits water (H2O) like chlorophyll does in photosynthesis. The light-absorbing reaction removes an electron. What you get is H+ and O2 (O2 being the good clean oxygen we breathe). Where the other H goes is not clear; presumably it also gives up an electron, without toxic oxidizing radicals, which real photosynthesis produces.
(2) The electrons travel down the nanowires to a source of CO2, in the presence of photocathode wires. But here’s where we need the bacteria, Sporomusa ovata. The bacteria use an ancient evolved pathway of metabolism to reduce (add electrons to) the CO2, making acetic acid, CH3CO2H. Notice a lot of extra steps in there.
The use of Sporomusa ovata in a fuel cell was actually shown a few years ago by Derek Lovley’s lab, which has a long history of pioneering work in bacterial electricity. Lovley’s students used an electrical current to make Sporomusa bacteria reduce CO2 to acetate, a building block for various industrial products.
Here are Lovley’s electrode biofilms of Sporomusa. The implication is that hybrid nanoelectrode-bacterial fixation of carbon could directly make fuels and plastics, without the usual production of biomass (sugar polymers etc.) and thus without all the carbon waste along way.
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Ultraphyte 
A bionic leaf–how weird. I didn’t know that photosynthesis produced toxic oxidizing radicals–what’s that about? Why is this better than trees? (Aside from the free acetate).
Or is it all about creating nano-chlorophyll? You’re right–it’s exciting to think about!
Photosynthesis splits water to form O2, but inbetween there are all sorts of intermediates like hydrogen peroxide and peroxide radicals (unpaired electrons). That is why plants make so many antioxidants (and wh dark-colored plants are good for us to eat). The plants make antioxidants to counteract their own oxidants produced in their cells. Most of these molecules convert to oxygen, however, before leaving the plant.
The acetate production is better for industry than trees, because acetate is the precise “feedstock” needed to build fuel and plastics. By contrast, “biofuels” that come from plant biomass are made from a huge mix of plant stuff, most of which gets lost as waste.
Thanks, Joan. I wish I knew more about this stuff–it’s so interesting. The world is too big and life is too short.
And what about the titanium oxide–how renewable is that stuff?
I don’t know about the titanium. I hope people are working on electrodes from carbon nanotubes and other recyclables.
Reblogged this on Xper Dunn Is Here.
Thank you. I am sharing this here in Perú with my science-teacher niece and my microbiologist (long-retired) husband. Really fun stuff.