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.