Nick Carne
US chemists have reported a “milestone” in their quest to combine bacteria and nanowires in a hybrid system that uses energy from sunlight to convert carbon dioxide (CO2) and water into building blocks for organic molecules.
It’s been an eight-year project to date, and the team – from the University of California, Berkeley (UCB) and the Lawrence Berkeley National Laboratory – isn’t short on vision.
Such a biohybrid could pull CO2 from the air on Earth to make organic compounds and address climate change, they say, and also help humans colonise Mars, by providing for the on-planet manufacture of fuels, drugs and other organic compounds.
“On Mars, about 96% of the atmosphere is CO2. Basically, all you need is these silicon semiconductor nanowires to take in the solar energy and pass it on to these bugs to do the chemistry for you,” says project leader Peidong Yang, from UCB.
You also need sunlight and water which, Yang says, is relatively abundant in Mars’s polar ice caps and likely also lies frozen underground.
The system works much like photosynthesis in plants, he suggests.
Nanowires are silicon wires about one-hundredth the width of a human hair and can be used as electronic components, sensors and solar cells.
In a paper in the journal Joule, the researchers report packing a species of bacteria (Sporomusa ovata) onto a “forest” of such wires to achieve a record efficiency: 3.6% of the incoming solar energy was converted and stored in carbon bonds, in the form of acetate molecules.
That’s quite an improvement on the 0.4% they were getting five years ago.
Yang says when tried to increase efficiency by packing more bacteria onto the nanowires – which transfer electrons directly to the bacteria for the chemical reaction – the bacteria separated from the nanowires, breaking the circuit.
They eventually discovered that as the bugs produced acetate they decreased the acidity of the surrounding water, which made them detach.
Keeping the water more acidic to get the pH right solved the problem, allowing bacteria to pack tightly around the nanowires. Close packing gives more efficient conversion of solar energy to carbon bonds.
In experiments, the nanowires are used only as conductive wires, not as solar absorbers. An external solar panel provides the energy.
In a real-world system, however, the nanowires would absorb light, generate electrons and transport them to the bacteria glommed onto the nanowires.
The bacteria take in the electrons and, similar to the way plants make sugars, convert two carbon dioxide molecules and water into acetate and oxygen.
“These silicon nanowires are essentially like an antenna,” says Yang. “They capture the solar photon just like a solar panel.
“Within these silicon nanowires, they will generate electrons and feed them to these bacteria. Then the bacteria absorb CO2, do the chemistry and spit out acetate.”
The oxygen is a side benefit, he adds. On Mars, it could replenish colonists’ artificial atmosphere.