To colonize Mars, settlers will need to manufacture on-planet a huge range of organic compounds, from fuels to drugs, that are too expensive to ship from Earth. Researchers have been working on a hybrid system combining bacteria and nanowires that can capture the energy of sunlight to convert carbon dioxide and water into building blocks for organic molecules. Nanowires are thin silicon wires about one-hundredth the width of a human hair, used as electronic components and as sensors and solar cells.
The only other requirement, besides sunlight, is water, which on Mars is relatively abundant in the polar ice caps and likely lies frozen underground over most of the planet. The biohybrid can also pull carbon dioxide from the air on Earth to make organic compounds and simultaneously address climate change, which is caused by an excess of human-produced CO2 in the atmosphere.
The researchers packaged the bacteria (Sporomusa ovata) into a forest of nanowires. More than 3% of the incoming solar energy is converted and stored in carbon bonds in the form of a two-carbon molecule called acetate; essentially, acetic acid or vinegar. Acetate molecules can serve as building blocks for a range of organic molecules, from fuels and plastics to drugs. Many other organic products could be made from acetate inside genetically engineered organisms, such as bacteria or yeast.
The system works like photosynthesis, which plants naturally employ to convert carbon dioxide and water to carbon compounds — mostly sugar and carbohydrates. Plants, however, have a fairly low efficiency, typically converting less than one-half percent of solar energy to carbon compounds. The new system is comparable to the plant that best converts CO2 to sugar: sugar cane, which is 4 to 5% efficient.
The researchers initially tried to increase the efficiency by packing more bacteria onto the nanowires, which transfer electrons directly to the bacteria for the chemical reaction. But the bacteria separated from the nanowires, breaking the circuit. The team eventually discovered that the bugs, as they produced acetate, decreased the acidity of the surrounding water — that is, increased a measurement called pH — and made them detach from the nanowires. The team found a way to keep the water slightly more acidic to counteract the effect of rising pH as a result of continuous acetate production. This allowed them to pack many more bacteria into the nanowire forest, increasing efficiency by a factor of nearly 10. They were able to operate the reactor, a forest of parallel nanowires, for a week without the bacteria peeling off.
In a real-world system, 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. The oxygen is a side benefit and on Mars, could replenish colonists’ artificial atmosphere, which would mimic Earth’s 21% oxygen environment.
The system has been tweaked to embed quantum dots in the bacteria’s own membrane that act as solar panels, absorbing sunlight and obviating the need for silicon nanowires. These cyborg bacteria also make acetic acid.