(CN) — Chemists from University of California, Berkeley, and Lawrence Berkeley National Laboratory have developed a plan to aid humans in the eventual effort to settle on Mars, according to a study released Tuesday.
For human settlers to accomplish the feat of colonizing Mars, something must be done to manufacture on-planet a huge range of organic compounds, from fuels to drugs, that will be far too expensive to ship from Earth. These university chemists have comprised a plan in their paper to be published Tuesday in the journal Joule.
Over the past eight decades, the UC Berkeley and Lawrence Berkeley Lab researchers have been working on a hybrid system that combines bacteria and nanowires to capture the energy of sunlight and 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 and are excellent to use for electronic components, sensors and solar cells.
The biohybrid is also capable of pulling carbon dioxide from the air on Earth to make organic compounds, as well as address climate change caused by an excess of human-produced CO2 in the atmosphere.
"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," said project leader Peidong Yang, professor of chemistry and the S.K. and Angela Chan Distinguished Chair in Energy at UC Berkeley. "For a deep-space mission, you care about the payload weight, and biological systems have the advantage that they self-reproduce: You don't need to send a lot. That's why our biohybrid version is highly attractive."
Conveniently, the only other requirement for this process, besides sunlight, is water which is relatively abundant on Mars near the polar ice caps and likely lies frozen underground over most of the planet, said Yang, who is a senior faculty scientist at Berkeley Lab and director of the Kavli Energy Nanoscience Institute.
In their paper, the researchers report a breakthrough in packing these bacteria (Sporomusa ovata) into a "forest of nanowires" to achieve a record efficiency. According to their findings, 3.6% 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 serve as building blocks for various organic molecules, playing a part in fuels and plastics to drugs. Several other organic products could potentially be made from acetate inside genetically engineered organisms, including bacteria or yeast.
The process functions the same as photosynthesis, in which plants naturally convert carbon dioxide and water to carbon compounds, mostly sugar and carbohydrates. The systems differ because plants have a fairly low efficiency, and typically convert less than 0.5% of solar energy to carbon compounds. On the other hand, Yang’s system is comparable to sugar cane, which is the plant that best converts CO2 to sugar with 4-5% efficiency.
One of Yang’s other projects involves working on systems to efficiently produce sugars and carbohydrates from sunlight and CO2, with the potential to provide food for the colonists on Mars.
Five years ago, Yang and his colleagues first demonstrated their nanowire-bacteria hybrid reactor and only saw a solar conversion efficiency rate of 0.04%. It was comparable to plants but still low when compared to typical efficiencies of silicon solar panels that convert light to electricity at 20%. Notably, Yang was one of the first to turn nanowires into solar panels.
At first, the researchers attempted increasing the efficiency by packing more bacteria onto the nanowires, which would then transfer electrons directly to the bacteria for the chemical reaction. But this was not successful as the bacteria separated from the nanowires, breaking the circuit.
The team then discovered that as the bugs produced acetate, they decreased the acidity of the surrounding water and made them detach from the nanowires. Yang and his students then 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 technique allowed them to pack many more bacteria into the nanowire forest, upping the efficiency nearly by a factor of 10. With this development, they were able to operate the reactor, a forest of parallel nanowires, for a week without the bacteria peeling off.
For this particular experiment, the nanowires only functioned as conductors with an external solar panel to produce energy and were not used as solar absorbers. However, in a real-world scenario, the nanowires would absorb light, generate electrons and transport them to the bacteria glommed onto the nanowires. From there, the bacteria take in electrons and converts two carbon dioxide molecules and water into acetate and oxygen, similar to the way plants make sugars.
"These silicon nanowires are essentially like an antenna: They capture the solar photon just like a solar panel," Yang said. "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 will be a side benefit on Mars, and could replenish the colonists' artificial atmosphere, mimicking Earth’s 21% oxygen environment.
Yang’s lab continues to search for even more ways to increase the efficiency of the biohybrid and is also currently exploring techniques for genetically engineering the bacteria to make them more versatile and therefore capable of producing a variety of organic compounds.
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