In keeping with humanity’s quest to explore new worlds and boldly go where no one has gone before, leading space agencies are gearing up to send crewed missions to Mars. One of the many challenges of this mission is the production of food and other life-support consumables on site without having to import them from Earth.
In a new study, scientists at the Center of Applied Space Technology and Microgravity (ZARM), at the University of Bremen, in Germany, showed that it is possible to grow Anabaena cyanobacteria under conditions that are a compromise between conditions on the Martian surface and optimal conditions for cyanobacterial productivity.
In an article titled, “A Low-Pressure, N2/CO2 Atmosphere Is Suitable for Cyanobacterium-Based Life-Support Systems on Mars,” published in the journal Frontiers in Microbiology, scientists show Anabaena cyanobacteria can be grown using local Martian gases, water, and other nutrients, at low pressure. These findings make it much easier to develop sustainable biological life support systems for a sustainable Mars exploration program.
“Here we show that cyanobacteria can use gases available in the Martian atmosphere, at a low total pressure, as their source of carbon and nitrogen. Under these conditions, cyanobacteria kept their ability to grow in water containing only Mars-like dust and could still be used for feeding other microbes. This could help make long-term missions to Mars sustainable,” said lead author Cyprien Verseux, PhD, an astrobiologist who heads the Laboratory of Applied Space Microbiology at ZARM.
Scientists have been trying to figure out how to grow cyanobacteria to drive life support on space missions, for a long time. It is an ideal candidate because all cyanobacterial species produce oxygen through photosynthesis and some can concoct nutrients from atmospheric nitrogen, a process called nitrogen fixation.
The inhospitable Martial atmosphere poses the major difficulty. Atmospheric pressure on Mars is less than 1% of Earth’s—6 to 11 hPa, too low for water to exist as a liquid. The partial pressure of nitrogen gas—0.2 to 0.3 hPa—is too low for fixation.
Recreating an Earth-like atmosphere is not feasible. An Earth-like culture system would be too expensive and too heavy to carry the distance. “Think of a pressure cooker,” Verseux said. So, researchers explored the middle ground: an atmosphere close to Mars’s that is good enough to grow cyanobacteria.
The team built a bioreactor called Atmos (Atmosphere Tester for Mars-bound Organic Systems), in which cyanobacteria are grown in artificial atmospheres. All inputs into the bioreactor were those that can be found on Mars: gases abundant in the Martian atmosphere (nitrogen and carbon dioxide), water that could be mined from ice, and nutrients found in the dusty and rocky surface of the planet (regolith) such as phosphorus, sulfur, and calcium.
Atmos is an array of nine, sterile, one-liter vessels made of glass and steel. Each contains constantly stirring cultures that are heated, pressure-controlled, and digitally monitored.
The authors chose Anabaena sp. PCC 7938, a strain of nitrogen-fixing cyanobacteria, because preliminary tests showed that it would be particularly good at using Martian resources and at helping to grow other organisms. Species related to Anabena are edible, suitable for genetic engineering, and can survive harsh conditions.
Verseux and his colleagues grew Anabaena for 10 days in a mixture of 96% nitrogen and 4% carbon dioxide and at a pressure of 100 hPa—ten times lower than on Earth. The cyanobacteria grew as well as under ambient air.
They then tested the combination of the modified atmosphere with regolith. Because no regolith has ever been brought from Mars, they used a substrate developed by the University of Central Florida instead to create a growth medium.
The cyanobacteria grew well on the Mars substrate, and the nitrogen- and carbon dioxide-rich mixture at low pressure, albeit slower than in standard medium optimized for cyanobacteria.
But this is still a major success: while standard medium would need to be imported from Earth, regolith is ubiquitous on Mars. “We want to use as nutrients resources available on Mars, and only those,” said Verseux.
Dried Anabaena biomass was ground, suspended in sterile water, filtered, and successfully used as a substrate for growing E. coli bacteria, proving that sugars, amino acids, and other nutrients can be extracted from them to feed other bacteria, which are less hardy but tried-and-tested tools for biotechnology. For example, E. coli could be engineered more easily than Anabaena to produce some food products and medicines on Mars that Anabaena cannot.
The researchers concluded that nitrogen-fixing, oxygen-producing cyanobacteria can be efficiently grown on Mars at low pressure under controlled conditions, with exclusively local ingredients.
“We want to go from this proof-of-concept to a system that can be used on Mars efficiently,” Verseux said. Further studies will be needed to fine-tune the combination of pressure, carbon dioxide, and nitrogen optimal for growth. Other genera of cyanobacteria will need to be tested and perhaps genetically tailored for space missions. A cultivation system for Mars also needs to be designed.
“Our bioreactor, Atmos, is not the cultivation system we would use on Mars: it is meant to test, on Earth, the conditions we would provide there. But our results will help guide the design of a Martian cultivation system. For example, the lower pressure means that we can develop a more lightweight structure that is more easily freighted, as it won’t have to withstand great differences between inside and outside,” said Verseux.