Finding life in outer space is one of the great endeavors of humankind. One approach is to find motile microorganisms that can move independently, an ability that is a solid hint for life. If movement is induced by a chemical and an organism moves in response, it is known as chemotaxis.
Now, researchers in Germany have developed a new and simplified method for inducing chemotactic motility in some of Earth's smallest life forms. They published their results in Frontiers in Astronomy and Space Sciences.
"We tested three types of microbes – two bacteria and one type of archaea – and found that they all moved toward a chemical called L-serine," said Max Riekeles, a researcher at the Technical University of Berlin. "This movement, known as chemotaxis, could be a strong indicator of life and could guide future space missions looking for living organisms on Mars or other planets."
Extreme survivors
The species included in the study were chosen due to their ability to survive in extreme environments. The highly motile Bacillus subtilis, in its spore form, can survive extreme conditions and endure temperatures of up to 100°C. Pseudoalteromonas haloplanktis, which is isolated from Antarctic waters, has an aptitude for growing in colder environments, between -2.5° and 29°C. The archaeon Haloferax volcanii (H. volcanii), belongs to a group similar to bacteria but is genetically different. Its natural habitats include the Dead Sea and other highly saline environments, so it, too, is well adapted to tolerate extreme conditions.
"Bacteria and archaea are two of the oldest forms of life on Earth, but they move in different ways and evolved motility systems independently from each other," Riekeles explained. "By testing both groups, we can make life detection methods more reliable for space missions."
L-serine, the amino acid the researchers used to get these species moving, has previously been shown to trigger chemotaxis in a wide range of species from all domains of life. It is also believed to exist on Mars. If life on Mars has a similar biochemistry to life on Earth, it is plausible that L-serine could attract potential Martian microbes.
Moving microbes
The results showed that L-serine worked as an attractor for all three species. "Especially the usage of H. volcanii broadens the scope of potential life forms that can be detected using chemotaxis-based methodologies, even when it is known that some archaea possess chemotactic systems," Riekeles explained. "Since H. volcanii is thriving in extreme salty environments, it could be a good model for the kinds of life we might find on Mars."
The researchers used a simplified approach, which might make the difference between it being feasible on future space missions or not. Instead of complex equipment, they used a slide with two chambers separated by a thin membrane. Microbes are placed on one side, and the chemical L-serine is added to the other. "If the microbes are alive and able to move, they swim toward the L-serine through the membrane," Riekeles explained. "This method is easy, affordable, and doesn't require powerful computers to analyze the results."
For this method to work on a space mission, however, some adjustments to the process would be needed, the researchers said. Smaller and more robust equipment that can survive the harsh conditions of space travel and a system that can work automatically without human intervention are two of them.
Once these difficulties are overcome, microbial movement could help detect microbes that might exist in outer space, for example, in the ocean of Jupiter's moon Europa. "This approach could make life detection cheaper and faster, helping future missions achieve more with fewer resources," concluded Riekeles. "It could be a simple way to look for life on future Mars missions and a useful addition for direct motility observation techniques."
