Movement is often one of the clearest signs of whether something is alive or not.
Taking advantage of this, a new test which prompts microbes to move could be adapted into a novel way to hunt for life on other planets.
The test uses a chemical called L-serine, which is one of the amino acids on which all Earth life is based.
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Scientists placed this chemical next to three different organisms and found that they all moved towards the L-serine.
"This movement, known as chemotaxis, could be a strong indicator of life and could guide space missions looking for living organisms on Mars or other planets," says Max Riekeles of the Technical University of Berlin, who led the study.
Testing L-serine for extra-terrestrial life
L-serine is known to inspire movement in a wide variety of organisms, but for this test the team chose three extremophiles – microbes well used to surviving in extreme conditions.
Two were bacteria: Bacillus subtilis, which can survive up to 100°C (212°F), and Pseudoalteromonas haloplanktis, which thrives even at sub-zero temperatures.
The third microbe was Haloferax volcanii, an archaea, which are superficially similar to bacteria but evolutionarily very distinct in many key ways.
This particular archaea typically lives in high-salinity environments, similar to what is found on Mars.
"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," says Riekeles.
"By testing both groups, we can make life-detection methods more reliable for space missions. Since Haloferax volcanii thrives in extreme salty environments, it could be a good model for the kinds of life we might find on Mars."
How it works
The test involves a simple method to look for signs of chemotaxis, using a special slide featuring two chambers, separated by a semi-permeable membrane.
The microbes were placed on one side, the amino acid on the other and then observed under a microscope.
"If the microbes are alive and able to move, they swim toward the L-serine through the membrane," says Riekeles.
"This approach could make life detection cheaper and faster, helping future missions achieve more with fewer resources. It could be a simple way to look for life on future Mars missions."
