A promising (and surprisingly simple) way to detect alien life

A central question for astrobiologists — researchers who, like me, explore the possibility of extraterrestrial life — seems deceptively simple: What’s the best way to detect and identify life?

So far, we’ve only managed to launch a single spacecraft mission dedicated to this problem, the Viking landers of the 1970s. The results of Viking’s biology experiments were generally thought to be inconclusive, and their interpretation remains controversial even today. What has become clear in the 50 years since, however, is that new methods are needed that can more definitively distinguish between soil samples with living and non-living content. One of the most promising of these methods is based on the motility of microbes — their ability to move in a liquid medium. 

Motility

Motility was actually suggested as a “biosignature” as early as the 1960s, but back then, the technology wasn’t advanced enough for automated microscopic observations on a Mars lander. Today’s much more powerful micro-computers make this an option not only on Mars but also on the icy moons of the outer Solar System.

Work conducted in my lab to advance motility as a method of life detection is headed by Max Riekeles, an engineer who applies algorithms to track the movement of microbes in liquid water. Using machine learning algorithms, we were able to distinguish microbial motility from the random movement of inorganic particles (such as sediment grains) with an accuracy greater than 99%. (That random movement, called Brownian motion, results when particles collide with each other, which happens more often as temperatures rise.)

We’ve followed up on this research in the last few years by exposing microbes to Mars-like conditions while using incentives to initiate movement. In a new paper, we report how the characteristic movement patterns of E. coli bacteria changed when exposed to various extreme environments, particularly high-salt solutions of the kind found on Mars.

Interestingly, we observed a short-lived increase in bacterial movement at certain salt concentrations, particularly with sodium chlorate and sodium perchlorate, which are toxic to cells. We interpret this as the organism’s effort to move away from stressfully high salt concentrations. Given that bacteria were already known to navigate harsh and changing environments, this holds promise as a key biosignature for pinpointing life on Mars.

There’s a catch, though. It can be difficult to get microbes to start moving away from stressors in the first place. They often like to sit around waiting for better times, meaning a return to benign environmental conditions with plenty of food. To work around this tendency, our group experimented with stressing the microorganisms while also inducing movement, which can be done by stimulating them with light, electric or magnetic fields, or certain chemicals. We used the amino acid L-serine as bait and found that all tested organisms — two different bacteria (Bacillus subtilis and Pseudoalteromonas haloplanktis) and one archaeon (Haloferax volcanii) — would move toward the L-serine. We think this kind of approach would be well-suited to a life-detection experiment on another planet because the setup is very small and easy to include on a spacecraft.

One big problem for a life-detection mission is finding a spot that’s accessible to a lander but where liquid water might also exist near the surface. We could get around this by taking water with us on a lander mission, exposing the Martian soil to a drop or two, and then seeing whether microbes move around in the liquid. Otherwise, we have to identify potential habitats where water already is present. These might include brines that only exist at very low temperatures and for short durations, or environments with sodium chloride-rich salt rocks that allow microbes to draw life-sustaining water from the atmosphere. The Southern Highlands of Mars would meet these conditions. A third possibility would be to land in topographically low places such as the floor of Valles Marineris or inside caves, where atmospheric pressures are sufficient to support liquid (salty) water.

While we think our approach could help distinguish life from non-life on Mars, we’ve been less successful in distinguishing various tested microbes from one another. Here we’ve only achieved an accuracy of 82% so far. Some microbes are easier to identify than others, however — specifically pathogens, because they move very fast and thrive in certain preferred temperatures.

Earthly applications

That leads to another possible application. We were able to simulate the movements of cholera bacteria in water and identify them by their characteristic movement patterns. Water-born pathogens like cholera are a huge health problem, particularly in developing countries, causing the deaths of more than three million people every year.  We are in the process of developing instruments to detect cholera bacteria in water and hope to expand the technique to other pathogens such as Salmonella and Legionella. Future applications might even be able to detect pathogens in other liquids such as blood.  

Once again, a technology developed for space exploration could have great value here on Earth.