• Dirk Schulze-Makuch

A Promising Method for Life-Detection

Microbial Motility

Imaging microbes in sediments would be done similarly in Martian soil. The large particles are sediment grains, while the small clustered ones are microbes.

The detection of life on neighboring planetary bodies is one of the most important goals of astrobiology. There are various ways in which we can attempt to detect extraterrestrial microbial life. In a round table discussion a few years ago, the point was made that no single methodology would be sufficient in proving the existence of extraterrestrial life. Instead, we should utilize several methods that are independent of each other and based on different characteristics of life.

But which methods should be used? One of the methods my research group uses is microbial motility, which is the characteristic movement pattern of microbes. We use automated algorithms for tracking their movements in liquid water. Our first success was when we were able to distinguish microbial motility from the random movement of inorganic particles with an accuracy larger than 99%, using machine learning algorithms. The random movement of minuscule particles occurs when particles collide with each other, which occurs statistically more often with an increase in temperature (the process is called Brownian motion). However, while we were successful in distinguishing microbial movement from the abiotic background, the algorithms were less successful in distinguishing the various tested microbes from each other. The accuracy of that automated identification rate was less than 82%. We published our preliminary results in the journal Life last year.

Currently, we are conducting field testing in Trinidad using self-built instrumentation. The goal is to detect microbial life in contaminated water and in liquid slurs emanating from mud volcanoes. One “Earth-bound” practical aspect of this research is identifying pathogens in water, such as the cholera bacterium, which has a specific motility pattern. However, a difficulty we often encounter is that microbes tend to be stationary in the water, clinging to sediment, and don´t exhibit any self-propelled movement. In our continued research, we will try to motivate them via positive stimuli like nutrients or specific amino acids and negative stimuli, like compounds that are toxic to microbial species. We will also have to keep in mind that different microbes react differently to environmental conditions or changes (such as temperature and light changes, or changes in the magnetic field). So, those are variables we need to look into as well. There is still lots of work to do.

Nevertheless, in principle, this methodology is promising. It could eventually be included as one of the suitable technologies in a life-detection mission to Mars or even the icy moons of the outer Solar System. It would be independent of other methodologies that may lean on identifying organic building blocks of life or that try to observe the metabolic interactions of putative extraterrestrial microbes with their environment. One method that could be paired with ours for a more reliable outcome is taking into account the morphology (size and shape) of the particles. This approach will benefit us greatly if we take a suitable microscope on a life-detection mission that has a resolution in the micrometer scale to track AND view putative microbial life.

Whatever methods are eventually used on a life-detection mission, we’re long overdue in undertaking one. Now we have so much more knowledge of the environmental conditions on Mars, and our methods for life-detection are much more sophisticated than the Viking life-detection mission in the late 1970s (which came out to be inconclusive). When we attempt a life-detection mission with instrumentation on Mars, it would be sensible to take sterile water with us to make a solution and observe any resulting movement in the solution microscopically. The icy moons are another promising target, and we would not even have to bring water with us. We would just need to melt some water ice and analyze it!

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