The Search for Life is focused on the search for biosignatures. Planetary life leaves a chemical fingerprint on a planet’s atmosphere, and scientists are trying to work out which chemicals in what combinations and amounts are a surefire indicator of life. Martian methane is one they’re puzzling over right now.
But new evidence suggests that super-tiny amounts of DNA can be detected in Martian rocks if it’s there. And though it requires physical samples rather than remote sensing, it’s still an intriguing development.
DNA is the gold standard for biosignatures. “DNA is an incontrovertible biosignature whose sequencing aids in species identification, genome functionality, and evolutionary relationships,” a new paper states. Advances in DNA have led to all kinds of leaps in industry, medicine, paleontology, and even criminal justice. Now, it looks like the search for life might receive similar benefits.
The new paper is “DNA sequencing at the picogram level to investigate life on Mars and Earth.” It’s published in Nature Scientific Reports, and the lead author is Jyothi Basapathi Raghavendra. Raghavendra is a Ph.D. researcher in the Department of Planetary Sciences in the School of Geosciences at the University of Aberdeen.
“There is a slim chance that microbial life exists on Mars today, but to find it, we need to operate at the sample scale, and that’s where the size and power of the hardware that’s used in space exploration is a crucial factor,” said study co-author Javier Martin-Torres.
If this technology can be brought to bear on Martian soil samples, it could be a game-changer.
“Investigating active life forms in extremely low biomass environments is a topic of interest for expanding our knowledge of Earth’s biodiversity and the search for life on Mars,” the authors write.
Low biomass environments are samples with a very small number of desirable cells. The tiny amount of material makes them more difficult to study, and they present problems to researchers because they’re more easily contaminated. With such a tiny amount of genetic material, it’s also harder to accurately amplify them without errors. “Hence, for the study of low concentrations of DNA, there is a need for new technologies with improved efficiency, sensitivity, and specificity,” the authors write.
The new method is called nanopore technology. A nanopore is simply a really, really tiny hole. Basically, researchers pass an electric current through the nanopore, and if something—a strand of DNA, in this case—passes through the pore, it changes the current. Different changes tell scientists different things about the DNA. Group a couple thousand of these nanopores into one tool, and you’re really onto something.
A company named Oxford Nanopore Technologies developed a tool named MinION to sequence DNA in this way. They say they can sequence any fragment of DNA or RNA from short to ultra-long. They can also do it with as little as two picograms of material. (A picogram is one-trillionth of a gram.) “This result is an excellent advancement in sensitivity, immediately applicable to investigating low biomass samples,” the authors of the study write.
The device is remarkably compact.
Scientists often turn to analogue Martian environments here on Earth to test ideas and equipment. One of them is the Atacama Desert in Chile, “one of the best Martian analogue environments due to its extreme aridity,” according to the authors. On Mars, the Perseverance Rover is collecting samples for return to Earth. The Mars Sample Return (MSR) mission is a few years away, but scientists are already preparing by testing things like DNA detection in soil samples from Atacama.
“In preparation for this MSR research, in this work, we investigate the sensitivity of MinION sequencer to detect extremely low concentrations of DNA that could also be present in a regolith sample or a crushed rock at the level of picogram,” the authors explain.
The team set out to define new limits of concentration for DNA sequencing. The results could be applied to diverse fields, but the focus is on the eventual return of Mars Samples to Earth. There’s an assumption that any Mars life will use DNA just as Earth life does. In other words, “it relies on the same chemical processes as terrestrial organisms, and it codes its genetic information with the known bases that are ubiquitously used by life on Earth,” the authors explain.
The team incubated the trays, extracted samples, and analyzed them. The results were promising. “In this work, we successfully detected E. coli and S. cerevisiae DNA even at the level of 10 pg. (picograms.)” That’s extraordinarily sensitive. In fact, the tool’s sensitivity was revealed by some cross-contamination. “The negative control and the test samples detected traces of human DNA,” the paper states. The lab work was performed in an ISO 5 clean room, so any contamination was extremely minor, yet MinION picked it up.
The research team pushed MinION even further. They diluted some samples even further and tested them with the device. The results were even more impressive. “We ran two replicates per sample type, and the MinION sequencer successfully detected the microbial taxa at 2 pg in both experiments,” the paper states. “This is the lowest limit detected so far.”
So where do things stand? “Our study shows that MinION sequencer can unequivocally detect and characterize species with as little as 2 pg of DNA with just 50 active nanopores,” the team writes. That’s without amplification, a step that can introduce more errors in low-biomass samples.
This is a significant leap in sensitivity and bodes well for the eventual study of the Mars Samples. Finding as little as two picograms of DNA mass in a soil sample is remarkable.
It not only means that it can identify life forms in samples with extremely low biomasses, but it also means it can detect ambient contamination from Earth.
But the study goes even further. If dormant Mars organisms exist in the samples, they could be easily revived. “We suggest that a Martian soil, without any added nutrients, could support the growth or revival of microorganisms by mere exposure to atmospheric moisture with water activity below 0.85,” the researchers say in their conclusion. So exposure to Earth’s ambient moisture could be enough to bring them back to life.
What’s more, the system could be used to test how soil organisms grow under the extreme environmental conditions on Mars. The researchers say that future work could investigate how soil microorganisms grow in Mars’ low temperatures, ionizing radiation, salts, and oxidating radicals, which are all barriers to life.
This research shows how powerful nanopore technology is at detecting minuscule amounts of DNA. But it can be improved even more, and that’s something Oxford Nanopore Technologies (ONT) is working on.
Clive Brown is the Chief Technology Officer at ONT. In an interview, Brown said, “Sequencing technology could be adapted for extreme applications such as Mars — and beyond — providing the tools needed to study the extra-terrestrial samples. We aim to push the technology even further for when the Mars Sample Return mission returns in 2033.”