When the first samples from Mars return to Earth, scientists should be on the lookout for ancient sleeping bacteria, according to a new study.
In a groundbreaking study, a research team, including Brian Hoffman and Ajay Sharma of Northwestern University, found that ancient bacteria could survive near the surface of Mars much longer than previously thought. And – when bacteria are buried and therefore protected from galactic cosmic radiation and solar protons – they can survive much longer.
These findings strengthen the possibility that if life ever evolved on Mars, its biological remains could be revealed in future missions, including ExoMars (Rosalind Franklin rover) and Mars Life Explorer, which will carry drills to extract materials 2 meters below the surface. surface.
And because scientists have proven that certain strains of bacteria can survive despite Mars’ harsh environment, future astronauts and space tourists could inadvertently contaminate Mars with their own hitchhiking bacteria.
The article will be published Tuesday, October 25 in the journal Astrobiology.
“Our model organisms serve as a proxy for both direct contamination from Mars and retrograde contamination from Earth, both of which should be avoided,” said Michael Daly, professor of pathology at the Uniformed Services University of the Health. Sciences (USU) and member of the National Academies Committee on Planetary Protection, who led the study. “It is important to note that these findings also have biodefense implications, as the threat of biological agents, such as Anthrax, remains a concern for military and homeland defense.”
“We concluded that terrestrial contamination on Mars would be essentially permanent – over time periods of thousands of years,” said Hoffman, the study’s co-lead author. “This could complicate scientific efforts to search for Martian life. Likewise, if microbes evolved on Mars, they might be able to survive to the present day. This means that returning samples from Mars could contaminate Earth.
Hoffman is the Charles E. and Emma H. Morrison Professor of Chemistry and Professor of Molecular Biosciences at Northwestern’s Weinberg College of Arts and Sciences. He is also a member of the Chemistry of Life Processes Institute.
Simulate Mars
The environment on Mars is harsh and unforgiving. The arid and frigid conditions, which average -80 degrees Fahrenheit (-63 degrees Celsius) at mid-latitudes, make the Red Planet seem inhospitable to life. Worse still: Mars is also constantly bombarded by intense galactic cosmic radiation and solar protons.
To determine whether or not life could survive under these conditions, Daly, Hoffman and their collaborators first determined the ionizing radiation survival limits of microbial life. Next, they exposed six types of Earth bacteria and fungi to a simulated Martian surface – which is frozen and dry – and zapped them with gamma rays or protons (to mimic radiation in space).
“There is no running water or significant water in the Martian atmosphere, so cells and spores dry out,” Hoffman said. “The surface temperature on Mars is also known to be roughly similar to dry ice, so it is indeed deeply frozen.”
Ultimately, the researchers determined that certain terrestrial microorganisms could potentially survive on Mars on geologic time scales of hundreds of millions of years. In fact, researchers have discovered that a hardy microbe, Deinococcus radiodurans (affectionately nicknamed “Conan the Bacteria”), is uniquely adapted to survive the harsh conditions of Mars. In the new experiments, Conan the bacterium survived astronomical amounts of radiation in the freezing, arid environment – far exceeding Bacillus spores, which can survive on Earth for millions of years.
radical radiation
To test the effects of the radiation, the team exposed samples to high doses of gamma radiation and protons – typical of what Mars receives in the near subsurface – and to much lower doses, which would occur if a microorganism was deeply buried.
Next, Hoffman’s team at Northwestern used an advanced spectroscopy technique to measure the accumulation of manganese-based antioxidants in the cells of the irradiated microorganisms. According to Hoffman, the size of the radiation dose that a microorganism or its spores can survive correlates with the amount of manganese-based antioxidants it contains. Therefore, more manganese-based antioxidants mean more radiation resistance – and better survival.
In earlier studies, previous researchers found that Conan the bacterium, when suspended in liquid, can survive 25,000 radiation (or “gray”) units, the equivalent of about 1.2 million years just below the surface of Mars. But the new study found that when the hardy bacterium is dried, frozen and deeply buried – which would be typical of a Martian environment – it could withstand 140,000 grays of radiation. This dose is 28,000 times greater than what would kill a human.
Although Conan the Bacteria can only survive a few hours on the surface when bathed in ultraviolet light, its lifespan improves dramatically when shaded or located directly below the surface of Mars. Buried just 10 centimeters below the Martian surface, the survival period of Conan the bacterium increases to 1.5 million years. And, once buried 10 meters deep, the pumpkin-colored bacterium could survive for 280 million years.
For future assignments
This amazing survival feat is partly due to the bacteria’s genomic structure, the researchers found. Long suspected, researchers have discovered that the chromosomes and plasmids of Conan the bacterium are linked together, keeping them in perfect alignment and ready to be repaired after intense radiation.
This means that if a microbe, similar to Conan the bacterium, evolved at a time when water last flowed on Mars, its living remains could still be dormant in the deep underground.
“Although the D. radiodurans buried in the Martian subsoil could not survive in dormancy for the estimated 2 to 2.5 billion years since the disappearance of running water on Mars, these Martian environments are regularly weathered and melted by meteor impacts,” Daly said. “We suggest that periodic melting could allow intermittent restocking and dispersal. Moreover, if Martian life existed, even if viable life forms are not present on Mars, their macromolecules and viruses would survive much, much longer. This reinforces the likelihood that, if life ever evolved on Mars, it would reveal itself in future missions.
The study, “Effects of Desiccation and Freezing on Survivability of Microbial Ionizing Radiation: Considerations for Returning Samples to Mars,” was supported by the Defense Threat Reduction Agency (grant number HDTRA1620354) and the National Institutes of Health (grant number GM111097).
Effects of desiccation and freezing on microbial ionizing radiation survivability: considerations for returning samples to Mars, astrobiology
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