Saturday, April 1, 2023

Ancient bacteria could survive beneath Mars’ surface

New study finds the chances of uncovering life on Mars are better than previously expected.

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Mars has a harsh and unforgiving environment. The Red Planet appears to be uninhabitable to life due to the dry and icy temperatures, which average -80 degrees Fahrenheit (-63 degrees Celsius) at mid-latitudes. Even worse: Solar protons and powerful galactic cosmic radiation are continuously bombarding Mars.

In a groundbreaking investigation, a research team led by Brian Hoffman and Ajay Sharma of Northwestern University discovered that ancient bacteria might survive far longer than previously thought near the surface of Mars. Also, germs may survive considerably longer when buried because they are protected from solar protons and galactic cosmic radiation.

These findings strengthen the possibility that if life ever evolved on Mars, its biological remains might be revealed in future missions, including ExoMars (Rosalind Franklin rover) and the Mars Life Explorer, which will carry drills to extract materials from 2 meters below the surface.

The researchers also showed that some bacteria strains might endure the hostile climate on Mars, raising the possibility that future astronauts and space travelers may unintentionally introduce their microbes to the planet.

Michael Daly, a professor of pathology at Uniformed Services University of the Health Sciences (USU) and member of the National Academies Committee on Planetary Protection, who led the study, said, “Our model organisms serve as proxies for both forward contamination of Mars, as well as backward contamination of Earth, both of which should be avoided. Importantly, these findings have biodefense implications, too, because the threat of biological agents, such as Anthrax, remains a concern to military and homeland defense.”

Hoffman said, “We concluded that terrestrial contamination on Mars would essentially be permanent — over timeframes of thousands of years. This could complicate scientific efforts to look for Martian life. Likewise, if microbes evolved on Mars, they could be capable of surviving until the present day. That means returning Mars samples could contaminate Earth.”

nutrient agar plate
D. radiodurans growing on a nutrient agar plate. The red color is due to carotenoid pigment.

For their study, scientists begin by determining the ionizing radiation survival limits of microbial life. Then, they exposed six different kinds of Earthly bacteria and fungi to a dry, frozen simulation of Mars’ surface and blasted them with protons or gamma rays (to mimic radiation in space).

Hoffman said, “There is no flowing water or significant water in the Martian atmosphere, so cells and spores would dry out. It also is known that the surface temperature on Mars is roughly similar to dry ice, so it is indeed deeply frozen.”

In the end, the scientists concluded that some terrestrial microorganisms would be able to endure on Mars for geologic epochs of hundreds of millions of years. The scientists found that one hardy microbe, Deinococcus radiodurans, or “Conan the Bacterium,” is especially well-suited to survive the severe Mars conditions. Conan the Bacterium outlasted Bacillus spores, which may live on Earth for millions of years, by surviving huge quantities of radiation in the frigid, arid environment.

The scientists exposed samples to high doses of gamma radiation and protons, similar to what Mars would experience in the immediate subsurface, as well as much lower doses, similar to what would happen if a microorganism were buried deeply.

The accumulation of manganese antioxidants in the cells of the exposed bacteria was then measured by Hoffman’s team at Northwestern using a sophisticated spectroscopy technique. Hoffman found a correlation between the number of manganese antioxidants a microbe or its spores carry and the size of the radiation dosage it can sustain. Therefore, having more manganese antioxidants increases radiation resistance and improves lifespan.

In prior research, scientists discovered that Conan the Bacterium can withstand 25,000 units of radiation (or “greys”), or around 1.2 million years just below Mars’ surface, while held in liquid. However, the latest study discovered that the resilient Bacterium could withstand 140,000 greys of radiation when it was dried, frozen, and deeply buried—conditions that would be characteristic of a Martian climate. The human lethal dose is 28,000 times higher than this one.

Although Conan, the Bacterium, could only survive for a few hours at the surface while bathed in ultraviolet light, its lifetime improves dramatically when its shaded or located directly below Mars’ surface. Buried just 10 centimeters below the Martian surface, Conan the Bacterium’s survival period increases to 1.5 million years. And, when buried 10 meters down, the pumpkin-colored Bacterium could survive a whopping 280 million years.

Daly said, “Although D. radiodurans buried in the Martian subsurface could not survive dormant for the estimated 2 to 2.5 billion years since flowing water disappeared on Mars, such Martian environments are regularly altered and melted by meteorite impacts. We suggest that periodic melting could allow intermittent repopulation and dispersal. Also, if Martian life ever existed, even if viable lifeforms are not now present on Mars, their macromolecules and viruses would survive much, much longer. That strengthens the probability that, if life ever evolved on Mars, this will be revealed in future missions.”

Journal Reference:

  1. William H. Horne, Robert P. Volpe et al. Effects of Desiccation and Freezing on Microbial Ionizing Radiation Survivability: Considerations for Mars Sample Return. Astrobiology. DOI: 10.1089/ast.2022.0065
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