Microbial Ecology and Evolution

The Ability to Survive Gamma Radiation Determines Whether or Not Prokaryotic Bacteria Can Survive on Mars

Russian researchers from Lomonosov Moscow State University have discovered that the critical factor in bacterial survival on Mars is the organism’s ability to withstand gamma radiation.

Conditions on Mars are extreme.  Temperatures can reach-145oC at night, the atmospheric pressure is 100-1,000 times lower than that of Earth, and it receives strong ultraviolet and ionizing radiation.  Any of these factors could make microbial life impossible, but the Russian scientists wanted to figure out which ones were most important to improve future astrobiological space missions.

Since the archaea show us that extreme temperatures are not incompatible with life and deep ocean organisms have demonstrated that pressure differences can be accommodated, that left radiation.  The team studied the effect of high doses (100 kGy) of gamma radiation on prokaryote vitality and, in order to simulate Martian conditions and evaluate an organism’s ability to accommodate both low temperatures and differences in pressure, they also studied bacterial survival in ancient permafrost that has not melted in about two million years.  Permafrost is considered the terrestrial equivalent of regolith, the ground left after space weathering.  To evaluate all three extreme conditions together, they used a constant climate chamber that could maintain low temperature and low pressure while an organism was being irradiated.

After irradiation in the chamber, prokaryotic cells and metabolically active bacterial cells retrieved from permafrost remained at the control level while cells grown in culture medium decreased tenfold and metabolically active archaea decreased threefold.

Although the permafrost cells remained constant as far as numbers go and baseline samples revealed high levels of biodiversity, permafrost microbial community structure changed significantly after irradiation.  Actinobacteria populations of the genus Arthrobacter, in particular, became predominant, although they were not detected at baseline.  Previous studies have shown that these bacteria have high resistance to ultraviolet radiation.

“The results of the study indicate the possibility of prolonged cryoconservation of viable microorganisms in the Martian regolith,” said author Vladimir S. Cheptsov.  “The intensity of ionizing radiation on the surface of Mars is 0.05-0.076 Gy/year and decreases with depth.  Taking into account the intensity of radiation in the Mars regolith, [our] data makes it possible to assume that hypothetical Mars ecosystems could be conserved in an anabiotic state in the surface layer of regolith, protected from UV rays, for at least 1.3-2.0 million years, at a depth of 2 meters for no less than 3.3 million years, and at a depth of 5 meters for at least 20 million years.  The data can also assess the possibility of detecting viable microorganisms on other objects of the solar system and within small bodies in outer space.”

The research was conducted as part of the Noah’s Ark project supported by the Russian Science Foundation.

For more information, go to the October 9 issue of Extremophiles.

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