The staff of the Faculty of Biology of Lomonosov Moscow State University simulated the conditions of an increased radiation background in combination with low temperatures, close to the Martian ones, and studied the resistance of microorganisms to them. It turned out that some bacteria and archaea living in ancient Arctic frozen rocks can exist in such conditions for up to 20 million years in an inactive state.
The average temperature on Mars is -63 ° C, but at the poles at night it can drop to -145 ° C. Until now, it was not known what the limits of the resistance of microorganisms to such extreme factors are. Using these limits, scientists can assess the potential for the preservation of microorganisms and biomarkers in the composition of various objects in the solar system. This information is necessary to plan astrobiological space missions, for which it is important to carefully approach the selection of objects and regions of study and the development of methods for detecting life.
How microbes survive on Mars
In this work, the authors investigated the radioresistance of microbial communities in permafrost sedimentary rocks under conditions of low temperature and low pressure. These rocks are considered to be the terrestrial analogue of regolith - residual soil after space weathering. Scientists suggest that the potential biosphere of Mars can be preserved in a cryopreserved state and the main factor limiting the duration of its preservation is the accumulation of radiation damage by cells. Determination of the radioresistance limit of microorganisms will make it possible to estimate the duration of the retention of microorganisms in the regolith, including at different depths.
“We have investigated the combined effect of a number of physical factors (gamma radiation, low pressure, low temperature) on the microbial communities of ancient Arctic frozen sedimentary rocks. A unique natural object has been investigated - ancient frozen rocks that have not thawed for about two million years. In general, we carried out a model experiment that more fully reproduces the conditions of cryopreservation in the Mars regolith. It is also important that the study investigated the effect of high doses of gamma radiation (100 kGy) on the viability of prokaryotes, while previously living prokaryotes were not detected when irradiated with doses higher than 80 kGy, said one of the authors of the article, Vladimir Cheptsov, postgraduate student of the Department of Biology Soils of the Faculty of Biology, Moscow State University Lomonosov. The study was supported by the Russian Science Foundation (RSF) within the framework of the Noah's Ark project,his results are published in the journal Extremophiles.
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When simulating the impact of factors on organisms, scientists used an original climatic chamber, which allows maintaining low pressure and low temperature during gamma irradiation. The authors note that natural microbial communities, rather than pure cultures of microorganisms, were used as a model object.
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The studied microbial communities have shown high resistance to the effects of the simulated conditions of the Martian environment. After irradiation, the total number of prokaryotic cells and the number of metabolically active bacterial cells remained at the control level, the number of cultured bacteria (bacteria that grow on nutrient media) decreased tenfold, and the number of metabolically active archaeal cells decreased threefold. Moreover, the decrease in the number of cultured cells in the experiment was caused by a change in their physiological state, and not by death.
Cryopreservation: how to preserve life in ice
In an irradiated sample of permafrost, scientists found a high diversity of bacteria, although after irradiation, the structure of the microbial community changed significantly. In particular, populations of actinobacteria of the genus Arthrobacter, which were not detected in control samples, began to prevail in bacterial communities after exposure to model conditions. This was probably caused by a slight decrease in the number of cells of the dominant populations of bacteria, as a result of which scientists were able to detect actinobacteria of the genus Arthrobacter. The authors suggest that bacteria of this genus are more resistant to the effects of the studied conditions. There were also other studies, during which scientists proved that these bacteria exhibit a fairly high resistance to the effects of ultraviolet radiation and radiation,and their DNA is well preserved in ancient frozen sedimentary rocks for millions of years.
“The results of the study indicate the possibility of long-term cryopreservation of viable microorganisms in the Martian regolith. 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 regolith of Mars, our data suggest that hypothetical ecosystems of Mars are preserved in anabiotic state in the surface layer of regolith (protected from UV rays) for at least 1.3-2 million years, at a depth of two meters - at least 3.3 million years, at a depth of five meters - at least 20 million years. The data obtained can also be used to assess the possibility of detecting viable microorganisms on other objects in the solar system and inside small bodies in outer space,”the scientist added.
Conclusion
The authors were the first to prove the possibility of survival of prokaryotes when exposed to ionizing radiation at doses above 80 kGy. The data obtained indicate both a possible underestimation of the radioresistance of natural microbial communities and the need to study the synergistic effect of a combination of alien and space factors on living organisms and biomolecules in astrobiological model experiments.
The work was carried out in collaboration with scientists from the Space Research Institute of the Russian Academy of Sciences, the A. F. Ioffe RAS, Peter the Great St. Petersburg Polytechnic University, Ural Federal University and the B. P. Konstantinov of the National Research Center "Kurchatov Institute". The study was supported by a grant from the Russian Science Foundation "Scientific Foundations of the Creation of a National Depository Bank for Living Systems" (project "Noah's Ark").
Vasily Makarov