Abstract
Ionizing radiation is one of the main factors limiting the survival of microorganisms in extraterrestrial conditions. The survivability of microorganisms under irradiation depends significantly on the conditions, in which the irradiation occurs. In particular, temperature, pressure, oxygen and water concentrations are of great influence. However, the influence of factors such as the radiation intensity (in low-temperature conditions) and the type of mineral matrix, in which microorganisms are located, has been practically unstudied. It has been shown that the radioresistance of bacteria can increase after their exposure to sublethal doses and subsequent repair of damage under favorable conditions, however, such studies are also few and the influence of other factors of extraterrestrial space (temperature, pressure) was not studied in them. The viability of bacteria Arthrobacter polychromogenes, Kocuria rosea and Xanthomonas sp. after irradiation with gamma radiation at a dose of 1 kGy under conditions of low pressure (1 Torr) and low temperature (−50 °C) at different radiation intensities (4 vs. 0.8 kGy/h) with immobilization of bacteria on various mineral matrices (montmorillonite vs. analogue of lunar dust) has been studied. Native, previously non-irradiated strains, and strains that were previously irradiated with gamma radiation and subjected to 10 passages of cultivation on solid media were irradiated. The number of survived cells was determined by culturing on a solid medium. It has been shown that the radioresistance of bacteria depends significantly on the type of mineral matrix, on which they are immobilized, wherein montmorillonite contributes to an increased survivability in comparison with a silicate matrix. Survivability of the studied bacteria was found to increase with decreasing radiation intensity, despite the impossibility of active reparation processes under experimental conditions. Considering the low intensity of radiation on various space objects in comparison with radiobiological experiments, this suggests a longer preservation of the viable microorganisms outside the Earth than is commonly believed. An increase in bacterial radioresistance was revealed even after one cycle of irradiation of the strains and their subsequent cultivation under favourable conditions. This indicates the possibility of hypothetical microorganisms on Mars increasing their radioresistance.
Highlights
It has been shown that the radioresistance of bacteria depends significantly on the type of mineral matrix, on which they are immobilized, wherein montmorillonite contributes to an increased survivability in comparison with a silicate matrix
The assessment of the radioresistance of microorganisms makes it possible to estimate the potential duration of their cryopreservation in a viable state on various space objects, and, to select promising space bodies and their regions, as well as rocks and depths for sampling during space missions in order to study the potential habitability of these space bodies [6,9]
The observed complete preservation of CFU numbers of bacteria after exposure to low pressure and low temperature is similar with the previous results on the absence of a significant effect of 0.01 Torr pressure and −130 ◦ C temperature on the viability of bacteria immobilized in montmorillonite [17]
Summary
The assessment of the radioresistance of microorganisms makes it possible to estimate the potential duration of their cryopreservation in a viable state on various space objects, and, to select promising space bodies and their regions, as well as rocks and depths for sampling during space missions in order to study the potential habitability of these space bodies [6,9]. The concept of radioresistance of terrestrial microorganisms makes it possible to interpret the data obtained during space missions from the perspective of potential habitability of alien bodies in the past and to correct existing plans for colonization of extraterrestrial territories [10,11,12,13]
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