Effect of Sodium Perchlorate on the Viability of Bacillus subtilis in Conditions of Available Water Shortage

Perchlorate has been detected in the surface layer of Martian regolith and is believed to be globally distributed on the planet. Perchlorate has an inhibitory effect on microorganisms, however, due to its high hygroscopicity, at low water vapor pressure in the atmosphere, it is capable of forming solutions that remain liquid over a wide range of temperatures, creating the potential to develop and maintain cellular structures. However, perchlorate solutions cannot exist permanently in the conditions of modern Mars. At the same time, it is unknown whether perchlorate affects the survival of microorganisms after the solutions dry out, between periods of liquid water formation. We studied the effect of NaClO4 in concentrations up to 25% on the viability of Bacillus subtilis when there is a shortage of available moisture. Bacterial cells were immobilized in kaolinite, dried, mixed with sodium perchlorate and chloride, and stored at water activity <0.2 for two years. It was found that perchlorate in the absence of available water does not affect the number of viable microorganisms but leads to an increase in the potential metabolic activity of cells, which is probably a response to stress. The data obtained indicate the possibility of survival of terrestrial microorganisms between periods of formation of liquid perchlorate solutions on Mars.