ИСТИНА

The knowledge about limits of resistance of microbial communities to impact of ionizing radiation is still scarce. According to different authors, sterilizing doses vary from 15 to more than 50 kGy. However, in recent years, it has been shown that Cryomyces antarcticus fungi remain viable after exposure to gamma radiation at doses up to 117 kGy, and microbial communities of extreme habitats withstand exposure to the doses up to 1 MGy (if irradiated under low pressure and low temperatures). It confirms necessity of further investigation of the limits of microorganisms’ radioresistance. We irradiated ancient Arctic permafrost by gamma rays (60Co source) with dose gradient 159, 343, 463, 743, 1000, 1350 kGy in conditions close to normal (+ 16°C, 1 atm.) in order to estimate the limit of resistance of its microbial community to gamma radiation. Culturable bacteria were found in samples irradiated by doses of up to 463 kGy. Amongst them, representatives of the genera Microbacterium, Kocuria, Micrococcus, Tsukamurella, Dietzia (all related to Actinomycetales) and Sporosarcina (related to Bacillales) were identified. With increasing of doses, the number of CFUs decreased more rapidly than the total number of cells determined by epifluorescence microscopy. It allows to assume that part of the cells have entered to the viable but nonculturable state. Even at the minimal dose (159 kGy) the potential metabolic activity of the microbial community as shown by multisubstrate testing (MST) was drastically reduced and as was noted the only one substrate (peptone) was utilized. After irradiation with higher doses, metabolic activity by the MST method was not detected at all. In general, studied microbial community showed high resistance to gamma radiation, which is beyond the scope of existing knowledge. The obtained data confirm that the radioresistance of microorganisms being entrapped into the complex natural substrates is significantly underestimated.