Viability of the soddy–podzolic soil microbial community after 148–1250 kGy gamma irradiation

Abstract

Ionizing radiation is considered the main factor limiting preservation of putative microorganisms and biomarkers in cryogenic space bodies, restricting interplanetary transportation of living cells. Defining the radioresistance limits of Earth-bound life is challenging but necessary to assessing the likelihood of the existence of life outside of Earth for the purpose of planning space missions and developing methods for detecting life. The viability of native soil microbial communities under effects of ionizing radiation has not been sufficiently investigated. According to previous studies, soil should be sterilized with 15–100 kGy doses of gamma radiation, depending on the irradiation conditions. However, the limits of radioresistance of the cells in situ have not been reliably elucidated. We performed gamma irradiation of soddy–podzolic soil (Moscow region, Russia) at doses ranging from 148 kGy to 1.25 MGy to assess the radioresistance of the soil microbial community by means of culturing, epifluorescence microscopy (EFM), multisubstrate testing, and gas chromatography–mass spectrometry of lipid biomarkers. The soil microbial community exhibited high resistance to the effects of gamma radiation. Culturable bacteria were isolated after exposure to 148 kGy. After exposure to 320 kGy, there were no cells capable of proliferation on nutrient media, although many prokaryotic cells (∼3 × 106 cells/g), without visible signs of DNA damage, were observed in situ using EFM. These data suggest a transition into a viable but nonculturable state. Thus, the results obtained indicate that microbial communities radioresistance (and microorganisms in general) in natural habitats is significantly underestimated.