Radiation tolerance in tardigrades

Abstract

Tardigrades are invertebrates whose body length ranges from about 0.1 to 1.0 mm. Terrestrial tardigrades lose their body water almost completely and enter an ametabolic state, called as "anhydrobiosis". Anhydrobiotic tardigrades are known to tolerate extreme environmental conditions, such as high and low temperatures, high pressure and chemicals. In the present study, in order to know radio-tolerance in tardigrades, we examined the survival ability after exposure of heavy ions (⁴He²⁺; 12.5 MeV/amu, LET 16.2KeV/um) and gamma-rays in a tardigrade Milnesium tardigrdum in hydrated and anhydrobiotic states. Hydrated animals showed significantly higher LD₅₀ values (6200 Gy for heavy ions, 5000 Gy for gamma-rays) than anhydrobiotic ones (5200 Gy for heavy ions, 4400 Gy for gamma-rays) 48 hours after irradiation. It is difficult to explain why anhydrobiotic individuals are less radio-tolerant than hydrated ones, because anhydrobiotic animals with extremely low contents of water compared to hydrated ones had been expected to be less damaged by indirect action of radiation compared to hydrated ones. This finding suggests that M. tardigrdum has high ability to repair biological damages after irradiation. And, M. tardigrdum showed higher LD₅₀ values for heavy ions than those for gamma-rays. This result is interesting because high-LET heavy ions generally thought to be more detrimental to organisms than low-LET gamma-rays.