[Stable oxygen and carbon isotopes in biological studies]

Biogenic chemical elements in the Earth biosphere are presented both by light and heavy stable isotopes (16O and 18O, 12C and 13C). Because of inequality of these isotopes the light ones dominate in the exchange processes between the animate and inanimate nature and this leads to changes in the ratios of isotope modifications in the environment. A comparatively constant isotopic composition of the Earth atmosphere is primarily maintained by water. It is the heavy carbon isotope that inputs to the most isotope-heavy compounds such as marine carbonates within the atmosphere--hydrosphere system. As in nature, biogenic isotopes fractionate inside hermetically sealed habitats with the regenerative life-support systems; however, the mechanism of relative stability of the isotope composition is unknown. The problem of providing the best isotopic composition of biogenic chemical elements aboard manned space vehicles was first raised by Yu.Ye.Sinyak and A.I. Grigoriev in 1996. The review contains the data on the levels of oxygen isotopes and photosynthetically generated oxygen in water pools in different regions of the Earth. discussion of the ratios of the oxygen and carbon isotopes in the atmospheric carbon dioxide, and the range of isotopes variations and presumed "sites" of isotopes fractionating in cellular structures of autotrophs. The wealth of literary findings effectuates the conclusion that artificial environments for humans should reproduce the natural relationships and variations of biogenic isotopes. Yet, what biogenic isotopes ratios are desirable aboard manned space vehicles can be determined in extensive investigations of the effects of various modifications of stable isotopes on humans with due account for the unfavourable factors of space flight.