Isotope persisted down to the present because their rate of decay is so slow (e.g. uranium-238 and potassium-40). Post-primordial isotopes were created by cosmic ray bombardment as cosmogenic nuclides (e.g., tritium, carbon-14), or by the decay of a radioactive primordial isotope to a radioactive radiogenic nuclide daughter (e.g. uranium to radium). A few isotopes are naturally synthesized as nucleogenic nuclides, by some other natural nuclear reaction, such as when neutrons from natural nuclear fission are absorbed by another atom. As discussed above, only 80 elements have any stable isotopes, and 26 of these have only one stable isotope. Thus, about two-thirds
Stable-isotope probing organisms were consuming existing carbohydrates and which were using carbohydrates more recently produced from photosynthesis. When DNA is the biomarker, SIP theoretically can be performed using isotopically labeled C, H, O or N, though C is used most often. Cupples et al. (2007) demonstrated a weaker DNA buoyant density shift in when N- versus C-labeled substrates were used in pure culture, conversely a very strong buoyant density shift was observed when both labels were used. Stable-isotope probing Stable-isotope probing (SIP) is a technique in microbial ecology for tracing fluxes of nutrients in biogeochemical cycling by microorganisms. A substrate is enriched