Kinetic Isotope Effects of RuBisCO carbon, oxygen, and/or hydrogen, though currently only a significant isotope effect is seen for carbon isotope substitution. Isotopes are atoms that have the same number of protons but varying numbers of neutrons. "Lighter" isotopes (like the stable Carbon-12 isotope) have a smaller overall mass, and "heavier" isotopes (like the stable Carbon-13 isotope or radioactive Carbon-14 isotope) have a larger overall mass. Stable isotope geochemistry is concerned with how varying chemical and physical processes preferentially enrich or deplete stable isotopes. Enzymes like RuBisCO cause isotopic fractionation because molecules containing lighter isotopes have higher zero-point energies (ZPE), the lowest possible quantum energy
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