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
Hydrogen isotope biogeochemistry reconstructions. Since water is the primary hydrogen source for photoautotrophs, the hydrogen isotope composition of their biomass can be related to the composition of their growth water and thereby used to gain insight into some properties of ancient environments. Studying hydrogen isotopes can be very valuable, as hydrogen is more directly related to climate than other relevant stable isotope systems. However, hydrogen atoms bonded to oxygen, nitrogen, or sulfur atoms are exchangeable with environmental hydrogen, which makes this system less straightforward [ref to earlier H exchange section]. To study the hydrogen isotope composition of biomolecules, it is preferable to use