An important component of the environmental transport and distribution of elements is their cycling through biological compartments of ecosystems. This is also one of the most difficult processes to study. Even for the most heavily studied elements in biological compartments ( i.e., carbon, nitrogen, or phosphorus), the research has been painstaking, taxed the ingenuity of investigators, pushed to the limits the state-of-art of instrumental and chemical methods, and raised more research questions than it answered. Because the cycling of elements integrates organismal chemistry and biology, this subject plays a role in predicting the effects of elements on organisms, biological communities, and ecosystems.

The cycling of elements through biological compartments does not "theoretically" depend on whether a nuclide is stable or radioactive. A radioactive isotope cycles the same way as the stable isotope of the element. Thus, many studies on how elements cycle use radioactive isotopes as tracers. The quotation marks around theoretically suggest that perhaps the cycling of elements does sometimes depend on some particular property of an isotope, although it is unclear that nuclear stability is the property of interest. Differences in biological properties occur among isotopes of elements of low atomic weight. The substitution of deuterium ( ² H) for hydrogen in some molecules changes the physicochemical and biological properties of the molecule. Deuterium often increases the toxicity of a compound. Likewise tritium ( ³ H) also when substituted for hydrogen in molecules changes physical, chemical, and biological properties of the molecule.

The radioactive decay of a nuclide can produce a new nuclide of a different element. When that occurs, the new nuclide begins to cycle according to the chemical identity of the new element.