Stable-isotope probing (SIP) is a technique in microbial ecology for tracing uptake of nutrients in biogeochemical cycling by microorganisms.
A substrate is enriched with a heavier stable isotope that is consumed by the organisms to be studied.
The density shift is proportional to the change in density in the DNA, which depends on the difference in mass between the rare and common isotopes for a given element, and on the abundance of elements in the DNA.
Larger buoyant density shifts are observed when multiple isotope tracers are used.
[5] Because density shifts as a predictable function of the change in mass caused by isotope assimilation, stable isotope probing can be modeled to estimate the amount of isotope incorporation, an approach called quantitative stable isotope probing (qSIP),[6] which has been applied to microbial communities in soils,[7] marine sediments,[8] and decomposing leaves[9] to compare rates of growth and substrate assimilation among different microbial taxa.