Stable-isotope probing is a technique in microbial ecology for tracing fluxes of nutrients in biogeochemical cyling by microorganisms. A heavier stable isotope is can be enriched in a substrate that is consumed by the organisms to be studied. Biomarkers with the heavier isotope incorporated into them can be separated from biomarkers containing the more naturally abundant lighter isotope by isopycnic centrifugation. For example, 13CO2 can be used to find out which organisms are actively photosynthesizing or consuming new photosynthate. As the biomarker, DNA with 13C is then separated from DNA with 12C by centrifugation. Sequencing the DNA identifies which organisms were consuming existing carbohydrates and which were using carbohydrates more recently produced from photosynthesis.[1]