https://phys.org/news/2018-05-discern-microbes.html

The basis for connecting microbe and substrate are so-called carbon stable isotope ratios—the ratios between naturally occurring forms of carbon with different masses. Nature contains both carbon-12, the most abundant form, and carbon-13, which has one more neutron than carbon-12. Each material has a very specific ratio of these two isotopes, which essentially can be used as the fingerprint or signature of the material. The new algorithm links the carbon isotope ratios of the substrates that are available to microbes in a given environment to the ratios found in the microbes themselves.

Manuel Kleiner, an NC State assistant professor in the Department of Plant and Microbial Biology and corresponding author of a paper that reports the research, says that understanding microbial communities is necessary to better comprehend animal and plant health and disease, as well as important environmental processes such as decomposition of organic matter and nutrient cycling in soils and oceans.