All living organisms need the basic building blocks of life: carbon, nitrogen, phosphorus, and sulfur. These building blocks come to humans and other animals through a complex food web. At the base of that food web are microorganisms. Microbes are critical in transformations of nitrogen, for example. They fix nitrogen, create amino acids, mineralize nitrogen, and complete the cycle by denitrifying. We study these transformations using techniques such as isotope dilution and the microorganisms involved by measuring the composition and abundance of genes and transcripts that code for the enzymes critical in these transformations.