Protein stable isotope fingerprinting (P-SIF) is a method to measure the carbon isotope ratios of whole proteins separated from complex mixtures, including cultures and environmental samples. The goal of P-SIF is to expose the links between identity and function in microbial ecosystems by (i) determining the values of δ13C for different taxonomic divisions, and (ii) using those values as clues to the metabolic pathways employed by the respective organisms. This project measures >200 protein fractions and δ13C values for a sample of Cyanobacteria + Chloroflexi dominated microbial mat from Yellowstone National Park, USA.