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 a limited number of protein fractions and δ13C values for a sample of floating, mat-like microbial biomass of an intensely phototrophic layer from Mahoney Lake, BC Canada.