text: Amino acids are a key nutrient in ecosystems. Some are essential to animals, meaning that these organisms cannot synthesize them de novo. Instead, animals rely on their diet to acquire these molecules, creating strong interdependencies between animals and organisms with complete amino acid synthesis capabilities. In a study of bacteria and archaea at Antarctica's McMurdo Dry Valleys, the distribution of C between their amino acids reflected the biosynthetic pathways employed by these organisms. Autotrophs and heterotrophs had distinct isotopic fingerprints, as did organisms that employed alternatives to the citric acid cycle to ferment or produce acetate. Plants, fungi, and bacteria are also distinguishable by their amino acid carbon isotopes. The compositions of the essential amino acids, which have more complex biosynthetic pathways, are particularly informative. Lysine, isoleucine, leucine, threonine, and valine all had significantly different δC values between at least two of these groups. It is important to note that the fungi and bacteria in this study were grown on amino acid-free media to ensure that all the amino acids were synthesized by the organisms of interest.  Bacteria and fungi can also scavenge amino acids from the environment, complicating the interpretation of data from field samples. Nevertheless, researchers have successfully used these differences to identify the sources of amino acids in food webs. Terrestrial and marine producers in a mangrove forest had different patterns of C enrichment in their amino acids.  Fishes from a coral reef with diets containing different carbon sources also had variable amino acid δC values. Furthermore, one study observed distinct amino acid isotopic compositions for desert C, C, and CAM plants. These applications in diverse ecosystems highlight the versatility of compound-specific amino acid isotope analysis.