Plants, bacteria, and fungi produce essential amino acids (EAAs) with distinctive patterns of δ13C values that can be used as naturally occurring fingerprints of biosynthetic origin of EAAs in a food web. Because animals cannot synthesize EAAs and must obtain them from food, their tissues reflect δ13CEAA patterns found in diet, but it is not known how microbes responsible for hindgut fermentation in some herbivores influence the δ13C values of EAAs in their hosts' tissues. We examined whether distinctive δ13C fingerprints of hindgut flora are evident in the tissues of green turtles (Chelonia mydas), which are known to be facultative hindgut fermenters. We determined δ13CEAA values in tissues of green turtles foraging herbivorously in neritic habitats of Hawaii and compared them with those from green, olive ridley, and loggerhead turtles foraging carnivorously in oceanic environments of the central and southeast Pacific Ocean. Results of multivariate statistical analysis revealed two distinct groups that could be distinguished based on unique δ13CEAA patterns. A threeend- member predictive linear discriminant model indicated that δ13CEAA fingerprints existed in the tissues of carnivorous turtles that resembled patterns found in microalgae, which form the base of an oceanic food web, whereas herbivorous turtles derive EAAs from a bacterial or seagrass source. This study demonstrates the capacity for δ13C fingerprinting to establish the biosynthetic origin of EAAs in higher consumers, and that marine turtles foraging on macroalgal diets appear to receive nutritional supplementation from bacterial symbionts in their digestive system. © 2014 by the Ecological Society of America.