Carbon isotope fractionation of amino acids in fish muscle reflects biosynthesis and isotopic routing from dietary protein

P>1. Analysis of stable carbon isotopes is a valuable tool for studies of diet, habitat use and migration. However, significant variability in the degree of trophic fractionation (Delta 13C(C-D)) between consumer (C) and diet (D) has highlighted our lack of understanding of the biochemical and physiological underpinnings of stable isotope ratios in tissues. 2. An opportunity now exists to increase the specificity of dietary studies by analyzing the delta 13C values of amino acids (AAs). Common mummichogs (Fundulus heteroclitus, Linnaeus 1766) were reared on four isotopically distinct diets to examine individual AA delta 13C(C-D) variability in fish muscle. 3. Modest bulk tissue Delta 13C(C-D) values reflected relatively large trophic fractionation for many non-essential AAs and little to no fractionation for all essential AAs. 4. Essential AA delta 13C values were not significantly different between diet and consumer (Delta 13C(C-D) = 0 center dot 0 +/- 0 center dot 4 parts per thousand), making them ideal tracers of carbon sources at the base of the food web. Stable isotope analysis of muscle essential AAs provides a promising tool for dietary reconstruction and identifying baseline delta 13C values to track animal movement through isotopically distinct food webs. 5. Non-essential AA Delta 13C(C-D) values showed evidence of both de novo biosynthesis and direct isotopic routing from dietary protein. We attributed patterns in Delta 13C(C-D) to variability in protein content and AA composition of the diet as well as differential utilization of dietary constituents contributing to the bulk carbon pool. This variability illustrates the complicated nature of metabolism and suggests caution must be taken with the assumptions used to interpret bulk stable isotope data in dietary studies. 6. Our study is the first to investigate the expression of AA delta 13C(C-D) values for a marine vertebrate and should provide for significant refinements in studies of diet, habitat use and migration using stable isotopes.