Contributions of direct incorporation from diet and microbial amino acids to protein synthesis in Nile tilapia

1. Further advancement in the use of stable isotope analysis in animal ecology and physiology requires a better understanding of how organisms incorporate the macromolecular elements they consume into the tissues they biosynthesize. 2. Mixing models used to infer diets from isotopic data assume that assimilated macromolecules are dissembled into elements and then reassembled in animal tissues. 3. To test this assumption, we fed Nile tilapia diets with contrasting levels of protein and in which the carbon isotopic composition of protein differed from that of other macronutrients (carbohydrates and lipids). We then analysed the delta C-13 values of individual tilapia amino acids using compound-specific stable isotope analysis (CSIA). 4. The incorporation of dietary protein carbon was dependent on dietary protein content and on each amino acid's biosynthesis pathway. The delta C-13 values of glycolytic amino acids, such as glycine, serine and alanine, had roughly constant values that reflected a large contribution of dietary carbohydrates and a small contribution of dietary protein. The delta C-13 values of aspartate and glutamate that are biosynthesized from Krebs cycle intermediaries paralleled bulk diet. 5. The delta C-13 values of indispensable amino acids resembled that of carbohydrates at low protein intakes but tended asymptotically to that of the delta C-13 values of their corresponding dietary amino acid as protein intake increased. This pattern is consistent with assimilation of indispensable amino acids of microbial origin by tilapias fed low protein food. 6. Our results suggest that the assumptions of mixing models are sound in situations where omnivores consume protein-deficient diets, as the elemental constituents used to biosynthesize amino acids in tissues may be derived from various ingested macromolecules (e. g. protein, carbohydrates, or lipids) and their elemental components. In contrast, for omnivores that consume sufficient amounts of protein, macromolecular (e. g. protein) routing likely occurs and mixing model assumptions are violated. 7. Our study shows that CSIA is a novel method to quantify the contribution of symbiotic microbes to the amino acid homeostasis of animals.