Kelly Leona J, del Rio Carlos Martínez
Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming 82071-3166, USA.
Physiol Biochem Zool. 2010 May-Jun;83(3):473-80. doi: 10.1086/649628.
The application of stable isotope analysis to ecology requires estimating the contribution of different isotopic sources to the isotopic signatures of an animal's tissues using mixing models. These models make the physiologically unrealistic assumption that assimilated nutrients are disassembled into their elemental components and that these atoms are then reassembled into biomolecules. We quantified the extent to which mixing models yield erroneous results with an experiment using Nile tilapia (Oreochromis niloticus). The tilapia were fed synthetic diets that varied in protein content and in which the carbon isotopic composition of protein differed widely from that of carbohydrates and lipids. We predicted that dietary protein would contribute disproportionately to the carbon in muscle, whereas the nonprotein components of diet would contribute disproportionately to lipids. Our experiment also allowed us to test the conjecture of a positive correlation between the (15)N enrichment in tissues and protein intake. As predicted, the contribution of protein carbon to muscle was higher than that expected by the assumptions of isotopic mixing in all treatments except that with the lowest dietary protein content. We hypothesized that the unexpectedly high contribution of nonprotein carbon to muscle was the result of assimilating both dispensable and indispensable amino acids synthesized by the fishes' gut microbiota. Although we expected the contribution of carbon in nonprotein dietary ingredients to be higher than expected from a mixing model, we found that protein contributed more than expected, probably as a result of differences in amino acid composition between diet and tissues, which led to excess carbon used for lipid synthesis. Finally, our results verified the positive relationship between dietary protein content and the enrichment in tissue (15)N. Assuming perfect mixing in field isotopic studies can lead to erroneous inferences about the relative contributions of different sources to an animal's diet.
将稳定同位素分析应用于生态学需要使用混合模型来估计不同同位素源对动物组织同位素特征的贡献。这些模型做出了生理上不现实的假设，即同化的营养物质被分解成其元素成分，然后这些原子重新组装成生物分子。我们通过使用尼罗罗非鱼（Oreochromis niloticus）的实验，量化了混合模型产生错误结果的程度。给罗非鱼投喂蛋白质含量不同的合成饲料，其中蛋白质的碳同位素组成与碳水化合物和脂质的碳同位素组成差异很大。我们预测，膳食蛋白质对肌肉中碳的贡献将不成比例，而膳食中的非蛋白质成分对脂质的贡献将不成比例。我们的实验还使我们能够检验组织中（15）N富集与蛋白质摄入量之间正相关的推测。正如预测的那样，除了膳食蛋白质含量最低的处理外，在所有处理中，蛋白质碳对肌肉的贡献都高于同位素混合假设所预期的贡献。我们假设非蛋白质碳对肌肉的意外高贡献是鱼类肠道微生物群合成的必需和非必需氨基酸都被同化的结果。虽然我们预计非蛋白质膳食成分中碳的贡献会高于混合模型的预期，但我们发现蛋白质的贡献高于预期，这可能是由于膳食和组织之间氨基酸组成的差异导致用于脂质合成的碳过量所致。最后，我们的结果证实了膳食蛋白质含量与组织（15）N富集之间的正相关关系。在野外同位素研究中假设完全混合可能会导致对不同来源对动物饮食相对贡献的错误推断。
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