Data from: Amino acid-specific δ15N trophic enrichment factor in fish fed with formulated diets varying in protein quantity and quality

Compound-specific isotope analysis (CSIA) of amino acids (AAs) in consumer tissues is a developing technique with wide-ranging applications for identifying nitrogen (N) sources and estimating animal trophic level. Controlled experiments are essential for determining which dietary conditions influence variability in N stable isotopes (δ15N) trophic enrichment factors in bulk tissue (TEF bulk) and AAs (TEF AA). To date, however, studies have not independently evaluated the effect of protein quantity and quality (digestibility) on TEFs, complicating the application of AA-15N values for estimating trophic levels. We conducted a 98-d feeding experiment using five formulated isoenergetic feeds prepared with a high-quality protein source to evaluate the effect of protein quantity and quality on TEFs of liver and muscle tissues of juvenile Pacific yellowtail (Seriola lalandi), a carnivorous fish species. We decreased protein digestibility using well-established protocols that do not change AA profiles. Growth rates were higher in diets with higher protein content, and isotopic equilibrium was reached for both fish tissues and all treatments. Protein quantity and quality influenced isotope discrimination depending on tissue type and AA. In liver tissue, bulk TEFs showed a limited but significant relationship with protein quality, but did not differ with protein quantity nor quality in muscle. None of the pre-established source-AA (Lys, Met, Phe and Gly) TEFs varied significantly with protein quantity or quality in liver tissue. However, in muscle tissue TEFPhe increased significantly with protein content and decreased in response to reduced digestibility, indicating it may not serve as proxy for baseline isotopic values used to calculate trophic level. Among trophic AA, TEFLeu decreased significantly with increasing protein quantity in liver tissue, while both Leu and Ile TEFs decreased with lower protein digestibility in muscle tissue. Our results indicate that CSIA-AA in liver tissue provide more robust source and trophic AA-δ15N values than in muscle.

针对消费者组织内氨基酸（amino acids, AAs）的单体化合物同位素分析（Compound-specific isotope analysis, CSIA）是一项新兴技术，在识别氮（nitrogen, N）源以及估算动物营养级方面具备广泛应用前景。若要明确何种饮食条件会影响整体组织氮稳定同位素δ¹⁵N的营养富集因子（trophic enrichment factors in bulk tissue, TEF bulk）与氨基酸营养富集因子（trophic enrichment factors in AAs, TEF AA）的变异，受控实验是不可或缺的研究手段。然而迄今为止，尚无研究独立评估蛋白质含量与质量（消化率）对营养富集因子的影响，这一局限给利用氨基酸¹⁵N值估算动物营养级的应用带来了诸多困扰。本研究开展了一项为期98天的喂食实验，以肉食性鱼类幼年太平洋黄尾鲷（Seriola lalandi）为实验对象，采用5种以优质蛋白源配制的等能饲料，旨在探究蛋白质含量与质量对其肝脏与肌肉组织营养富集因子的影响。实验通过成熟且不会改变氨基酸组成的标准化方法降低了蛋白质消化率。结果显示，蛋白质含量更高的饲料组鱼类生长速率更快，且所有处理组的两种鱼组织均达到了同位素平衡状态。蛋白质含量与质量对同位素分馏的影响因组织类型与氨基酸种类而异。在肝脏组织中，整体组织营养富集因子与蛋白质质量仅存在微弱但显著的相关性，而肌肉组织的整体营养富集因子则不受蛋白质含量或质量的影响。在肝脏组织中，预先选定的源氨基酸（赖氨酸Lys、甲硫氨酸Met、苯丙氨酸Phe和甘氨酸Gly）的营养富集因子均未随蛋白质含量或质量发生显著变化。但在肌肉组织中，苯丙氨酸营养富集因子（TEF Phe）随蛋白质含量升高显著上升，却随消化率降低而下降，这表明其无法作为计算营养级所需基准同位素值的替代指标。在营养氨基酸方面，肝脏组织中的亮氨酸营养富集因子（TEF Leu）随蛋白质含量升高显著降低，而肌肉组织中的亮氨酸与异亮氨酸营养富集因子则随蛋白质消化率降低而下降。本研究结果表明，相较于肌肉组织，肝脏组织的单体化合物氨基酸同位素分析所得到的源氨基酸与营养氨基酸δ¹⁵N值更为稳健可靠。