Whiteman John P, Kim Sora L, McMahon Kelton W, Koch Paul L, Newsome Seth D
Department of Biology, University of New Mexico, Albuquerque, NM, 87131, USA.
Department of Life and Environmental Sciences, University of California, Merced, CA, 95343, USA.
Oecologia. 2018 Dec;188(4):977-989. doi: 10.1007/s00442-018-4276-2. Epub 2018 Oct 22.
Stable isotopes are important ecological tools, because the carbon and nitrogen isotopic composition of consumer tissue reflects the diet. Measurements of isotopes of individual amino acids can disentangle the effects of consumer physiology from spatiotemporal variation in dietary isotopic values. However, this approach requires knowledge of assimilation patterns of dietary amino acids. We reared leopard sharks (Triakis semifasciata) on diets of squid (Loligo opalescens; 1250 days; control sharks) or squid then tilapia (Oreochromis sp.; switched at 565 days; experimental sharks) to evaluate consumer-diet discrimination factors for amino acids in muscle tissue. We found that control sharks exhibited lower nitrogen isotope discrimination factors (∆N) than most previous consumer studies, potentially because of urea recycling. Control sharks also had large carbon isotope discrimination factors (∆C) for three essential amino acids, suggesting microbial contributions or fractionation upon assimilation. Compared to controls, experimental sharks exhibited higher ∆C values for four amino acids and ∆N values for seven amino acids, corresponding with differences between diets in δC and δN values. This suggests that not all amino acids in experimental sharks had reached steady state, contrary to the conclusion of a bulk isotope study of these sharks. Our results imply that (1) the magnitude of a shift in dietary δC and δN values temporarily influences the appearance of discrimination factors; (2) slow turnover of amino acid isotopes in elasmobranch muscle precludes inferences about seasonal dietary changes; (3) elasmobranch discrimination factors for amino acids may be affected by urea recycling and microbial contributions of amino acids.
稳定同位素是重要的生态学工具，因为消费者组织中的碳和氮同位素组成反映了其饮食情况。对单个氨基酸同位素的测量能够将消费者生理因素的影响与饮食同位素值的时空变化区分开来。然而，这种方法需要了解饮食氨基酸的同化模式。我们用鱿鱼（太平洋褶柔鱼；1250天；对照鲨鱼）或先喂鱿鱼后喂罗非鱼（尼罗罗非鱼；565天切换；实验鲨鱼）的饲料饲养豹鲨，以评估肌肉组织中氨基酸的消费者-饮食歧视因子。我们发现，对照鲨鱼的氮同位素歧视因子（∆N）比之前大多数消费者研究中的要低，这可能是由于尿素循环利用。对照鲨鱼对三种必需氨基酸也有较大的碳同位素歧视因子（∆C），这表明存在微生物贡献或同化过程中的分馏作用。与对照相比，实验鲨鱼对四种氨基酸的∆C值较高，对七种氨基酸的∆N值较高，这与不同饮食的δC和δN值差异相对应。这表明，与对这些鲨鱼的整体同位素研究结论相反，实验鲨鱼中的并非所有氨基酸都达到了稳态。我们的结果表明：（1）饮食δC和δN值的变化幅度会暂时影响歧视因子的表现；（2）软骨鱼类肌肉中氨基酸同位素的缓慢周转排除了对季节性饮食变化的推断；（3）软骨鱼类对氨基酸的歧视因子可能受尿素循环利用和氨基酸的微生物贡献影响。
