Tracing the carbon flow through food webs is a big challenge in ecological science. Previous studies have found that stable carbon isotope composition (δ¹³C) of amino acids (AAs) in organisms could record the sources of carbon in the food web (e.g. Larsen et al. 2009). However, its application is still limited because of the lack of quantity and reliability of δ¹³C_AA data in natural organisms, which is mainly due to experimental difficulties.
Using a new analytical method with high precision (Sun et al. 2020), we analyzed several kinds of marine consumers including fish and squid. δ¹³C compositions of AAs in different organisms show a similar pattern: δ¹³C_AA values have a variation up to 30‰, with glycine, serine and threonine being ¹³C-enriched and leucine and phenylalanine being ¹³C-depleted. The results support the hypothesis that δ¹³C_AA compositions in organisms are determined by their central biochemical pathways. To examine δ¹³C trophic discrimination factors, we also analyzed a cultured fish, Sebastes melanops, and its diet. Differences in δ¹³C_AA values between S. melanops and the diet were small for most AAs (–0.6‰ on average), indicating a direct isotopic routing from diet to consumer. This study provides a new assessment of δ¹³C_AA compositions and trophic discrimination factors in marine organisms, which will contribute to future food web studies.