Metabolic control of nitrogen isotope composition of amino acids in macroalgae and gastropods: implications for aquatic food web studies

We investigated the nitrogen isotopic compositions of individual amino acids from brown and red macroalgae and gastropods in a natural marine coastal environment, to further evaluate them as a tool for ecological studies and to understand the factor(s) controlling the isotopic compositions in terms of biosynthetic and metabolic processes. The isotopic compositions of 12 amino acids range from -2.1 to + 8.4 for brown algae, from -3.3 to +12.9 for red algae, and from -0.6 to +16.6 for gastropods. The isotopic distributions between algae and gastropods and within algal classes are consistent with those in previous reports, suggesting them to be quite useful for studying food web structure. The nitrogen isotopic variation between amino acids strongly reflects their biosynthetic and metabolic processes. In particular, we suggest here that metabolic fate is an important factor in producing distinct trophic relationships in nitrogen isotopic compositions between amino acids.