Carbon isotope ratios of proteinogenic amino acids in freshwater fish from Lake Biwa

Abstract

We analyzed stable carbon isotope ratios of bulk tissue and 16 amino acids in both formalin-fixed and frozen muscle samples from two freshwater fish species collected in Lake Biwa. The compound-specific isotope ratios varied widely within individuals but followed consistent patterns linked to biosynthetic origins. Glycolytic amino acids, glycine and serine, were enriched in ¹³C, while phenylalanine, tyrosine, valine, and methionine were relatively depleted compared to bulk tissue. Formalin fixation reduced the recoveries of several amino acids, including tyrosine, phenylalanine, histidine, and lysine, but their isotope ratios remained largely unchanged. These results support the use of preserved biological specimens in amino acid compound-specific isotope studies across diverse taxa and ecosystems.