Lipid extraction alters amino acid composition and bulk, but not amino acid, carbon and nitrogen isotope values

Concerns exist over observed shifts in value and variance of nitrogen isotopes following physicochemical extraction of lipids from organic matter. The mechanisms behind these apparent changes in bulk tissue δ¹⁵N values are not fully understood yet have major implications for analytical costs and integrity of data interpretations.

Changes in proximate analysis, amino acid composition, C:N ratios, bulk tissue and amino acid δ¹³C and δ¹⁵N values, and resulting isotope-based food web metrics were compared between lipid-intact and lipid-extracted muscle tissue of fishes spanning <1% to >20% muscle fat content to identify mechanisms of nitrogen isotope fractionation associated with physicochemical lipid extraction.

Bulk δ¹³C and δ¹⁵N values increased and %N, C:N ratios and crude protein content decreased following lipid extraction. Resulting bulk isotope niche spacing and overlap varied significantly between lipid-intact and lipid-extracted tissues. While amino acid composition significantly changed during lipid extraction, particularly for lipid-associated amino acids (e.g., Glu, Lys, Ser), individual amino acid δ¹³C and δ¹⁵N values, and their associated compound-specific isotope analysis of amino acids (CSIA-AA)-based food web metrics, did not.

Physicochemical lipid extraction caused significant tissue composition changes (e.g., leaching of amino acids and ¹⁵N-deplete nitrogenous waste) that affected δ¹³C and δ¹⁵N values and tissue %C and %N beyond simply removing lipids. However, lipid extraction did not alter individual amino acid δ¹³C or δ¹⁵N values or their associated CSIA-AA-based food web metrics.