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

Rationale: 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. Methods:
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. Results: 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. Conclusions: 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.