Effects of chemical preservation on bulk and amino acid isotope ratios of zooplankton, fish, and squid tissues

TitleEffects of chemical preservation on bulk and amino acid isotope ratios of zooplankton, fish, and squid tissues
Publication TypeJournal Article
Year of Publication2019
AuthorsHetherington E.D, Kurle C.M, Ohman MD, Popp BN
Date Published2019/05
Type of ArticleArticle
ISBN Number0951-4198
Accession NumberWOS:000466811100005
KeywordsBiochemistry & Molecular Biology; chemistry; delta-c-13; delta-n-15; food-web structure; fractionation; nitrogen; pacific subtropical gyre; position; samples; signatures; spectroscopy; stable-carbon; Trophic

Rationale It is imperative to understand how chemical preservation alters tissue isotopic compositions before using historical samples in ecological studies. Specifically, although compound-specific isotope analysis of amino acids (CSIA-AA) is becoming a widely used tool, there is little information on how preservation techniques affect amino acid delta N-15 values. Methods We evaluated the effects of chemical preservatives on bulk tissue delta C-13 and delta N-15 and amino acid delta N-15 values, measured by gas chromatography/isotope ratio mass spectrometry (GC/IRMS), of (a) tuna (Thunnus albacares) and squid (Dosidicus gigas) muscle tissues that were fixed in formaldehyde and stored in ethanol for 2 years and (b) two copepod species, Calanus pacificus and Eucalanus californicus, which were preserved in formaldehyde for 24-25 years. Results Tissues in formaldehyde-ethanol had higher bulk delta N-15 values (+1.4, D. gigas; +1.6 parts per thousand, T. albacares), higher delta C-13 values for D. gigas (+0.5 parts per thousand), and lower delta C-13 values for T. albacares (-0.8 parts per thousand) than frozen samples. The bulk delta N-15 values from copepods were not different those from frozen samples, although the delta C-13 values from both species were lower (-1.0 parts per thousand for E. californicus and -2.2 parts per thousand for C. pacificus) than those from frozen samples. The mean amino acid delta N-15 values from chemically preserved tissues were largely within 1 parts per thousand of those of frozen tissues, but the phenylalanine delta N-15 values were altered to a larger extent (range: 0.5-4.5 parts per thousand). Conclusions The effects of preservation on bulk delta C-13 values were variable, where the direction and magnitude of change varied among taxa. The changes in bulk delta N-15 values associated with chemical preservation were mostly minimal, suggesting that storage in formaldehyde or ethanol will not affect the interpretation of delta N-15 values used in ecological studies. The preservation effects on amino acid delta N-15 values were also mostly minimal, mirroring bulk delta N-15 trends, which is promising for future CSIA-AA studies of archived specimens. However, there were substantial differences in phenylalanine and valine delta N-15 values, which we speculate resulted from interference in the chromatographic resolution of unknown compounds rather than alteration of tissue isotopic composition due to chemical preservation.

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