|Title||Effects of chemical preservation on bulk and amino acid isotope ratios of zooplankton, fish, and squid tissues|
|Publication Type||Journal Article|
|Year of Publication||2019|
|Authors||Hetherington E.D, Kurle C.M, Ohman MD, Popp BN|
|Type of Article||Article|
|Keywords||Biochemistry & 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.