Carbon isotopic fractionation in eider adipose tissue varies with fatty acid structure: implications for trophic studies

Abstract

SUMMARYCarbon isotopic fractionation was investigated in fatty acids (FA) of adipose tissue and blood serum of threatened Stellerʼs eiders (Polysticta stelleri) and spectacled eiders (Somateria fischeri) relative to the FA in their diets. Captive eiders were fed a known diet for 180 days with serum sampled at 60, 120 and 180 days immediately after a 12 h fast; adipose was collected at 180 days. Essential FA (EFA) in the adipose showed varying degrees of isotope fractionation (0-4‰), depending on FA structure. The d13 C values of long-chain FA 20:5n-3 and 22:6n-3 did not differ from those in the diet, while those of 18:2n-6 and 18:3n-3 were ~2‰ greater than in the diet. The d13 C values of free FA (FFA) in serum were not consistent within individuals or sampling dates; fractionation varied randomly, suggesting that FFA were arising from diet, rather than mobilization from adipose tissue. Discrimination factors were used in combination with a mixing model incorporating FA and lipid concentrations to estimate the diet of eiders fed a binary mixture with contrasting isotopic signatures. Diet estimates varied with FA but mean values closely approximated the actual proportions consumed. By tracking EFA, this study avoided the complications in interpretation arising from isotopic routing of carbon in bulk isotope analyses and serves as a basis for the development of compound-specific isotopic methods to trace dietary input in wild eiders. However, our understanding of the processes contributing to the variation in isotopic signatures of FA in nature is currently limited, and we recommend that future research directions focus on elucidating these mechanisms.Key words: diet estimate, discrimination, lipid, compound-specific isotope analysis, stable isotope, Steller's eider, spectacled eider. THE JOURNAL OF EXPERIMENTAL BIOLOGY 3791Carbon isotopes of fatty acids in eiders of higher predators (e.g. Thiemann et al., 2007;Williams et al., 2008;Cooper et al., 2009), including captive eiders . FA are the primary components of most lipids and have a variety of structures. Because of biochemical restrictions, many dietary polyunsaturated FA (PUFA) are incorporated into marine animals with little modification to structure, making these FA valuable as indicators, or biomarkers, of their source. However, these biomarker techniques can only offer general information on temporal and spatial variation in consumer diets; much more complex models (i.e. quantitative fatty acid signature analysis, QFASA) (Iverson et al., 2004) are necessary to estimate proportions and types of prey species in predator diets using FA.Recently, we demonstrated the use of stable carbon isotopes of individual FA to trace carbon flow from prey to predator across multiple trophic levels in the Arctic (Budge et al., 2008 (Focken, 2004;Codron et al., 2011) and environmental factors (Bloomfield et al., 2011), and techniques such as lipid removal before isotopic analysis attempt to minimize its impact; however, it remains a complication in the interpretation of bul...

Metabolism and foraging strategies of mid‐latitude mesozooplankton during cyanobacterial blooms as revealed by fatty acids, amino acids, and their stable carbon isotopes

“…Such differences have mainly been attributed to FA-specific trophic fractionation of 13 C isotope that depends on the individual FA structure (Bec et al, 2011;Budge, Wang, Hollmen, & Wooller, 2011). However, we still found that in mesozooplankton δ 13 C values of essential FAs with 18C are significantly different from essential δ 13 C-EPA and δ 13 C-DHA values, while both EPA and DHA have similar δ 13 C values to bulk δ 13 C in POM.…”

Stable Isotope Composition of Fatty Acids in Organisms of Different Trophic Levels in the Yenisei River

“…In a cultured diatom algae Thalassiosira pseudonana 20:5n-3 and 22:6n-3 were less enriched compared to 18:2n-6 and 18:1n-9 [41]. Budge et al [8] depicted the parabolic pattern for sea birds, eiders, fed on controlled diet.…”

“…However, experimental tests are still rare and did not necessarily support this essential assumption [6]–[8]. Moreover, FA δ 13 C values of zooplankton and fish were not directly related to those of particulate organic matter in a marine field study [4].…”

Ice Algae-Produced Carbon Is Critical for Overwintering of Antarctic Krill Euphausia superba

“…The FA-specific stable isotope composition of carbon was measured in FAME derivatives of the diatom-associated FAs 16:1n-7 and 20:5n-3, and the dinoflagellate-associated FAs 18:4n-3 and 22:6n-3 in POM and krill (CSIA; Kohlbach et al, 2016). During transesterification, a methyl group is added to the FA, which can result in a difference in δ 13 C values between the free FA and the corresponding FAME (e.g., Budge et al, 2011). In a preparatory experiment for this study, however, we did not find significant differences between the δ 13 C values of the free FA and the FAME (e.g., 16:0 FA: −28.56 ± 0.12‰, 16:0 FAME: −28.57 FIGURE 1 | Map of the sampling area during "Polarstern" expedition PS81 in the Weddell-Scotia Confluence Zone (14 August to 16 October 2013) with sea ice concentration (Spreen et al, 2008).…”

“…The mixing models can account for different isotopic turnover rates in the consumers. However, krillspecific trophic enrichment of 13 C is unknown, and was thus assumed to be zero for both BSIA and CSIA models (Budge et al, 2011;Graham et al, 2014;Wang et al, 2015). For the BSIA models, trophic enrichment of 15 N was assumed to be 3.4‰ per trophic level (Minagawa and Wada, 1984).…”