Assessing the reliability of fatty acid–specific stable isotope analysis for trophic studies

Abstract

Summary1. Fatty acid-specific stable isotope analysis (FA-SIA) is expected to encompass most of the limits encountered when using more classical trophic markers such as bulk tissue stable isotope or fatty acid analyses. However, an essential premise for using FA-SIA for trophic studies is that the individual FA d 13C values of the consumer reflect those of its diet. Field studies using FA-SIA have so far made this assumption, which is not necessarily supported by the rare experimental tests. 2. A feeding experiment was conducted on Daphnia to test whether the d 13C values of individual fatty acids in Daphnia were actually related to those of its food. 3. Only the stable isotope composition of polyunsaturated (PUFA) and branched fatty acids (BrFA) was globally transmitted from the diet to Daphnia lipids, with however a significant isotope fractionation that varied depending on the considered diet source. 4. A model was constructed to evaluate how such variability may affect the reliability of FA-SIA to track the dietary sources of consumers' PUFA and BrFA in the field. Results suggest that provided the endmembers are substantially isotopically different, FA-SIA could provide valuable insights into the pathways conveying these particular FA to consumers in the field.

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

“…The first half of this parabolic tendency, namely the significant depletion of the essential 18:2n-6 and especially 18:3n-3 compared to their less saturated precursors, is in a good agreement with the ideas, that the kinetic isotope effect make more unsaturated fatty acids lighter, than their precursors [6], [9], [11]. In contrast, the other half of the found tendency (18:3n-3<18:4n-3<20:5n-3≈22:6n-3) evidently contradicts the above postulate.…”

“…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].…”

“…However, even the essential fatty acids were found to be generally 13 C-depleted compared with their counterpart in the corresponding diet in field and laboratory pelagic food webs [4], [6]. Thus, variations in isotope fractionation in some fatty acids are difficult to explain with current theories of biosynthesis and isotope fractionation, and more work is needed.…”

How pollen organic matter enters freshwater food webs

“…At a compoundspecific level, seston FA d 13 C values ranged from 225.3% down to 238.2%. This high intermolecular variability (i.e., 13%) would corroborate the multiple origins of carbon in seston Bec et al 2011 (Jones and Grey 2011). In our oxbow lake, the important organic matter loading in the shallow sediments would probably provide methane to methane-oxidizing bacteria to produce depleted CO 2 , which could result in depleted phytoplankton d 13 C values.…”

“…In a previous experimental study, such a pollen-based microbial food web was shown to transfer pollen carbon and upgrade its quality to promote zooplankton growth, especially by adding some long-chain PUFA, such as eicosapentaenoic acid (20:5v3; Masclaux et al 2011). Trophic upgrading of food quality by microorganisms has been associated with increasing digestibility and essential lipid compound production (such as C20 and C22-PUFA; Bec et al 2006). Indeed, long-chain PUFA and especially 20:5v3 are key essential compounds for metazoans, and their dietary availability may strongly determine secondary production in aquatic ecosystems, as metazoans cannot synthesize them de novo (Arts et al 2001;Mü llerNavarra et al 2004).…”