skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


Title: Meta‐analysis of primary producer amino acid δ 15 N values and their influence on trophic position estimation
Abstract Compound‐specific stable isotope analysis of individual amino acids (CSIA‐AA) has emerged as a transformative approach to estimate consumer trophic positions (TPCSIA) that are internally indexed to primary producer nitrogen isotope baselines. Central to accurate TPCSIAestimation is an understanding of beta (β) values—the differences between trophic and source AA δ15N values in the primary producers at the base of a consumers’ food web. Growing evidence suggests higher taxonomic and tissue‐specificβvalue variability than typically appreciated.This meta‐analysis fulfils a pressing need to comprehensively evaluate relevant sources ofβvalue variability and its contribution to TPCSIAuncertainty. We first synthesized all published primary producer AA δ15N data to investigate ecologically relevant sources of variability (e.g. taxonomy, tissue type, habitat type, mode of photosynthesis). We then reviewed the biogeochemical mechanisms underpinning AA δ15N andβvalue variability. Lastly, we evaluated the sensitivity of TPCSIAestimates to uncertainty in meanβGlx‐Phevalues and Glx‐Phe trophic discrimination factors (TDFGlx‐Phe).We show that variation inβGlx‐Phevalues is two times greater than previously considered, with degree of vascularization, not habitat type (terrestrial vs. aquatic), providing the greatest source of variability (vascular autotroph = −6.6 ± 3.4‰; non‐vascular autotroph = +3.3 ± 1.8‰). Within vascular plants, tissue type secondarily contributed toβGlx‐Phevalue variability, but we found no clear distinction among C3, C4and CAM plantβGlx‐Phevalues. Notably, we found that vascular plantβGlx‐Lysvalues (+2.5 ± 1.6‰) are considerably less variable thanβGlx‐Phevalues, making Lys a useful AA tracer of primary production sources in terrestrial systems. Our multi‐trophic level sensitivity analyses demonstrate that TPCSIAestimates are highly sensitive to changes in bothβGlx‐Pheand TDFGlx‐Phevalues but that the relative influence ofβvalues dissipates at higher trophic levels.Our results highlight that primary producerβvalues are integral to accurate trophic position estimation. We outline four key recommendations for identifying, constraining and accounting forβvalue variability to improve TPCSIAestimation accuracy and precision moving forward. We must ultimately expand libraries of primary producer AA δ15N values to better understand the mechanistic drivers ofβvalue variation.  more » « less
Award ID(s):
1907144 2049307
PAR ID:
10377312
Author(s) / Creator(s):
 ;  ;  ;  
Publisher / Repository:
Wiley-Blackwell
Date Published:
Journal Name:
Methods in Ecology and Evolution
Volume:
12
Issue:
10
ISSN:
2041-210X
Format(s):
Medium: X Size: p. 1750-1767
Size(s):
p. 1750-1767
Sponsoring Org:
National Science Foundation
More Like this
  1. ; (, Marine Ecology Progress Series)
    null (Ed.)
    We investigated the response of an open-ocean plankton food web to a major ecosystem perturbation event, the Hawaiian lee cyclonic eddy Opal, using compound-specific isotopic analyses of amino acids (CSIA-AA) of individual zooplankton taxa. We hypothesized that the massive diatom bloom that characterized Opal would lead to a shorter food chain. Using CSIA-AA, we differentiated trophic position (TP) changes that arose from altered transfers through protistan microzooplankton, versus metazoan carnivory, and assessed the variability at the base of the food web. Contrary to expectation, zooplankton TPs were higher in the eddy than in ambient control waters (up to 0.8 trophic level), particularly for suspension feeders close to the food-web base. Most of the effect was due to increased trophic transfers through protistan consumers, indicating a general shift up, not down, of grazing and remineralization in the microbial food web. Eucalanus sp., which was 15-fold more abundant inside compared to outside of the eddy, was the only taxon observed to be a true herbivore (TP = 2.0), consistent with a high phenylalanine (Phe) δ 15 N value indicating feeding on nitrate-fueled diatoms in the lower euphotic zone. Oncaea sp., an aggregate-associated copepod, had the largest (1.5) TP difference, and lowest Phe δ 15 N, suggesting that detrital particles were local hot spots of enhanced microbial activity. Rapid growth rates and trophic flexibility of protistan microzooplankton apparently allow the microbial community to reorganize to bloom perturbations, as microzooplankton remain the primary phytoplankton grazers—despite the dominance of large diatoms—and are heavily preyed on by the mesozooplankton. 
    more » « less
  2. ; ; ; ; ; ; (, Rapid Communications in Mass Spectrometry)
    RationaleNitrogen isotopic compositions (δ15N) of source and trophic amino acids (AAs) are crucial tracers of N sources and trophic enrichments in diverse fields, including archeology, astrobiochemistry, ecology, oceanography, and paleo‐sciences. The current analytical technique using gas chromatography‐combustion‐isotope ratio mass spectrometry (GC/C/IRMS) requires derivatization, which is not compatible with some key AAs. Another approach using high‐performance liquid chromatography‐elemental analyzer‐IRMS (HPLC/EA/IRMS) may experience coelution issues with other compounds in certain types of samples, and the highly sensitive nano‐EA/IRMS instrumentations are not widely available. MethodsWe present a method for high‐precision δ15N measurements of AAs (δ15N‐AA) optimized for canonical source AA‐phenylalanine (Phe) and trophic AA‐glutamic acid (Glu). This offline approach entails purification and separation via high‐pressure ion‐exchange chromatography (IC) with automated fraction collection, the sequential chemical conversion of AA to nitrite and then to nitrous oxide (N2O), and the final determination of δ15N of the produced N2O via purge‐and‐trap continuous‐flow isotope ratio mass spectrometry (PT/CF/IRMS). ResultsThe cross‐plots of δ15N of Glu and Phe standards (four different natural‐abundance levels) generated by this method and their accepted values have a linear regression slope of 1 and small intercepts demonstrating high accuracy. The precisions were 0.36‰–0.67‰ for Phe standards and 0.27‰–0.35‰ for Glu standards. Our method and the GC/C/IRMS approach produced equivalent δ15N values for two lab standards (McCarthy Lab AA mixture and cyanobacteria) within error. We further tested our method on a wide range of natural sample matrices and obtained reasonable results. ConclusionsOur method provides a reliable alternative to the current methods for δ15N‐AA measurement as IC or HPLC‐based techniques that can collect underivatized AAs are widely available. Our chemical approach that converts AA to N2O can be easily implemented in laboratories currently analyzing δ15N of N2O using PT/CF/IRMS. This method will help promote the use of δ15N‐AA in important studies of N cycling and trophic ecology in a wide range of research areas. 
    more » « less
  3. ; ; (, New Phytologist)
    Summary While plant δ15N values have been applied to understand nitrogen (N) dynamics, uncertainties regarding intraspecific and temporal variability currently limit their application. We used a 28 yr record of δ15N values from two Mojave Desert populations ofEncelia farinosato clarify sources of population‐level variability.We leveraged > 3500 foliar δ15N observations collected alongside structural, physiological, and climatic data to identify plant and environmental contributors to δ15N values. Additional sampling of soils, roots, stems, and leaves enabled assessment of the distribution of soil N content and δ15N, intra‐plant fractionations, and relationships between soil and plant δ15N values.We observed extensive within‐population variability in foliar δ15N values and found plant age and foliar %N to be the strongest predictors of individual δ15N values. There were consistent differences between root, stem, and leaf δ15N values (spanningc. 3‰), but plant and bulk soil δ15N values were unrelated.Plant‐level variables played a strong role in influencing foliar δ15N values, and interannual relationships between climate and δ15N values were counter to previously recognized spatial patterns. This long‐term record provides insights regarding the interpretation of δ15N values that were not available from previous large‐scale syntheses, broadly enabling more effective application of foliar δ15N values. 
    more » « less
  4. ; ; (, Limnology and Oceanography)
    Abstract Eukaryotic microalgae play critical roles in the structure and function of marine food webs. The contribution of microalgae to food webs can be tracked using compound‐specific isotope analysis of amino acids (CSIA‐AA). Previous CSIA‐AA studies have defined eukaryotic microalgae as a single functional group in food web mixing models, despite their vast taxonomic and ecological diversity. Using controlled cultures, this work characterizes the amino acidδ13C (δ13CAA) fingerprints—a multivariate metric of amino acid carbon isotope values—of four major groups of eukaryotic microalgae: diatoms, dinoflagellates, raphidophytes, and prasinophytes. We found excellent separation of essential amino acidδ13C (δ13CEAA) fingerprints among four microalgal groups (mean posterior probability reclassification of 99.2 ± 2.9%). We also quantified temperature effects, a primary driver of microalgal bulk carbon isotope variability, on the fidelity ofδ13CAAfingerprints. A 10°C range in temperature conditions did not have significant impacts on variance inδ13CAAvalues or the diagnostic microalgalδ13CEAAfingerprints. Theseδ13CEAAfingerprints were used to identify primary producers at the base of food webs supporting consumers in two contrasting systems: (1) penguins feeding in a diatom‐based food web and (2) mixotrophic corals receiving amino acids directly from autotrophic endosymbiotic dinoflagellates and indirectly from water column diatoms, prasinophytes, and cyanobacteria, likely via heterotrophic feeding on zooplankton. The increased taxonomic specificity of CSIA‐AA fingerprints developed here will greatly improve future efforts to reconstruct the contribution of diverse eukaryotic microalgae to the sources and cycling of organic matter in food web dynamics and biogeochemical cycling studies. 
    more » « less
  5. ; ; ; (, Rapid Communications in Mass Spectrometry)
    RationaleIt 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 acidδ15N values. MethodsWe evaluated the effects of chemical preservatives on bulk tissueδ13C andδ15N and amino acidδ15N 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 pacificusandEucalanus californicus, which were preserved in formaldehyde for 24–25 years. ResultsTissues in formaldehyde‐ethanol had higher bulkδ15N values (+1.4,D. gigas; +1.6‰,T. albacares), higherδ13C values forD. gigas(+0.5‰), and lowerδ13C values forT. albacares(−0.8‰) than frozen samples. The bulkδ15N values from copepods were not different those from frozen samples, although theδ13C values from both species were lower (−1.0‰ forE. californicusand −2.2‰ forC. pacificus) than those from frozen samples. The mean amino acidδ15N values from chemically preserved tissues were largely within 1‰ of those of frozen tissues, but the phenylalanineδ15N values were altered to a larger extent (range: 0.5–4.5‰). ConclusionsThe effects of preservation on bulkδ13C values were variable, where the direction and magnitude of change varied among taxa. The changes in bulkδ15N values associated with chemical preservation were mostly minimal, suggesting that storage in formaldehyde or ethanol will not affect the interpretation ofδ15N values used in ecological studies. The preservation effects on amino acidδ15N values were also mostly minimal, mirroring bulkδ15N trends, which is promising for future CSIA‐AA studies of archived specimens. However, there were substantial differences in phenylalanine and valineδ15N 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. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email