Compound‐specific stable isotope analysis (
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 acid
- Award ID(s):
- 2049307
- NSF-PAR ID:
- 10419140
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Limnology and Oceanography
- Volume:
- 68
- Issue:
- 6
- ISSN:
- 0024-3590
- Page Range / eLocation ID:
- p. 1331-1345
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract CSIA ) of amino acids (AA ) has rapidly become a powerful tool in studies of food web architecture, resource use, and biogeochemical cycling. However, applications to avian ecology have been limited because no controlled studies have examined the patterns inAA isotope fractionation in birds. We conducted a controlledCSIA feeding experiment on an avian species, the gentoo penguin (Pygoscelis papua ), to examine patterns in individualAA carbon and nitrogen stable isotope fractionation between diet (D) and consumer (C) (Δ13CC‐Dand Δ15NC‐D, respectively). We found that essentialAA δ 13C values and sourceAA δ 15N values in feathers showed minimal trophic fractionation between diet and consumer, providing independent but complimentary archival proxies for primary producers and nitrogen sources respectively, at the base of food webs supporting penguins. Variations in nonessentialAA Δ13CC‐Dvalues reflected differences in macromolecule sources used for biosynthesis (e.g., protein vs. lipids) and provided a metric to assess resource utilization. The avian‐specific nitrogen trophic discrimination factor (TDF Glu‐Phe= 3.5 ± 0.4‰) that we calculated from the difference in trophic fractionation (Δ15NC ‐D) of glutamic acid and phenylalanine was significantly lower than the conventional literature value of 7.6‰. Trophic positions of five species of wild penguins calculated using a multi‐TDFG lu‐Pheequation with the avian‐specificTDFG lu‐Phevalue from our experiment provided estimates that were more ecologically realistic than estimates using a singleTDFG lu‐Pheof 7.6‰ from the previous literature. Our results provide a quantitative, mechanistic framework for the use ofCSIA in nonlethal, archival feathers to study the movement and foraging ecology of avian consumers. -
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Abstract Animals often consume resources from multiple energy channels, thereby connecting food webs and driving trophic structure. Such ‘multichannel feeding’ can dictate ecosystem function and stability, but tools to quantify this process are lacking. Stable isotope ‘fingerprints’ are unique patterns in essential amino acid (EAA) δ13C values that vary consistently between energy channels like primary production and detritus, and they have emerged as a tool to trace energy flow in wild systems. Because animals cannot synthesize EAAs de novo and must acquire them from dietary proteins, ecologists often assume δ13C fingerprints travel through food webs unaltered. Numerous studies have used this approach to quantify energy flow and multichannel feeding in animals, but δ13C fingerprinting has never been experimentally tested in a vertebrate consumer.
We tested the efficacy of δ13C fingerprinting using captive deer mice
Peromyscus maniculatus raised on diets containing bacterial, fungal and plant protein, as well as a combination of all three sources. We measured the transfer of δ13C fingerprints from diet to consumer liver, muscle and bone collagen, and we used linear discriminant analysis (LDA) and isotopic mixing models to estimate dietary proportions compared to known contributions. Lastly, we tested the use of published δ13C source fingerprints previously used to estimate energy flow and multichannel feeding by consumers.We found that EAA δ13C values exhibit significant isotopic (i.e. trophic) fractionation between consumer tissues and diets. Nevertheless, LDA revealed that δ13C fingerprints are consistently routed and assimilated into consumer tissues, regardless of isotopic incorporation rate. Isotopic mixing models accurately estimated the proportional diets of consumers, but all models overestimated plant‐based protein contributions, likely due to the digestive efficiencies of protein sources. Lastly, we found that δ13C source fingerprints from published literature can lead to erroneous diet reconstruction.
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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‐specificβ value variability than typically appreciated.This meta‐analysis fulfils a pressing need to comprehensively evaluate relevant sources of
β value 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 andβ value variability. Lastly, we evaluated the sensitivity of TPCSIAestimates to uncertainty in meanβ Glx‐Phevalues and Glx‐Phe trophic discrimination factors (TDFGlx‐Phe).We show that variation in
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β values are integral to accurate trophic position estimation. We outline four key recommendations for identifying, constraining and accounting forβ value 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 ofβ value variation. -
Midwater zooplankton are major agents of biogeochemical transformation in the open ocean; however their characteristics and activity remain poorly known. Here we evaluate midwater zooplankton biomass, amino acid (AA)-specific stable isotope composition (δ15N values) using compound-specific isotope analysis of amino acids (CSIA-AA), trophic position, and elemental composition in the North Pacific Subtropical Gyre (NPSG). We focus on zooplankton collected in the winter, spring, and summer to evaluate midwater trophic dynamics over a seasonal cycle. For the first time we find that midwater zooplankton respond strongly to seasonal changes in production and export in the NPSG. In summer, when export from the euphotic zone is elevated and this ‘summer pulse’ material is transported rapidly to depth, CSIA-AA indicates that large particles (> 53 μm) dominate the food web base for zooplankton throughout the midwaters, and to a large extent even into the upper bathypelagic zone. In winter, when export is low, zooplankton in the mid-mesopelagic zone continue to rely on large particle basal resources, but resident zooplankton in the lower mesopelagic and upper bathypelagic zones switch to include smaller particles (0.7–53 μm) in their food web base, or even a subset of the small particle pool. Midwater zooplankton migration patterns also vary with season, with migrants distributed more evenly at night through the euphotic zone in summer as compared to being more compressed in the upper mixed layer in winter. Deeper zooplankton migration within the mesopelagic zone is also reduced in late summer, likely due to the increased magnitude of large particle material available at depth during this season. Our observed seasonal change in activity and trophic dynamics drives modestly greater biomass in summer than winter through the mesopelagic zone. In contrast midwater zooplankton carbon (C), nitrogen (N), and phosphorus (P) composition does not change with season. Instead we find increasing C:N, C:P, and N:P ratios with greater depths, likely due to decreases in proteinaceous structures and organic P compounds and increases in storage lipids with depth. Our study highlights the importance and diversity of feeding strategies for small zooplankton in NPSG midwaters. Many small zooplankton, such as oncaeid and oithonid copepods, are able to access small particle resources at depth and may be an important trophic link between the microbial loop and deep dwelling micronekton species that also rely on small particle-based food webs. Our observed midwater zooplankton trophic response to export-driven variation in the particle field at depth has important implications for midwater metabolism and the export of C to the deep sea.more » « less
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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
δ 15N values.Methods We evaluated the effects of chemical preservatives on bulk tissue
δ 13C andδ 15N and amino acidδ 15N values, measured by gas chromatography/isotope ratio mass spectrometry (GC/IRMS), of (a) tuna (Thunnus albacares Dosidicus gigas Calanus pacificus Eucalanus californicus Results Tissues in formaldehyde‐ethanol had higher bulk
δ 15N values (+1.4,D. gigas T. albacares δ 13C values forD. gigas δ 13C values forT. albacares δ 15N values from copepods were not different those from frozen samples, although theδ 13C values from both species were lower (−1.0‰ forE. californicus C. pacificus δ 15N values from chemically preserved tissues were largely within 1‰ of those of frozen tissues, but the phenylalanineδ 15N values were altered to a larger extent (range: 0.5–4.5‰).Conclusions The effects of preservation on bulk
δ 13C values were variable, where the direction and magnitude of change varied among taxa. The changes in bulkδ 15N values associated with chemical preservation were mostly minimal, suggesting that storage in formaldehyde or ethanol will not affect the interpretation ofδ 15N values used in ecological studies. The preservation effects on amino acidδ 15N values were also mostly minimal, mirroring bulkδ 15N trends, which is promising for future CSIA‐AA studies of archived specimens. However, there were substantial differences in phenylalanine and valineδ 15N 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.
