Consumer d15N

You Are What You Eat… Sort Of

d15NTuna = d15NPrey + 2–5‰D

d15N = 12‰

d15N = 15‰

14NH3

Biochemical–Physiological Processes:Deamination: Lose 14N

14NH3

8.5

9.0

9.5

10.0

10.5

11.0

11.5

12.0

12.5

13.0

-80 -40 0 40 80 120 160 200 240 280 320 360 400 440

d15 N

Days Since Birth

Anne (Mom)

Tessa (Daughter)

Jakle et al. unpublished

Tessa

Anne

Quinn (n=2)

Amino Acids

Peptide Bonds

Amino Acid

Glucose

3-P-Glycerate

Pyruvate

Acetyl - CoA

Glycolysis

These 15 amino acids represent ~85–90% of amino acids in animal tissues

Essential AAsLeucine

IsoleucinePhenylalanine

ThreonineValineLysine

*Tyrosine

Alanine

Glycine

Serine

Proline*

a-Ketoglutarate Glutamate

Glutamine

Oxaloacetate

AspartateAsparagine

TCACycle

Arginine*

Assimilation:Isotopic Incorporation & Protein Routing

Biosynthesis:Protein, Lipids, Carbohydrates

Excretion(12CO2)

Tissue-DietDiscrimination

(D13C)

Dietd13CProtein

d13CCarbohydrates

d13CLipids

Tissues (Protein)(13C-enriched)

AminoAcid Pool

BodyProtein

GlucosePool

Glycogen NeutralFat

GlycerolFattyAcids

Food

AcetylGroup

Lipid Synthesis

TCA Cycle

12CO2

D13C Discrimination:Decarboxylation (Lose 12CO2)

D13C: You Are What You Eat +0–2‰

212CO213C

3-Phosphoglycerate

Pyruvate

Acetyl CoA

Glycolysis

Assimilation:Isotopic Incorporation & Protein Routing

Biosynthesis:Amino Acids, Nucleic Acids

Excretion(14NH3)

Tissue-DietDiscrimination

(D15N)

Dietd15NProtein

Tissues (Protein)(15N-enriched)

AminoAcid Pool

BodyProtein

GlucosePool

Glycogen NeutralFat

GlycerolFattyAcids

Food

3-Phosphoglycerate

PyruvateGlycolysis

TCA Cycle

Acetyl CoA

D15N Discrimination:Amino Acid Catabolism

Deamination (Lose 14NH3)14NH3

Urea

Glutamate

Glutamine 15N

D15N: You Are What You Eat +2–5‰

NH3+ C H

COO-

R

DietProtein

orBody

Protein

C O

COO-

CH2

CH2

COO-

a-Ketoglutarate NH3+ C H

COO-

CH2

CH2

COO-

Glutamate

C O

COO-

R

a-KetoAcids

Transamination

14NH3

Deamination

Direction influenced by [glutamate], [a-ketoglutarate], [NH3], [enzymes]

NH2O

NH3+ C H

COO-

CH2

CH2

C

Glutamine(tissues)

Deamination

NH3+ C H

COO-

CH3

Alanine

C O

COO-

CH3

Pyruvate

Deamination

14NH3Urea – (14NH2)2CO

Uric Acid – C5H414N4O3

Ammonia – NH3

Urea – (NH2)2CO

• More complex to synthesize, but relatively nontoxic

• Excreted by mammals and some marine fish (sharks)

• Results in large D15N

• Most complicated synthesis, but least toxic form

• Excreted by birds, reptiles, and some insects

• Results in large D15NUric AcidC5H4N4O3

• Simplest form to produce, but highly toxic

• Excreted by fully aquatic animals and invertebrates

• Results in small D15N

Vanderklift and Ponsard 2003

Mammals BirdsReptiles

InvertsFish

BatsSpiders

D15

N

Uric Acid – C5H4N4O3

Ammonia – NH3

Urea – (NH2)2CO

Consumers have higher isotopic values than their foodAnimals preferentially retain 15N and excrete 14N

‘Sapivorous’ Insects

1. Dietary Protein Content

2. Dietary Protein Quality

3. Animal Nutritional Status

Vanderklift and Ponsard 2003

D15

N

Kelly and Martinez del Rio 2010

Percent Dietary Protein

Nile Tilapia

Oreochromis niloticus

Robbins et al. 2005

D15 N

Biological Value (%)

Mammals

d15 N

Whisker Length (mm)

Lubcker Whitemande Bruyn

Lubcker et al. 2016, 2017, 2020

Mirounga leonina

Marion Island, South AfricaPost-Weaning Fast

Catabolic

IndependentForaging

Anabolic

-5

0

5

10

15

20

25

30

Ala Asp Glu Ile Leu Val Pro Gly Ser Phe Lys Tyr

Amino Acid

d15 N

Fasting (endogenous)Independent (exogenous)

*

*

*

*

* **

Asp Pro Gly Ser Phe Lys Tyr

Lubcker et al. 2020

Exogenous(dietary protein)

Endogenous(skeletal muscle)

d15N

+

–

Ala, Ser, Gly, Glu, Pro, Asp

Phe, Tyr(glucogenic)

Lys(ketogenic)

(plasma)

15N-AAs 14N-Urea

(pee)

D15N varies with dietary protein content, protein quality, and catabolism:D15N increases positively with dietary protein contentD15N decreases with increasing dietary protein quality

D15N increases with increasing catabolism of endogenous protein (muscle).

Consumers have higher isotopic values than their foodAnimals retain 15N, excreting 14N via NH3, urea, and uric acid.

Interpretation of isotope data relies on knowing which factors controlthe assimilation, synthesis, and incorporation of isotopes into tissues.