4.1d Organismal Isotopes

Baseline (11)

What was the animal eating?

• Autotroph baseline values affected by

  • Source of inorganic carbon

    • Marine DIC ~ 2 δ13C

    • Atmospheric CO2 ~ -7 δ13C

  • photosynthetic pathways

    • Photosynthesis prefers 12C because it will diffuse quicker and react quicker

    • C3 plants and marine phytoplankton have similar fractionation

    • Macroagae & seagrasses use C3 but because of their DIC source, they have δ13C values closer to C4

    • C3 plants - fractionation about 20‰ lower than source C

    • C4 plants - fractionation about 6‰ lower than source C

      • Bundle sheath cell makes the process much more efficient

Uses

• Generalists vs. specialists - generalists may change between C3 and C4 sources depending on availability & season, while specialists only eat certain plants that are all C3 or C4

• Habitat use - ex. phytoplankton have lower δ13C than benthic algae, so you can use this to assess inshore (benthic) vs offshore (plankton) feeding

• Trophic position

Isotopic Incorporation (11)

When was the food consumed?

• Changes in diet take time to be reflected in tissues

• Different tissues have different turnover times

  • Metabolically Active

    • breath > Plasma, liver > Muscle, Red blood cells > Bone Collagen

  • Metabolically Inert

    • (short timeframe) Fur, feathers, egg shell, fingernails, breastmilk

    • (Accretionary) Teeth, hair, whiskers, baleen, otoliths

Physiology (12)

How did the animal incorporate the food?

Trophic fractionation

• Commonly generalized but there are large amounts of variation in these numbers:

• ∆13C +0.4 ‰ per trophic level

• ∆15N +3.4 ‰ per trophic level

N - Protein Cycling

• High dietary protein - convert protein to fat/glucose, high N excretion rates

• Low dietary protein - essential AA deficiency, use carbs & lipids for non-essential AA synthesis, low N excretion rates

• Amino Acids

  • The amino acid pool is comprised of protein already in the body, protein from the diet, and the synthesis of new amino acids from synthesis from the diet.

  • AA go into body protein, N compounds (enzymes?), and waste products

  • Transamination - transfer of an amino group from one molecule to another, especially from an amino acid to a keto acid.

    • diet/body protein transformed into alpha-keto acids

  • Deamination - removal of an amino group from an amino acid or other compound

• Quality of diet

  • a high-quality diet suggests that the AA composition of diet matches consumer so less fractionation will occur

  • herbivore - lower protein content, lower excretion rate, lower ∆15N

  • Carnivore - higher protein, higher excretion, higher TDF

• Mode of excretion

  • ∆15N highest in Urea producers (mammals) > Uric acid (insects) > Ammonia (fish) > Guanine (arachnids) > Amino acids (babies)

  • Urea excretion is complex in mammals, lots of opportunity for fractionation

• Nutritional Status

  • starving animal starts to catabolize its own endogenous protein stores

  • Catabolism - the breakdown of body molecules to form simpler ones

  • as animal loses weight its ∆15N decreases

The Omnivore's Dilemma

• Perfect Mixing - 50/50 split

• Perfect routing - 100% from one source