Fractionation in Animals

Question 1

Why do animals vary in their isotopic composition?

Answer 1

• different dietary sources contribute different proportions to the animal’s dietary signature

carbon isotopes vary considerably between dietary sources, but less so among trophic levels === varies with diet source

nitrogen isotopes vary significantly between trophic levels (“biomagnify”), but less so between dietary sources === varies with diet source and trophic position

Question 2

Which isotope is referred to ‘you are what you eat + a little bit more’ ? Explain

Answer 2

nitrogen stable isotope

the little bit more is the ~3.5 ppt fractionation that occurs between trophic levels

the N stable isotope varies a bit between dietary sources, but more between trophic levels (differences between TPs)

ex. a herbivore that consumes a plant with a d15N = 5 ppt will have a d15N = 8.5 ppt, a carnivore that eats that herbivore will have d15N = 13 ppt

Question 3

Which isotope is referred to ‘you are more or less what you eat’ ? Explain

Answer 3

carbon stable isotopes

they vary a lot between dietary sources, but not among trophic levels - ex. a carnivore d13C will reflect the d13C of the base of the food web (no difference between trophic positions)

Question 4

What is the assumption for big delta Dtrophic for d15N? for d13C? where Dtrophic = danimal - ddiet. Are these assumptions valid?

Answer 4

Dtrophic for d15N = ~3.5 ppt (an average)

Dtrophic for d13C = 0-1 ppt (if there is variation) because there is very little difference between trophic positions in d13C

Question 5

How does Dtrophic = danimal - ddiet compare to Dphotosynthesis = d13CCO2 - d13Cleaf?

Answer 5

D(A-B) = dA - dB where A = substrate and B = product

the Dtrophic equation just reverses the substrate and product to keep the values easy to work with

Question 6

Why does Dtrophic happen at all?

Answer 6

Nitrogen can exist in different forms - ex. excretion can be urea, ammonium, etc.

fractionation of d15N occurs at all trophic positions - food is chewed, digested, and nutrition is extracted and sent to muscles or tissues with specific biochemistry

Question 7

Why does Dtrophic for d15N change and d13C doesn’t change?

Question 8

Why might Dtrophic of d13C occur?

consider the graph with 4 animals fed different herbivorous diets that looks at the d13C in their respired CO2, feces, adult body, and diet

Answer 8

There are different forms which it can take (ex. respiration and feces, body tissues, and compared to the diet)

Question 9

Why is respired CO2, feces, adult body, and diet considered in measuring d13C of different organisms?

Answer 9

respired CO2 = d13C excreted as waste - what left - lighter C would be lost first

feces = d13C excreted in waste - what left

adult body = d13C assimilated into animal tissues
- what stayed

diet = d13C of diet source

Question 10

Why does animal d15N get increasingly heavier with increased trophic position?

Answer 10

N fractionation is a kinetic reaction, so the lighter isotopes react and are lost first
- light isotopes can be lost in respiration or excretion
- so whatever is incorporated into an animal that is consuming another organism will be heavier than what was lost in the waste - ex. NH4+

Question 11

What did the study about zooplankton and their NH4+ waste tell us about d15N?

Answer 11

d15N in the body is ~3-4 ppt heavier than what is excreted in the waste as ammonium

ammonium d15N excreted in waste is light relative to zooplankton body d15N

== overall, this tells us that d15N gets heavier with increasing trophic position

Question 12

What did Deniro and Epstein report for variation in Dtrophic between individuals on the same diet? how did they explain this?

Answer 12

there is interspecific and intraspecific variation in Dtrophic for organisms with the same diet

Question 13

Looking at the 2 graphs for carbon and nitrogen isotopes: what kind of variation do we see?

Answer 13

carbon d13C:
- variation amongst individuals of a species

d15N:
- variation between species
- variation within species

Question 14

Explain why the Dtrophic for d15N ~3.5 ppt glosses over a lot of information and same for why the Dtrophic for d13C ~0-1 ppt?

Answer 14

these values are averages for the ranges that exist
nature and food webs are highly complex and so these values do not and cannot include the level of complexity which exists

the 3.5 ppt is an average value calculated from a huge range (0.5-5.5 ppt)

the 0-1 ppt for Dtrophic of d13C was calculated from -3 to 4 ppt

Question 15

What are 2 major reasons Dtrophic for d15N have such a huge range (0.5-5.5 ppt)?

Answer 15

1. nitrogen excretion mechanisms are strongly correlated with Dtrophic
2. isotopes vary among tissues types

Question 16

Explain how nitrogen excretion mechanisms are strongly correlated with Dtrophic and how this affects Dtrophic for d15N

Answer 16

different animals have different metabolism pathways which converts types of N for excretion (urea, uric acid, ammonia, guanine) - the mechanism for metabolism and N excretion (physiology) has a huge influence on Dtrophic

Question 17

Explain how isotopes vary among tissues types and how this affects Dtrophic for d15N - what implications does this have for studying isotopes?

Answer 17

different tissues have different biochemical compositions - consider the brain, heart, hair, liver, lungs, muscles

1. different tissues have different growth rates
   - ex. hair and nails grow much faster than bone or muscle
2. different tissues have different isotopic turnover rates - how long will it take after switching a diet to see a change?
3. isotopic routing - specific diet components are routed to specific tissues - ex. in bone, collagen reflects isotopic signature of diet PROTEINS only but apatite reflects d13C of diet as a whole
4. biochemical composition (tissue synthesis and enzyme fractionation) - the concentrations of lipids v. glycogen v. proteins varies in different tissues depending on the function, environment, and animal behaviour - ex. bears preparing for hibernation will have different composition than post-hibernation

researchers have to consider what they are trying to learn and which tissues make the most sense to sample
- ex. sampling d15N from bone tissue would provide information about the early life stages of a bird (when bones were developing), but sampling from feathers may provide a more current representation, breath (respiration) even more short-term

Question 18

If studying the Dtrophic for the overall diet of an organism using tissues, why might apatite be a better choice than bone collagen?

Answer 18

isotopic routing causes tissues to vary from the diet source

collagen is a reflection of diet proteins only, but apatite (rich in carbonates) reflects the d13C of the diet as a whole

Question 19

Why do biochemicals vary in isotopic signatures?

Answer 19

different biochemical pathways for incorporation

• some directly incorporated from the diet; ex. fatty acids
• some are synthesized by the animal; ex. fractionation by enzymes

Question 20

How does dietary protein (quality of diet) affect Dtrophic?

Answer 20

if an animal is starving for proteins, the body starts using up fat reserves for survival and eventually will break down muscles if needed = essentially, eating feeding on itself

this causes d15N to artificially increase because it’s consuming heavy isotopes, fractionating them, and incorporating heavier isotopes

ex. Daphnia fed on same kind of algae but they modified the [protein] in the diets to study starvation impacts on d15N –> not changing trophic position, but Dtrophic and d15N animal increased!

Question 21

What 4 major things causes animal fractionation? give examples from class for each

Answer 21

1. excretion or respiration of light isotopes from the animal increases (heavier) the isotopic signature of the animal compared to its diet

• ex. zooplankton excreted lighter NH4 (ammonium) than was in their body

2. interspecific differences between animals caused by differences in excretion or respiration (metabolic) pathways or biochemical/tissue composition, sizes, etc.

• different N excretion mechanisms - ex. excreting urea causes the heaviest Dtrophic v. excreting guanine was much lower

3. intraspecific differences caused by variation in traits (age, size, starvation, sex, behaviour) and variation among tissues

• ex. Daphnia fed on algae with varying [protein], those starved had inflated d15N in their bodies causing an increased Dtrophic

4. tissue variation caused by differences in isotopic turnover, routing, biochemical synthesis pathways

• ex. bone collagen v. apatite or breath gives different information