Why extract lipids from egg yolk?

Using isotope ratios has become an important technique to investigate nutrient pathways in ornithological studies. One popular application of this technique is to quantify the origin of nutrients allocated to eggs. Dry egg yolk contains > 50% lipids, which are known to be depleted in carbon isotope ratios compared to protein. This can lead to ambiguous results of isotope studies unless lipids in egg yolk are accounted for specifically.

Currently, most researchers remove lipids from egg yolk with a chemical lipid extraction procedure before analyzing the isotopic composition of protein in egg yolk. We found that this procedure significantly altered nitrogen and sulfur isotope ratios in eggs of 20 wild King Eiders and 15 captive Spectacled Eiders housed at the Alaska SeaLife Center. We therefore do not recommend that nitrogen or sulfur isotope ratios of lipid-extracted yolk samples are used for inference in isotopic studies.

This result indicates that yolk samples need to be analyzed in duplicate if reliable isotope measurements for carbon and for nitrogen or sulfur are required. As this approach doubles the cost and effort of isotope studies we explored whether an arithmetic correction could be used to correct the carbon isotope ratio of egg yolk for the amount of lipid contained in yolk. This calculation can be performed using standard output of isotope ratio mass spectrometry, and we provide a spreadsheet that demonstrates the calculation and can be used by researchers for their own data sets.

The arithmetic corection worked extremely well for captive Spectacled Eider eggs, because the birds had been fed a constant diet throughout their life. The correction was unreliable for wild migratory King Eider eggs, because King Eiders may transfer macronutrients (fat and protein) between isotopically distinct environments.

The graph on the right indicates the error of the arithmetic correction (in ‰) in relation to the C:N ratio of the whole yolk. The C:N ratio is indicative of the lipid content.

In wild migratory birds that may use stored body reserves for egg production the arithmetic correction is unreliable because the use of isotopically distinct body reserves violates one of the key assumptions of the model.

The schematic graph on the left demonstrates how the parameter D may assume values that differ from a constant value (here assumed to be 5‰) if lipids and proteins are derived from different environments.

We caution researchers to carefully consider the origin of lipid and protein components in egg yolk before using our model to correct yolk carbon isotope ratios for lipid content.


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Click here to download a spreadsheet to correct your whole yolk carbon isotope ratios for lipid content.

 

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