Factors Controlling the Stable Nitrogen Isotopic Composition (δ(15)N) of Lipids in Marine Animals

Lipid extraction of biomass prior to stable isotope analysis is known to cause variable changes in the stable nitrogen isotopic composition (δ(15)N) of residual biomass. However, the underlying factors causing these changes are not yet clear. Here we address this issue by comparing the δ(15)N of bulk and residual biomass of several marine animal tissues (fish, crab, cockle, oyster, and polychaete), as well as the δ(15)N of the extracted lipids. As observed previously, lipid extraction led to a variable offset in δ(15)N of biomass (differences ranging from -2.3 to +1.8 ‰). Importantly, the total lipid extract (TLE) was highly depleted in (15)N compared to bulk biomass, and also highly variable (differences ranging from -14 to +0.7 ‰). The TLE consisted mainly of phosphatidylcholines, a group of lipids with one nitrogen atom in the headgroup. To elucidate the cause for the (15)N-depletion in the TLE, the δ(15)N of amino acids was determined, including serine because it is one of the main sources of nitrogen to N-containing lipids. Serine δ(15)N values differed by -7 to +2 ‰ from bulk biomass δ(15)N, and correlated well with the (15)N depletion in TLEs. On average, serine was less depleted (-3‰) than the TLE (-7 ‰), possibly due to fractionation during biosynthesis of N-containing headgroups, or that other nitrogen-containing compounds, such as urea and choline, or recycled nitrogen contribute to the nitrogen isotopic composition of the TLE. The depletion in (15)N of the TLE relative to biomass increased with the trophic level of the organisms.