STABLE ISOTOPE ANALYSIS OF BONE COLLAGEN

Sequentially deposited tissues, such as teeth in marine mammals and bone in marine turtles, contain records of habitat use and diet in the form of stable isotopes (δ13C, δ15N) that can be used to study the ecology of individuals throughout their lifetime. However, bone tissue consists of two distinct components, collagen and bioapatite, that are synthesized through different biosynthetic pathways and from different components of an animal's diet (collagen: protein only; bioapatite: protein, carbohydrates, and lipids), which results in isotopic fractionation differing between the two tissue types. As a result, stable isotope analyses utilizing bone tissue typically isolate the collagen component of bone using an acidification method that removes lipids, minerals, and other contaminants. However, this method requires quantities of sample greater than what can be collected from individual sea turtle growth increments.

We developed standardized protocols to test for the effects of pre-SIA acidification on the SI values of bone growth layers to: (1) identify a simplified method to process cortical bone samples in order to obtain accurate, protein-based δ13C and δ15N values; and (2) estimate potential SI correction factors that can be used when limited amounts of bone material prevent pre-SIA acidification. We also assessed the need to lipid extract cortical bone samples.

We found that acidification of micromilled bone samples from marine turtles (Northwest Atlantic & Pacific loggerheads, Pacific greens) does not affect their δ15N values, but that there was a small effect on δ13C values that could be mathematically corrected for. In addition, the lipid content of cortical bone was low, as indicated by C:N ratios being under 3.5. Therefore, pre-acidification and lipid removal are not recommended once a species-specific correction factor is developed.

The primary collaborators for this project included: Calandra N. Turner Tomaszewicz (UCSD), Jeffrey Seminoff (NOAA SWFSC), and Carolyn M. Kurle (UCSD).