Friday, April 11, 2014

Investigation of Alternate Techniques for Stable Isotope Analysis of Sea Turtle Bone Collagen

Turner-Tomaszewicz, C. 1, Ramirez, M.D.2, Seminoff, J.A. 3, Avens, L.4, Shakya, A.1, McCartha, M.5, Heppell, S.2 and Kurle, C.K.1

1University of California San Diego, La Jolla, California, USA
2Oregon State University, Corvallis, Oregon, USA
3NOAA – National Marine Fisheries Service, Southwest Fisheries Science Center, La Jolla, California, USA
4NOAA – National Marine Fisheries Service, Southeast Fisheries Science Center, Beaufort, North Carolina USA
5University of Washington, Tacoma, Washington, USA

Summary:
Stable isotope analysis (SIA) of skeletal structures is increasingly used to reconstruct historical marine animal foraging ecology and life history. Nitrogen isotope values (δ15N) are used to examine trophic relationships as there is a predictable 3-5‰ increase in δ15N values with increasing trophic levels. Carbon isotope values (δ13C) from animal tissues can be used to reconstruct animal movement patterns due to spatial variation across productivity gradients and geographic regions. In order to accurately reconstruct sea turtle diet and habitat use through SIA of bone tissue, potential differences in the isotope values of the two main components of bone (70% bioapatite and 30% collagen by weight) must be addressed. The δ13C values from bone collagen reflect diet-based carbon, whereas the carbon source in inorganic apatite can vary and is therefore typically removed via acidification prior to SIA. Our research involves extracting samples from the individual growth layer rings of sea turtle humerus bones, which yields very small amounts of bone dust (~1.5mg), far less than is required for traditional SIA collagen preparation. The primary objective of this study was to test alternative methods of collagen extraction from small amounts of sea turtle bone. We measured differences in the δ13C and δ15N values in bulk bone and isolated collagen to determine the most accurate method for measurement of bone δ13C and δ15N values. We measured this in two sea turtle species representing three distinct populations across two ocean basins (North Pacific loggerhead (n=15, CCL 51-73cm), East Pacific green (n=15, CCL 53-73cm), and Northwest Atlantic loggerhead (n=20, CCL 70-96 cm)). SIA was performed on paired bulk bone and bone collagen samples that had been acid-treated (0.25M HCl) to remove inorganic carbon (total n=50). We measured differences in the δ13C and δ15N values in bulk bone and isolated collagen to determine the most accurate method for measurement of bone δ13C and δ15N values. We measured this in two sea turtle species representing three distinct populations across two ocean basins (North Pacific loggerhead (n=15, CCL 51-73cm), East Pacific green (n=15, CCL 53-73cm), and Northwest Atlantic loggerhead (n=20, CCL 70-96 cm)). SIA was performed on paired bulk bone and bone collagen samples that had been acid-treated (0.25M HCl) to remove inorganic carbon (total n=50). We tested a second method that involved sampling individual annual growth layers of 10 Pacific turtle bone sections, previously processed for skeletochronology. Effect of pre-SIA treatment on the δ13C and δ15N values was tested against ecologically significant “detection-differences” that are commonly used in migration and foraging studies (δ13C: 1‰ and δ15N: 3‰). At these detection differences, neither δ13C nor δ15N were ecologically significantly affected by either of the pre-SIA treatment methods, but there was a consistent depletion effect on δ13C.



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