Stable isotopic analysis of the major biochemically important elements is an increasingly utilized tool in the study of ecology. Patterns of isotopic fractionation in carbon and nitrogen are used to determine trophic linkages, nutrient pathways, and sources of primary production in numerous contexts. Traditional techniques rely on measurements made of bulk samples such as tissue, but emerging methods using individual chemical compounds provide a means of achieving deeper understanding in a variety of inquiries.
Amino acids, as the building blocks of proteins, are the dominant nitrogen-bearing biomolecules and are a major constituent of all life. Patterns of isotopic fractionation during synthesis and transformations of these compounds record a variety of information about their environmental history. The purpose of this study was to utilize amino acid-specific isotopic analysis to address a variety of questions in paleoecology and trophic ecology, with an eye towards overcoming limitations inherent in bulk analyses applied to these fields.
Organic matter preserved in shells provides an archive of compounds that are typically lost quickly from the environment, such as proteins. It can be used as an analog of the soft body parts typically used isotope-based environmental measures and therefore provide a window on past environmental conditions, if it can be demonstrated that the two are chemically equivalent. Compositional differences between tissue and shell organic matter can obscure this relationship in bulk analyses, so a compound-specific approach is needed to accurately test this idea. Comparison of amino acid δ13C values between hinge muscle tissue and shell organic matter in Crassostrea virginica sampled along an estuarine salinity gradient in Rookery Bay, Florida, demonstrated functional equivalency between them. While minor isotopic offsets were observed between them, likely due to differences in turnover time between tissues, the ability to resolve location within the estuary was identical between them. This suggests that amino acid-specific isotopic measurements from shell organic matter can be effectively used in paleoreconstructions.
A complicating factor in using shell organic matter as a surrogate for body tissues arises from interspecies differences in organic matrix composition. This manifests itself as species-specific isotopic offsets in bulk analyses, making meaningful comparisons across species difficult. Amino acid nitrogen isotope analysis of a suite of mollusks from St Joe Bay, Florida, was used to infer trophic positions from shell organics and the results obtained were compared to equivalent tests using body tissue. Despite the inability to reconstruct trophic position from bulk isotopic compositions, shell organic extracts produced an identical description of trophic levels to that obtained using body tissues. CSIA, therefore, can be used to eliminate compositional biases in interspecies comparisons using organic matter preserved in biominerals.
Trophic level determinations derived from measurements of amino acid δ 15N benefit from that fact that a subset of these compounds directly record ecosystem isotopic baseline values. It is therefore possible to parse the source of observed changes in bulk isotopic compositions into contributions from both baseline variation and trophic shifts. By correcting for the possibility of baseline changes, unambiguous assignments of trophic position can therefore be made. Application this technique to a collection of Bairdiella chrysoura spanning multiple year classes was used to demonstrate the timing of ontogenetic diet shifts in this species. Juveniles (<80 mm standard length) were found to consistently occupy a lower trophic level than adults (>80 mm) based upon Δ15Nglutamic acid-glycine . Results obtained were compatible with previous estimates of ontogenetic effects on prey preferences derived from gut-content analysis.
Amino acid 15N-based trophic determinations assume near-constant trophic fractionation in each compound, regardless of an organism's health or nutritional status. The fact that this fractionation is influenced by internal nitrogen processing within the organism argues against such consistency, however. Effects of condition on the magnitude of compound-specific fractionation and resulting trophic estimates were tested by comparing dry season and wet season samples of Anchoa mitchilli subject to seasonal starvation in the latter period from the Alafia River, Florida. Despite significantly lower measures of condition (as measured by length:weight ratios) in the wet season fish, no differences in amino acid fractionation were detectable between them. This suggests that amino acid-specific fractionation factors are in fact robust to changes in organism health, although more rigorous assessment of this in culture is required.
|Commitee:||Herbert, Gregory, Peebles, Ernst, Robbins, Lisa, Van Vleet, Edward|
|School:||University of South Florida|
|School Location:||United States -- Florida|
|Source:||DAI-B 73/09(E), Dissertation Abstracts International|
|Subjects:||Chemistry, Biochemistry, Nuclear chemistry|
|Keywords:||Amino acids, Carbon, Isotopic fractionation, Nitrogen, Proteins, Stable isotopes|
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