Amino acid racemization in four species of ostracodes: Taxonomic,environmental, and microstructural controls

Here we quantified the aspartic acid and glutamic acid racemization rates of the four main ostracode species (Herpetocypris reptans, Candona neglecta, Ilyocypris gibba and Cyprideis torosa) present in several Iberian Peninsula localities covering a wide chronological range (ca. 1 Ma to present). At low D/L values (at Asp D/L < 0.40; and Glu D/L = 0.09–0.18), H. reptans racemized at higher rates than C. neglecta, C. torosa and I. gibba. In contrast, for Asp D/L > 0.4 and Glu D/L > 0.18, H. reptans, C. neglecta and C. torosa showed similar racemization rates. I. gibba exhibited the lowest D/L values in old samples (Middle and Lower Pleistocene). We attribute these differences in amino acid racemization rates mainly to variations in valve protein composition. We found that the microstructure of the valves of each species (size, morphology, and arrangement of crystals) differed, but did not appear to change over time (at least for the last ca. 1 Ma). Such differences may also be linked to the type of proteins involved in the respective calcification processes of these organisms. On the basis of our results, and given that other studies have demonstrated that the majority of inter-crystalline proteins are leached early after death (a few centuries or millennia), we propose that the degradation rates of the most resistant inter- and intra-crystalline proteins in each species differ depending on the protein composition of the valves. Although further research is required, we suggest that amino acid racemization in each ostracode species might be related to valve microstructure.