| 23 Apr 2026
Ground-truthing the application of compound-specific stable isotopes of amino acids to planktic foraminifera tests from Santa Barbara Basin
Abstract. Planktic foraminifera form shells that are preserved in ocean sediments and are used in a variety of paleoproxy and biostratigraphic applications. However, species-specific ecology can complicate the interpretation of planktic foraminifera-based proxies, and limited options exist for examining the ecology of extinct species. Here we apply test-bound compound-specific stable isotopes of amino acids (CSI-AA) to examine the trophic ecology of extant planktic foraminifera. We measure CSI-AA in planktic foraminifera shells collected in sediment traps from the Santa Barbara Basin, CA, specifically the three most abundant species in this region: Globigerina bulloides, Neogloboquadrina incompta, and Turborotalita quinqueloba. The nitrogen CSI-AA of all three species suggest that planktic foraminifera have metazoan-like metabolisms, and that trophic position estimates using CSI-AA are appropriate for planktic foraminifera. All three species had trophic positions near 2 (primary consumer), with no evidence for mixotrophy or photosymbionts. Carbon CSI-AA, in combination with a Bayesian stable isotope mixing model, indicates that the three species occupied separate niches based on diet. Globigerina bulloides fed opportunistically on all groups of phytoplankton available in Santa Barbara Basin, adjusting its diet with seasonal changes in phytoplankton assemblage. Turborotalita quinqueloba specialized in diatoms and heterotrophic bacteria. Neogloboquadrina incompta consumed heterotrophic bacteria and some phytoplankton. Our results align with the current understanding of each species’ ecology while further defining their niches. Our findings suggest that CSI-AA is a promising tool for understanding the trophic ecology of planktic foraminifera, and we make recommendations for future applications of CSI-AA to fossil specimens.
This manuscript demonstrates the compound-specific isotope analysis (CSIA) is a new tool for evaluating the trophic position and trophic niche of planktonic foraminifera species. I expect that the Authors worked hard on the present study, and I recognize that the data reported here are novel and valuable. Although I have no major concern in this manuscript, I feel that its quality could be improved before publication, because several parts of manuscript are insufficiently explained (which are described below).
Concerns:
1. The Authors measured the isotope ratios of amino acids preserved in foraminifera tests, however, this manuscript does not describe the procedure how the tests were purified. I think that the purification procedure should be included in this manuscript. Is there any fractionation of C and N isotopes of amino acids during the purification process?
2. The Authors compared the isotope ratios of Glx and Ala (and TP-Glx and TP-Ala), based on the cap-delta Glu and Ala reported in Décima et al., 2017; Chikaraishi et al., 2009. Although those cap-delta are likely common for major metazoans with trophic levels lower than 3 (because there are many literatures), I doubt whether these cap-delta values can be used generally for major protist species. The generality should be discussed in this manuscript, because it is critical to applying the findings of the present study to future studies.
3. On line 43, the Authors wrote ‘The foraminifera themselves can also assimilate inorganic nitrogen’. Does this mean that the foraminifera can biosynthesize many amino acids including essential amino acids, de novo? If so, how does this biosynthetic ability impact the isotope results of the present study? Please explain what this sentence means.