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.