Direct pathway of incorporating dietary nitrogen in shell-bound matrix of the planktic foraminifera Trilobatus sacculifer

Abstract

The stable isotopes of organic nitrogen (N) preserved within the fossil tests of foraminifera have been used to reconstruct past changes in surface ocean nitrogen cycling processes. Modern observations show temporal and spatial covariations of δ¹⁵N between planktic foraminifera and particulate organic matter in the surface ocean, suggesting that heterotrophic foraminifera record the N isotopic compositions of their diet. However, little is known about the underlying mechanisms of N translocation from diet into foraminiferal biomass (i.e., intrashell protoplasm) and finally embedded within their mineralized shells. We investigate the pathways of N uptake from diet into foraminiferal calcite tests by feeding the planktic, dinoflagellate-bearing Trilobatus sacculifer with two strains of brine shrimp (Artemia) with different ¹⁵N/¹⁴N isotopic compositions, and monitoring the δ¹⁵N evolution of both biomass and shell-bound matrix. The two feeding groups show comparable results, that δ¹⁵N of both the biomass and shell-bound matrix evolve towards their diet sources without expressing trophic enrichment in δ¹⁵N. However, we observe that the δ¹⁵N of the biomass and shell-bound matrix exhibit distinct mixing behaviors. Biomass δ¹⁵N is well modelled as a mixture between the original biomass and newly metabolised dietary intake, suggesting T. sacculifer has a closed N system with minimum N leakage. Meanwhile, shell-bound δ¹⁵N quickly approaches the δ¹⁵N of the diet, and is offset from the biomass δ¹⁵N. This indicates a direct pathway of N incorporation from the diet into shell-bound organics during calcification, without significant exchange with the overall biomass pool.