Ammonium is the preferred source of nitrogen for planktonic foraminifer and their dinoflagellate symbionts

The symbiotic planktonic foraminifera Orbulina universa inhabits open ocean oligotrophic ecosystems where dissolved nutrients are scarce and often limit biological productivity. It has previously been proposed that O. universa meets its nitrogen (N) requirements by preying on zooplankton, and that its symbiotic dinoflagellates recycle metabolic 'waste ammonium' for their N pool. However, these conclusions were derived from bulk ¹⁵N-enrichment experiments and model calculations, and our understanding of N assimilation and exchange between the foraminifer host cell and its symbiotic dinoflagellates remains poorly constrained. Here, we present data from pulse-chase experiments with ¹³C-enriched inorganic carbon, ¹⁵N-nitrate, and ¹⁵N-ammonium, as well as a ¹³C- and ¹⁵N- enriched heterotrophic food source, followed by TEM (transmission electron microscopy) coupled to NanoSIMS (nanoscale secondary ion mass spectrometry) isotopic imaging to visualize and quantify C and N assimilation and translocation in the symbiotic system. High levels of ¹⁵N-labelling were observed in the dinoflagellates and in foraminiferal organelles and cytoplasm after incubation with ¹⁵N-ammonium, indicating efficient ammonium assimilation. Only weak ¹⁵N-assimilation was observed after incubation with ¹⁵N-nitrate. Feeding foraminifers with ¹³C- and ¹⁵N-labelled food resulted in dinoflagellates that were labelled with ¹⁵N, thereby confirming the transfer of ¹⁵N-compounds from the digestive vacuoles of the foraminifer to the symbiotic dinoflagellates, likely through recycling of ammonium. These observations are important for N isotope-based palaeoceanographic reconstructions, as they show that δ¹⁵N values recorded in the organic matrix in symbiotic species likely reflect ammonium recycling rather than alternative N sources, such as nitrates.