Sources of carbon and nitrogen in modern planktonic foraminifera: the role of algal symbionts as determined by bulk and compound specific stable isotopic analyses

Juvenile specimens of the symbiont-bearing planktonic foraminifera, Orbulina universa, and the symbiont-free planktonic foraminifera, Globigerina bulloides, were collected by scuba divers off the coast of Santa Catalina Island, California, U.S.A. These specimens were cultured under controlled laboratory conditions and the foraminifera were fed daily a single Artemia nauplii of known isotopic composition. The bulk 15N data shows the typical 3‰, enrichment for the non-symbiont bearing species, G. bulloides. The symbiont-bearing species, O. universa, are enriched in 15N by 1.5‰, indicating a contribution of nitrogen to the foraminifera by the endosymbionts. The carbon isotope values show that both species are depleted by up to 3.5‰ in 13C relative to their A. nauplii diet. Compound-specific stable isotope data on the fatty acids are also depleted in 13C relative to those from the A. nauplii. These values reflect the characteristic isotope fractionations associated with biosynthesis of these compounds, indicating possible synthesis by both species of foraminifera. The stable carbon isotopic values of the majority of amino acids from the foraminifera are also consistent with biosynthesis. However, amino acids, such as glutamic acid, valine and isoleucine are directly incorporated from the diet. These isotopic data provide insight into the source of metabolic carbon and nitrogen used for amino acid and fatty acid synthesis in both species of foraminifera. Our data suggest that diet is the sole source of metabolic carbon and nitrogen used for amino acid and fatty acid synthesis in G. bulloides, whereas the isotopic data from O. universa are consistent with two isotopically distinct sources of carbon and nitrogen. The isotopic data from O. universa are indicative of the transfer of isotopically heavy metabolic carbon and nitrogen from its symbionts and relatively lighter carbon and nitrogen from the diet.