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, Or bulina universa, and the symbiont-free planktonic foraminifera, Globig erina bulloides, were collected by scuba divers off the coast of Santa Catalina Island, California, U.S.A. These specimens were cultured und er controlled laboratory conditions and the foraminifera were fed dail y a single Artemia nauplii of known isotopic composition. The bulk N-1 5 data shows the typical 3 parts per thousand, enrichment for the non- symbiont bearing species, G. bulloides. The symbiont-bearing species, O. universa, are enriched in N-13 by 1.5 parts per thousand, indicatin g a contribution of nitrogen to the foraminifera by the endosymbionts. The carbon isotope values show that both species are depleted by up t o 3.5 parts per thousand in C-13 relative to their A. nauplii diet. Co mpound-specific stable isotope data on the fatty acids are also deplet ed in C-13 relative to those from the A. nauplii. These values reflect the characteristic isotope fractionations associated with biosynthesi s of these compounds, indicating possible synthesis by both species of foraminifera. The stable carbon isotopic values of the majority of am ino acids from the foraminifera are also consistent with biosynthesis. However, amino acids, such as glutamic acid, valine and isoleucine ar e directly incorporated from the diet. These isotopic data provide ins ight into the source of metabolic carbon and nitrogen used for amino a cid 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 isotopical ly distinct sources of carbon and nitrogen. The isotopic data from O. universa are indicative of the transfer of isotopically heavy metaboli c carbon and nitrogen from its symbionts and relatively lighter carbon and nitrogen from the diet. (C) 1997 Elsevier Science Ltd.