The fate of nitrogen in the Orbulina universa foraminiferasymbiont system determined by nitrogen isotope analyses of shell-bound organic matter
Limnol. Oceanogr., 44(8), 1999, 1968-1977 | DOI: 10.4319/lo.19126.96.36.1998
ABSTRACT: To trace the flow of nitrogen through the foraminifera-symbiont system, juvenile specimens of the symbiont-bearing planktonic foraminifera, Orbulina universa were collected by SCUBA divers off the coast of Santa Catalina Island, California, and grown in filtered seawater solutions spiked with 15N-labeled nitrate. Our work isolated each pathway of this symbiotic system to determine the amount of nitrogen translocated to the foraminifera from its endosymbionts and its captured diet. Our model results show that when the nitrate uptake by the symbionts is at a maximum, between 50 and 57% of the foraminiferal nitrogen was translocated from the symbionts and the remainder was derived from the captured diet. In nitrate-deficient environments, when the symbiont nitrate uptake was at a minimum, ~90-100% of the nitrogen was transferred to the foraminifer by the symbionts from the recycled nitrogen (NH4+) pool. In nitrate-deficient conditions, the primary role of the captured diet may be to provide the system with phosphorous. Highly efficient nutrient use is particularly important to the foraminifera-symbiont system, especially when nutrient concentrations are low. The results indicate a limited correlation between the isotopic composition of the NO3- in the culture solutions and the δ15n values of the foraminifera. However, the δ15n values of the foraminifera much more strongly reflect the δ15n values of the NH4+ recycled through the system. It appears that recycled nitrogen (NH4+) is a more important source of nitrogen to the symbionts and the foraminifera than nitrate; therefore, the δ15n values of the foraminifera may not reflect the isotopic composition of the surface-water nutrients. Understanding the nitrogen flow within modern foraminifera-symbiont associations is also important to ancient marine systems, because symbiont-bearing foraminifera are ubiquitous in the fossil record. This study shows that analysis of individual, symbiont-bearing foraminifera species is a necessary first step toward the development of more reliable use of nitrogen isotopes for paleoceanographic reconstructions.