Me. Uhle et al., The fate of nitrogen in the Orbulina universa foraminifera-symbiont systemdetermined by nitrogen isotope analyses of shell-bound organic matter, LIMN OCEAN, 44(8), 1999, pp. 1968-1977
To trace the flow of nitrogen through the foraminifera-symbiont system, juv
enile specimens of the symbiont-bearing planktonic foraminifera, Orbulina u
niversa were collected by SCUBA divers off the coast of Santa Catalina Isla
nd, California, and grown in filtered seawater solutions spiked with N-15-l
abeled 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 n
itrate uptake by the symbionts is at a maximum, between 50 and 57% of the f
oraminiferal nitrogen was translocated from the symbionts and the remainder
was derived from the captured diet. In nitrate-deficient environments, whe
n the symbiont nitrate uptake was at a minimum, similar to 90-100% of the n
itrogen was transferred to the foraminifer by the symbionts from the recycl
ed nitrogen (NH,I) 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-symbio
nt system, especially when nutrient concentrations are low. The results ind
icate a limited correlation between the isotopic composition of the NO, in
the culture solutions and the delta(15)N values of the foraminifera. Howeve
r, the delta(15)N values of the foraminifera much more strongly reflect the
delta(15)N values of the NH4+ recycled through the system. It appears that
recycled nitrogen (NH4+) is a more important source of nitrogen to the sym
bionts and the foraminifera than nitrate; therefore, the delta(15)N values
of the foraminifera may not reflect the isotopic composition of the surface
-water nutrients. Understanding the nitrogen flow within modern foraminifer
a-symbiont associations is also important to ancient marine systems, becaus
e symbiont-bearing foraminifera are ubiquitous in the fossil record. This s
tudy shows that analysis of individual, symbiont-bearing foraminifera speci
es is a necessary first step toward the development of more reliable use of
nitrogen isotopes for paleoceanographic reconstructions.