Symbiont-influenced isotopic disequilibrium in Heterostegina depressa

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Abstract
Stable oxygen and carbon isotopic determinations on living specimens of H. depressa d''Orbigny reveal the magnitude of isotopic disequilibrium due to the interaction between the host foraminifer and its intracellular symbionts. Analyses were performed on specimens from in situ collections over a range of water depths (0-55 m) and light conditions off Oahu, Hawaii [USA] and from laboratory culturing experiments with varying light conditions. The calcite tests of H. depressa become increasingly depleted in 18O and 13C with increased illumination levels. The magnitude of this light-induced isotopic effect is similar in the laboratory and in situ specimens, and suggests that the photosynthetic activity of the intracellular symbionts exerts substantial control over the isotopic composition of H. depressa.