Abstract
Stable isotopic analyses were performed in cultured and in situ collected specimens of symbiont-bearing Heterostegina depressa to investigate the effect of symbiont photosynthesis on isotopic fractionation in foraminifera. Sibling groups from four different clones were cultured under identical conditions except for different light intensities. Living H. depressa were collected from water depths of 0.5–75 m off Oahu, Hawaii, for comparison with the culturing experiments. H. depressa individuals from both the cultures and natural environmental exhibit a marked vital effect in their oxygen and carbon isotopic composition (i.e, disequilibrium fractionation). The magnitude of this vital effect varied as a function of light intensity (increased photosynthesis by the symbionts) as well as age (possible metabolic effects). The correlation between isotopic composition and intensity of symbiont photosynthesis may be due to enhanced uptake of metabolic CO 2 during calcification, and, in addition, to recycling of photosynthetically fixed carbon (depleted in 13C and 18O) between the symbionts and the host foraminiferan. These relationships could also account for the vital effects observed in symbiont-bearing planktonic foraminifera.
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