Symbiotic photosynthesis in a planktonic foraminiferan, Globigerinoides sacculifer (Brady), studied with microelectrodes1

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Abstract
The chemical environment and the symbiotic photosynthesis of the spinose foraminiferan, Globigerinoides sacculifer, from the Gulf of Aqaba, northern Red Sea, were studied with O2 and pH microelectrodes at a resolution of 50–100 µm. In the dark, the foraminiferan respiration lowered the O2 concentration at the shell surface to 50% of air saturation, while pH was lowered from the ambient value of 8.23 to 8.15. In the light, the O2 increased to 2.5 times air saturation while pH reached 8.62. These steep chemical gradients were established over a diffusive boundary layer between the spines, which partly separated the animal from the bulk seawater. The O2 pool around the shell was very dynamic, with residence times down to 5 s in the light. Photosynthetic rates were mapped around and within the animal. The compensation light intensity of the symbiont‐host system was 26–30 µEinst m−2 s−1, while the light saturation intensity, Ik, was 160–170 µEinst m−2 s−1. The action spectrum of symbiont photosynthesis showed peaks at 450 and 670 nm due to chlorophyll a as well as a broad peridinin peak at 500–550 nm. Total respiration of one symbiont‐host system was 3.0 nmol O2 h−1 and was not significantly different between dark and light. Gross photosynthesis at light saturation was 18.1 nmol O2 h−1 per foraminiferan. The high net photosynthesis showed that the zooxanthellae could theoretically provide all organic carbon required for growth and respiration of the host. Model calculations indicated, however, that the foraminifera were diffusion‐limited in the uptake of dissolved nitrogen and phosphorus. The capture of prey was therefore a necessary source of these nutrients, while symbiotic photosynthesis could cover the energy requirements and allow efficient internal recycling of nutrients.