227 103 103 2 2 B. ter Kuile J. Erez E. Padan The Interuniversity Institute of Eilat The Hebrew University of Jerusalem P.O. Box 469 88103 Eilat Israel Department of Geology The Hebrew University of Jerusalem 91904 Jerusalem Israel The Division of Microbial and Molecular Ecology, Institute of Life Sciences The Hebrew University of Jerusalem 91904 Jerusalem Israel Unit for Tropical Diseases The International Institute for Cellular and Molecular Pathology Avenue Hippocrate 74.39 B-1200 Brussels Belgium Abstract The processes of photosynthesis and calcification in the symbiont-bearing foraminifer Amphistegina lobifera compete for inorganic carbon (Ci). This conclusion is based on kinetic carbon uptake experiments performed in spring 1987 on specimens from the Gulf of Eilat, Red Sea, that were preincubated in Ci-free seawater in order to empty their internal carbon pools. Photosynthetic rates at 0.8 and 2.2 m M external Ci concentration were initially high and decreased with time while calcification rates started low and increased with time. At Ci=4.2 m M , when inward diffusion of Ci is not rate limiting, both rates were high from the beginning of the experiment. Calcification rates of specimens incubated with 3(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), to inhibit photosynthesis, were high and constant at all Ci levels. Addition of carbonic anhydrase (CA) to the medium, which catalyses the conversion of HCO 3 - to CO 2 , stimulated photosynthesis but inhibited calcification, probably because it caused the collapse of the internal Ci pool for calcification. Attempts to measure natural CA activity in the foraminifera gave negative results. These observations cannot be explained by models featuring mutual enhancement of photosynthesis and calcification, but instead suggest competition for Ci between these two processes.
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