Microsensor measurements of CO 2 , O 2 , pH and Ca 2' in the vicinity of the symbiont-bearing planktonic foraminifer Orbulina universa showed major light-modulated changes in the chemical microenvironment due to symbiont photosynthesis, respiration of the holobiont, and precipitation of the calcite shell. Under saturating light conditions, microprofiles measured towards the shell surface showed an O 2 increase of up to 220% air saturation, a decrease in CO 2 concentration to 4.9 mM, and a pH increase to 8.8 due to symbiont photosynthesis. The Ca 2' concentration decreased to Â/9.6 mM in two specimens, while it increased to 10.2 Á/10.8 mM in three other specimens kept in light. In darkness, the respiration of the community decreased the O 2 concentration to 82% of air saturation, CO 2 increased up to 15 mM, the pH decreased to 8.0, and the Ca 2' concentration increased up to 10.4 mM. These data, and derived calculations of the distribution of HCO 3 ( and CO 3 2( near the shell, showed that the carbonate system in the vicinity of O. universa was significantly different from conditions in the surrounding seawater, both in light and darkness, due to the metabolism of the foraminifer and its associated algae. Experimental light Á/dark cycles indicated a sufficient CO 2 supply sustaining high carbon fixation rates of the symbiotic algae via conversion of HCO 3 ( or via CO 2 release from calcification and host respiration. Our findings on irradiance-dependent CO 2 and pH changes in the vicinity of symbiont-bearing planktonic foraminifera give direct experimental evidence for the predictions of isotope fractionation models used in palaeoclimatology stating that metabolic processes affect the isotopic carbon signal (d 13 C) in foraminifera.
The pH on the frustule of individual cells of the marine centric diatoms Coscinodiscus granii and Coscinodiscus wailesii (Bacillariophyceae) was measured with pH microsensors in culture media with increasing pH values of 8.04, 8.14, and 8.22, respectively. In 85–96% of the C.granii cells the pH on the frustule was up to 0.4 units higher than that of the medium, reaching a maximum pH 8.95. Only in 2–3% the surface pH exceeded that of the medium by up to 0.7 pH units. These results strongly suggest that diatoms in batch cultures differ, at least temporarily, in their individual photosynthetic activities. Infection experiments with the parasitoid nanoflagellate Pirsonia diadema (Stramenopile) showed that flagellates failed to infect when the culture pH was 8.8 and above. pH measurements on freshly infected C. granii showed that the prevalence of infection was higher in tendency on diatoms with low surface pH. Application of these results to parasitoid-diatom interactions in natural waters suggests that within phytoplankton populations a strong photosynthetic activity might prevent diatom cells temporarily from infection by pH-sensitive parasitoids.
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