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Abstract

Oxygen and pH microelectrodes were used to investigate the microenvironment of the planktonic foraminifer Orbulina universa and its dinoflagellate endosymbionts. A diffusive boundary layer surrounds the foraminiferal shell and limits the O-2 and proton transport from the shell to the ambient seawater and vice versa, Due to symbiont photosynthesis, high O-2 concentrations of up to 206% air saturation and a pH of up to 8.8, i.e. 0.5 pH units above ambient seawater, were measured at the shell surface of the foraminifer at saturating irradiances. The respiration of the host-symbiont system in darkness decreased the O-2 concentration at the shell surface to < 70% of the oxygen content in the surrounding air-saturated water. The pH at the shell surface dropped to 7.9 in darkness. We measured a mean gross photosynthetic rate of 8.5 +/- 4.0 nmol O-2 h(-1) foraminifer(-1). The net photosynthesis averaged 5.3 +/- 2.7 nmol O-2 h(-1). In the light, the calculated respiration rates reached 3.9 +/- 1.9 nmol O-2 h(-1), whereas the dark respiration rates were significantly lower (1.7 +/- 0.7 nmol O-2 h(-1)). Experimental light-dark cycles demonstrated a very dynamic response of the symbionts to changing light conditions. Gross photosynthesis versus scalar irradiance curves (P vs E-o curves) showed light saturation irradiances (E-k) Of 75 and 137 mu mol photons m(-2) s(-1) in two O. universa specimens, respectively. No inhibition of photosynthesis was observed at irradiance levels up to 700 mu mol photons m(-2) s(-1). The light compensation point of the symbiotic association was 50 mu mol photons m(-2) s(-1). Radial profile measurements of scalar irradiance (E-o) inside the foraminifera showed a slight increase at the shell surface up to 105% of the incident irradiance (E-d).