Microsensor studies of photosynthesis and respiration in larger symbiotic foraminifera. I - The physico-chemical microenvironment of Marginopora vertebralis, Amphistegina lobifera and Amphisorus hemprichii

The physico-chemical microenvironment of larger benthic foraminifera was st udied with microsensors for O-2, CO2, PH, Ca2+ and scaler irradiance. Under saturating light conditions, the photosynthetic activity of the endosymbio tic algae increased the O-2 up to 183% air saturation and a pH of up to 8.6 was measured at the foraminiferal shell surface. The photosynthetic CO2 fi xation decreased the CO2 at the shell down to 4.7 muM. In the dark, the res piration of host and symbionts decreased the O-2 level to 91% air saturatio n and the CO2 concentration reached up to 12 muM. pH was lowered relative t o the ambient seawater pH of 8.2. The endosymbionts responded immediately t o changing light conditions, resulting in dynamic changes of O-2, CO2 and p H at the foraminiferal shell surface during experimentally imposed light-da rk cycles. The dynamic concentration changes demonstrated for the first tim e a fast exchange of metabolic gases through the perforate, hy aline shell of Amphistegina lobifera. A diffusive boundary layer (DBL) limited the solu te exchange between the foraminifera and the surrounding water. The DBL rea ched a thickness of 400-700 mum in stagnant water and was reduced to 100-30 0 mum under flow conditions. Gross photosynthesis rates were significantly higher under Row conditions (4.7 nmol O-2 cm(-3) s(-1)) than in stagnant wa ter (1.6 nmol O-2 cm (-3) s(-1)), whereas net photosynthesis rates were una ffected by flow conditions. The Ca2+ microprofiles demonstrated a spatial v ariation in sites of calcium uptake over the foraminiferal shells. Ca2+ gra dients at the shell surface showed total Ca; uptake rates of 0.6 to 4.2 nmo l cm(-2) h(-1) in A. lobifera and 1.7 to 3.6 nmol cm(-2) h(-1) in Marginopo ra vertebralis. The scattering and reflection of the foraminiferal calcite shell increased the scalar irradiance at the surface up to 205% of the inci dent irradiance. Transmittance measurements across the calcite shell sugges t that the symbionts are shielded from higher light levels, receiving appro ximately 30% of the incident light for photosynthesis.