The photosynthesis and respiration of the larger foraminifera Amphistegina lobifera and Amphisorus hemprichii was studied with O², CO², and pH microsensors, and with a miniature gas exchange chamber. The diffusive transport of O² and CO² through both perforate (A. lobifera) and imperforate (A. hemprichii) calcite shells of the foraminifera was fast and allowed investigations of endosymbiont photosynthesis by microsensor measurements at the shell surface. Gross photosynthesis versus scalar irradiance (P vs. E⁰) curves showed onset of light saturation (E^k) at 95-198 pmol photons m_-2 s_-1. No photoinhibition was observed up to an irradiance of 2000 pmol photons m_-2 s_-1. Net photosynthesis (at saturating irradiance) and dark respiration rates were 3.7-22.7 and 5.6-14.3 nmol O² foraminifer_-1 h_-1, respectively. Simultaneous CO², pH and O² measurements at the shell surface of A. hemprichii during experimental light-dark cycles showed rapid concentration changes of all three variables upon light-dark or dark-light shifts. The dynamics of O² and CO² at the shell surface of A. hemprichii showed unequal net conversion rates of O² and CO² during experimental light-dark cycles. The molar O²/CO² conversion ratio at the shell surface of the foraminifera was ~2 in darkness and ~6 at saturating irradiance, pointing to a large internal supply of CO² in the host-symbiont association and the use of bicarbonate as source for inorganic carbon. The carbonate chemistry in the vicinity of symbiont-bearing larger foraminifera is thus strongly affected by the combined action of photosynthesis, respiration and calcification, and cannot be considered in equilibrium with the surrounding sea water. This has important implications for paleoenvironmental analysis and interpretation of the stable isotope composition of foraminiferal calcite shells and the derived models used for reconstructing climate and productivity in the past.