Foraminifera are the most important source of information foroceanographic- and climatic reconstruction on glacial-interglacialas well as on much longer time-scales. The information is containedin the chemical composition, especially the isotopic ratios, of thecalcitic shells (e.g. delta11B, delta13C, delta18O).Based on the assumption that our understanding of the major parameterscontrolling stable isotope incorporation is complete, these geochemicalproxies have been used to reconstruct glacial ice volumes, seasurface and deep water temperatures, ocean circulation changes andshifts between carbon reservoirs. However, recent laboratory experimentshave demonstrated that the delta13C and delta18O are not only stronglydependent on the carbonate chemistry of the culture medium but thatthe so-called `vital-effects' are probably mediated via perturbationsof the local carbonate system. These findings have an important impacton the interpretation of isotope data. For instance, the carbonate systemof the glacial ocean was quite different from that of the Holocene andsince the onset of the industrial revolution the carbonate chemistry ofthe surface water must have changed drastically.As a first step towards a better understanding of the isotopicfractionation processes we present results of a diffusion-reactionmodel of the carbonate system (CO2, HCO3-, CO32-, H+, OH-, B(OH)3,B(OH)4-) in the micro-environment (the diffusive boundary layer)of living planktic foraminifera. The carbon fluxes associated with themain life processes (calcification, respiration and symbiontphotosynthesis) lead to substantial perturbations in pH and significantshifts in the concentrations of CO2, CO32- and other components inthe vicinity of the foraminifer. Consequently, the carbonate chemistryof the ambient environment is quite different from that of the bulksea water. Comparison with pH-micro-electrode measurements confirm ournumerical results. Our results further demonstrate that the symbiontsmust use bicarbonate as an additional carbon source for photosynthesisas the calculated CO2 fluxes are not sufficient to support measuredrates of oxygen evolution. The simulations also show that for thefast calcification of Globigerinoides sacculifer the supply of carbonateions is insufficient and therefore use of bicarbonate or an internal poolfor carbon is required, whereas no such pool is necessary for the muchslower calcification in Orbulina universa.