Foraminifera are the most important source of information for oceanographic and climatic reconstruction on glacial-interglacial as well as on much longer time-scales. The information is contained in the chemical composition, especially the isotopic ratios, of the calcitic shells (e.g., δ11B, δ13C, δ18O). Based on the assumption that our understanding of the major parameters controlling stable isotope incorporation is complete, these geochemical proxies have been used to reconstruct glacial ice volumes, sea surface and deep water temperatures, ocean circulation changes and shifts between carbon reservoirs. However, recent laboratory experiments have demonstrated that the δ13C and δ18O are not only strongly dependent on the carbonate chemistry of the culture medium but that the so-called 'vital-effects' are probably mediated via perturbations of the local carbonate system. These findings have an important impact on the interpretation of isotope data. For instance, the carbonate system of the glacial ocean was quite different from that of the Holocene and since the onset of the industrial revolution the carbonate chemistry of the surface water must have changed drastically. As a first step towards a better understanding of the isotopic fractionation processes we present results of a diffusion-reaction model of the carbonate system (CO2, HCO3/-, CO3/2-, H+, OH-, B(OH)3, B(OH)4/-) in the microenvironment (the diffusive boundary layer) of living planktic foraminifera. The carbon fluxes associated with the main life processes (calcification, respiration and symbiont photosynthesis) lead to substantial perturbations in pH and significant shifts in the concentrations of CO2, CO3/2- and other components in the vicinity of the foraminifer. Consequently, the carbonate chemistry of the ambient environment is quite different from that of the bulk sea water. Comparison with pH-microelectrode measurements confirm our numerical results. Our results further demonstrate that the symbionts must use bicarbonate as an additional carbon source for photosynthesis as the calculated CO2 fluxes are not sufficient to support measured rates of oxygen evolution. The simulations also show that for the fast calcification of Globigerinoides sacculifer the supply of carbonate ions is insufficient and therefore use of bicarbonate or an internal pool for carbon is required, whereas no such pool is necessary for the much slower calcification in Orbulina universa.