The stable boron isotope composition (d11B) of fossil foraminiferalshells is used as a paleo-pH recorder and is therefore one of themost promising paleo-carbonate chemistry proxies ('paleoacidimetry').One crucial question regarding this proxy is whether foraminiferarecord the pHof the bulk seawater or the pH of the microenvironment (diffusiveboundary layer, ~500mum) which is strongly influenced bylife processes. Here we present a novel, theoretical approach toaddress this question by using a diffusion-reaction model. Model resultsindicate that the d11B in planktonic foraminifera is primarilycontrolled by the pH of the microenvironment. We therefore predictthat the d11B of different species (e.g. symbiont-bearing vs.symbiont-barren) or of foraminifera grown in the dark andin the light should be offset from the d11B of inorganic calcite.This theoretical prediction was experimentally confirmed while thispaper was written (Hoenisch et al., in revision).Most importantly, the model predicts that this offset is constantover a wide pH range. Thus, the use of d11B as a paleo-pHindicator is not compromised through vital effects as modeled here.