Carbon dioxide concentration (pCO2) in the ocean is steadily increasing causing a drop of pH, consequently turning the surface seawater more acidic. Due to possible adaptation mechanisms some marine organisms can cope better with high pCO2 and low pH than others. The ocean quahog Arctica islandica is widely distributed in the North Atlantic region. Populations of this species are also well established in the high fluctuating environment of the Kiel Bight in the Western Baltic Sea and show high tolerance to environmental parameters like salinity, temperature and low oxygen levels. In my thesis I am interested in the performance of A. islandica from Kiel Bight to build and maintain its shell in a high pCO2 environment and the general aspects of bivalve shell properties as well as the site of calcification within the bivalve as a prerequisite for a mechanistic understanding of the biomineralization process. This thesis summarizes i) A. islandica from Kiel Bight populations is resistant and most likely pre-adapted towards elevated pCO2 over a short period of time (90 days) and contributes to the fundamental understanding of ii) single organic shell-compounds identified as pigment polyenes, that are not habitat related and may contribute to shell formation and iii) the calcification process itself is not happening inside the bulk EPF but rather within a supersaturated microsite created by active ion pumping by the OME. Understanding the biomineralization process and all components involved is crucial and thus the next challenge in order to estimate the robustness of A. islandica and other marine calcifiers in a high pCO2 world.