Marine plankton plays an essential role in the global carbon cycle. 75% of the preindustrial vertical gradients of the dissolved inorganic carbon in the sea are due to biological processes. Model simulations show that the atmospheric CO₂ concentration doubles when the photosynthetic fixation of carbon is shut down. Marine plankton is not only a key player in the global carbon cycle, it reacts at the same time on changes in the CO₂ concentration in surface waters. Currently several direct effects (`direct' in contrast to effects of increased atmospheric CO₂  mediated by climatic changes affecting ocean circulation) are known that include the CO₂  dependence of algal growth  rates, carbon isotopic fractionation, elementary composition (C:N:P) and CO₃^2-  dependent carbon and oxygen isotopic fractionation as well as calcification in planktonic foraminifera. These dependencies can be used to reconstruct oceanic paleoenvironments from proxies in the sediments. Some of the direct effects are of potential relevance for the biological carbon pump in the oceans. The investigation of such effects and their consequences as well as development and validation of proxy relationships lies in the focus of our interdisciplinary group at the Alfred Wegener Institute.