Description: The human induced global climate change has severe consequences for the marine systems. Oceans have absorbed 50% of anthropogenic carbon dioxide emissions, consequently, attenuating global atmospheric warming. However, once entering the oceans, CO2 loses its inert characteristics. By the reaction with water it forms carbonic acid resulting in the phenomenon latterly referred to as ocean acidification. In the last two centuries, with the beginning of the industrial revolution, the global mean surface pH was already reduced by 0.1 units. Within the next 90 years the acidity level is believed to drop by another 0.35 units. Based on a simple causality a given atmospheric partial pressure of CO2 can easily be transferred into surface ocean carbon chemistry. Nevertheless, implications for the marine biota caused by increasing ocean acidification are complex and difficult to assess. Although the effects of rising pCO2 have been shown on single species of corals, pteropods, foraminifera, diverse phytoplankton species and larvae of echinoderms and fishes, research is far from understanding correlations between single species response and ecosystem functioning. Based on the importance as the most prominent pelagic calcifier and their hypothesised functioning as carbon export ballast, coccolithophores are among the best evaluated species with respect to ocean acidification. Calcification and photosynthesis has turned out to be sensitive to future conditions, however, with highly variable responses among species and species strains. For most analysed species calcification declined with rising pCO2, also the production of organic matter usually decreased, but turned out to rise for Gephyrocapsa oceanica. Surprisingly, the species Coccolithus braarudii appeared to be insensitive to an elevation of pCO2 from 380 µatm to 750 µatm. Based on this intriguing picture of coccolithophore response to ocean acidification this dissertation was concerned with the following questions: Does the insensitivity of C. braarudii to ocean acidification hold true for higher CO2 concentrations and what is the reason for the different sensitivities of various species? Does the total population carbon accumulation in the stationary phase reveal what could have been expected from the physiological ocean acidification response while undergoing exponential growth and are these results, gathered in the stationary phase dependent on the applied nutrient ratio? Higher diversity is known to positively affect stability and resistance of ecosystems. Is it possible to extrapolate single species responses to ocean acidification to multi-species responses? The study presented in chapter I affirms the insensitivity of C. braarudii for a pCO2 range up to 800 µatm. Further increases to values of 2500 µatm, however, revealed a decrease in calcification. The biomass production by photosynthesis, showed an optimum at 1600 µatm pCO2. An optimum response has been observed earlier for the coccolithophore Calcidiscus leptoporus, however, for calcification. Based on these findings, an increase in dissolved inorganic carbon, i.e. increase in the substrate for calcification and photosynthesis was discussed as to be an advantage in the first place. At a certain CO2 concentration the linked reduction in pH might negatively affect physiological processes and antagonise the positive effects of increasing substrate. For these reasons, the variable sensitivities of coccolithophores might not arise from different intracellular mechanisms but rather different optima for CO2, 〖"HCO" 〗_3^- and H+. Studies on coccolithophores in the exponential growth phase and their response to ocean acidification have proven to be useful to analyse underlying physiological mechanisms. Additional experiments concerning total population carbon accumulation in the stationary phase might allow for first estimates on ecosystem functioning of single species populations with respect to ocean acidification. Chapter II describes an experiment, allowing Emiliania huxleyi and Gephyrocapsa oceanica to deplete nutrients and stay for three days in the stationary phase. Under the stress of phosphate limitation and three different ocean acidification scenarios cells revealed an increase in cell size in the stationary phase. This increase was attenuated with rising pCO2. Also the accumulations of calcite and organic carbon on a population level showed pronounced responses to increasing ocean acidification. These responses, however, were significantly dependent on the nutrient ratio the cells had to face. The biomass decrease for Emiliania huxleyi and increase for Gephyrocapsa oceanica as well as the decrease in calcite of both species were more pronounced for cells growing under a high N:P ratio compared to cells facing a Redfield ratio. This was contrary to the response on a population level. Due to changing nutrient uptake the more sensitive “High N:P” treatments were able to produce more cells with rising pCO2, resulting in an attenuated calcite and biomass accumulation decrease. Based on these results, estimations of the future influence of coccolithophores on both atmospheric pCO2 feedback and carbon export should take the affects of nutrient limitation on cell physiology stronger into consideration. Despite their relevance for the physiological mechanisms of coccolithophores, hitherto published studies are far from assessing whole ecosystem functioning with respect to ocean acidification. Diversity is known to have positive effects on ecosystem stability and resistance, nevertheless, community interactions of coccolithophores and their collective response to rising pCO2 were so far neglected. To gain first indications of the potential community interaction of coccolithophores, the experiment presented in chapter III allows three species, namely G. oceanica, E. huxleyi and C. braarudii to grow alone and within a community. Under the stress of nutrient limitation and three different pCO2 single species cultures revealed a decrease in the population calcite and biomass accumulation. In contrary, the multiple species approach showed no significant variation in photosynthesis and calcification. This suggests a higher resilience caused by community interactions. This dissertation underlines the importance of single species approaches to understand the underlying physiological processes with respect to ocean acidification. But it also shows our ignorance of marine ecosystem resilience and therefore, suggests that it might not be appropriate to extrapolate the changes in calcification rates of single species to ocean acidification onto a global scale. In near future it needs further experiments to evaluate the role of diversity and nutrient ratios as a positive factor for resilience and resistance of coccolithophore ecosystems.