The impact of climate change on phytoplankton bacterioplankton interactions Global warming has already and is continuing to impact the global oceans. Half of the global primary production is performed by phytoplankton in the oceans and heterotrophic marine bacteria channel a substantial amount of primary organic carbon through the microbial loop. Understanding the influence of climate change on these important processes is therefore essential for an assessment of the vulnerability of the carbon cycle and possible feedbacks. This thesis reports results from investigations on the temperature dependent coupling between phytoplankton and bacterioplankton with respect to additional effects of light intensity and inorganic nutrient concentrations. During four consecutive years mesocosm experiments with natural Kiel Fjord winter plankton communities investigated the influences of increasing water temperatures of up to T +6ʿC and different light intensities between 16 and 100% of natural incident light. In an additional microcosm experiment with a single algal species and the natural bacterial community two inorganic nutrient concentrations were used in order to evaluate the combined effects of temperature and substrate on the algal bacterial coupling. Summarising the results from all experiments it can be concluded that increasing temperatures generally led to an increased heterotrophic bacterial organic substrate utilisation relative to primary production. In combination with a further brightening the supplemental promotion of primary production would increase the absolute amounts of cycled organic matter. Future increasing P limitation in coastal waters would lead not only to an enhanced absolute amount of cycled carbon but additionally to an increased relative amount of remineralised organic carbon through the microbial loop. An enhanced organic matter transfer through the microbial loop has the potential to alter the whole structure and functioning of the marine food web and the biological sequestration of carbon to depth. Additionally a substantial rise of CO2 emissions through enhanced respiration represents a positive feedback loop to the global climate change problem.