Diazotrophic cyanobacteria play an important role in the marine nitrogen cycle due to their ability to convert atmospheric N2 to bioavailable N species. With ongoing climate change, N2 fixers are subject to an array of perturbations in their environment. Several previous experiments suggested the abundant N2 fixer Trichodesmium to respond sensitively to ocean acidification as well as the concurrent changes in other environmental factors. The aim of this thesis was to improve understanding of the underlying mechanisms of these responses and determine whether they can be generalized to other diazotrophs. Key processes in the CO2 response of Trichodesmium were shown to include a reallocation of energy between the carbon concentrating mechanisms and N2 fixation, as well as intracellular cycling of inorganic carbon. Comparison of pCO2 effects on functionally different N2 fixers, in turn, revealed high variability in responses between different groups and even species of diazotrophs.