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To predict effects of climate change and possible feedbacks, it is crucial to understand the mechanisms behind CO 2  responses of biogeochemically relevant phytoplankton species. Previous experiments on the abundant  N 2   fixers  Trichodesmium  demonstrated strong  CO 2   responses, which were attributed to an energy reallocation between its carbon (C) and nitrogen (N) acquisition. Pursuing this hypothesis, we manipulated the cellular energy budget by growing  Trichodesmium erythraeum   IMS101  under different  CO 2   partial pressure ( pCO 2  ) levels (180, 380, 980 and 1400 µatm) and N sources ( N 2   and  NO 3   − ). Subsequently, biomass production and the main energy‐generating processes (photosynthesis and respiration) and energy‐consuming processes ( N 2   fixation and C acquisition) were measured. While oxygen fluxes and chlorophyll fluorescence indicated that energy generation and its diurnal cycle was neither affected by  pCO 2   nor N source, cells differed in production rates and composition. Elevated  pCO 2   increased  N 2   fixation and organic C and N contents. The degree of stimulation was higher for nitrogenase activity than for cell contents, indicating a  pCO 2   effect on the transfer efficiency from  N 2   to biomass.  pCO 2  ‐dependent changes in the diurnal cycle of  N 2   fixation correlated well with C affinities, confirming the interactions between N and C acquisition. Regarding effects of the N source, production rates were enhanced in  NO 3   −  grown cells, which we attribute to the higher N retention and lower  ATP  demand compared with  N 2   fixation.  pCO 2   effects on C affinity were less pronounced in  NO 3   −  users than  N 2   fixers. Our study illustrates the necessity to understand energy budgets and fluxes under different environmental conditions for explaining indirect effects of rising  pCO 2  .

Combined effects of different CO 2 levels and N sources on the diazotrophic cyanobacterium Trichodesmium