In view of the current increase in atmospheric pCO2 and concomitant changes in the marine environment, it is crucial to assess, understand and predict future responses of ecologically relevant phytoplankton species. The N2 fixing cyanobacteria Trichodesmium spp. contribute about half of all marine N2 fixation and thus can play a prominent role in the biogeochemical cycling of carbon and nitrogen. In this study, the effect of pCO2 and light on growth and production rates of Trichodesmium was investigated and underlying processes such as photosynthesis, C as well as N acquisition were studied.