Recent studies on the diazotrophic cyanobacteria Trichodesmium erythraeum (IMS101) showed that increasing pCO2 availability enhances N2 fixation and growth. However, uncertainties remain regarding the overall processes causing these responses. We examined the responses of Trichodesmium IMS101 grown under a matrix of low and high levels of pCO2 (150 and 900 µatm) and irradiance (50 and 200 µmol photons m-2 s-1). Growth rates as well as cellular C and N content increased with increasing pCO2 and light levels in the cultures. The pCO2-dependent stimulation in organic C and N production was highest under low-light. High pCO2 stimulated rates of N2 fixation and prolonged the duration while high-light affected maximum rates only. Gross photosynthesis increased with light, but did not change with pCO2. HCO3- was identified as the predominant carbon source taken up in all treatments showing no trend with pCO2, while gross CO2 uptake was enhanced under elevated pCO2. A comparison between measured carbon fluxes and those derived from 13C fractionation revealed high internal carbon cycling within the high light and low pCO2 treatment. Our data confirm the pronounced pCO2 effect on N2 fixation and growth in Trichodesmium and indicate a strong modulation of these effects by light intensity. We attribute these responses to changes in the allocation of photosynthetic energy between carbon acquisition and the assimilation of C and N under elevated pCO2.