We examined changes in carbon fluxes in the marine unicellular diazotrophic cyanobacterium (UCYN) Crocosphaera watsonii in response to irradiance. We estimated changes in the total organic carbon and nitrogen contents, incorporation of carbon into reserves, and exudation processes of extracellular exopolymeric substances (EPS), as well as in the formation of transparent exopolymeric particles (TEP). Variability of cellular carbon pools and extracellular EPS and TEP under conditions of different irradiance and the significance of carbon release in the environment is discussed. Our results show a relationship between growth rate, metabolic processes of carbon incorporation in cells, exudation and irradiance. When growth is light-limited, both carbon production and exudation increase with light. No inhibition occurs at growth-saturating irradiances and half of the total particulate organic carbon is then found as intracellular carbohydrates. While the overall growth becomes light saturated, carbon fixation is further stimulated by higher irradiances, leading to an increased EPS exudation. We found that EPS exudation in C. watsonii is not only a way for cells to release excess carbon, but also occurs when growth is light-limited, even at the lowest irradiance tested. The formation of TEP increases with irradiance, with TEPcarbon amounting to up to 30% of the total particulate organic carbon. Our data will help to quantify the role of unicellular diazotrophs in the export of both carbon and nitrogen through and below the euphotic zone in the marine environment.