Diazotrophic cyanobacteria play an important role in biogeochemical cycles of carbon and nitrogen and, hence, in oceanic productivity in the tropical and subtropical regions of the ocean. Although many studies have examined the impact of iron (Fe) limitation on particulate primary production and dinitrogen (N₂) fixation in the colonial cyanobacterium Trichodesmium, none have looked at the impact of Fe limitation on the percentage extracellular release (PER) and secondary production (SP) in Fe-limited cultures of this cyanobacterium. Here, we present the results of a series of culture experiments during which we examined the impact of 3 concentrations of dissolved iron (DFe) on total primary production (TPP = dissolved + particulate primary production, i.e. DPP + PPP), PER and on SP. Under severe Fe limitation (5 nM DFe), biomass, growth rates, TPP and N₂ fixation were strongly reduced, while PER increased relative to the rates ob served at the highest Fe concentration. Moreover, reducing Fe concentration induced an increase in the percentage of photosynthetically fixed C used for algal growth, while the percentage of C used to support algal respiration decreased. Reduced Fe concentrations also induced a decrease in SP and in the SP:DPP ratio, indicating that the efficiency of transfer of fixed carbon from autotrophic to heterotrophic processes is reduced. This suggests that Fe, either directly through influencing cellular processes or indirectly through influencing organic matter structure or nitrogen availability, is controlling SP and, thus, microbial carbon utilization. These results suggest that the amount of carbon entering into the microbial loop may be reduced under Fe limitation, thus leading to an accumulation of dissolved organic carbon with potentially important impacts on microbial carbon cycling and, ultimately, on the biological carbon pump