We examined the effect of iron (Fe) and Fe-light (Fe-L) co-limitation on cellular silica (BSi), carbon (C) and nitrogen (N) in two marine diatoms, the small oceanic diatom Thalassiosira oceanica and the large coastal species Ditylum brightwellii. We showed that C and N per cell tend to decrease with increasing Fe limitation (i.e. decreasing growth rate), both under high light (HL) and low light (LL). We observed an increase (T. oceanica, LL), no change (T. oceanica, HL) and a decrease (D. brightwellii, HL and LL) in BSi per cell with increasing degree of limitation. The comparison with literature data showed that the trend in C and N per cell for other Fe limited diatoms was similar to ours. Interspecific differences in C and N quotas of Fe limited diatoms observed in the literature seem thus to be mostly due to variations in cell volume. On the contrary, there was no global trend in BSi per cell or per cell volume, which suggests that other interspecific differences than Fe-induced variations in cell volume influence the degree of silicification. The relative variations in C:N, Si:C and Si:N versus the relative variation in specific growth rate (i.e. mu:mu(max)) followed the same patterns for T. oceanica and D. brightwellii, whatever the irradiance level. However, the variations of C:N under Fe limitation reported in the literature for other diatoms are contrasted, which may thus be more related to growth conditions than to interspecific differences. As observed in other studies, Si:C and Si:N ratios increased by more than 2-fold between 100% and 40% of mu(max). Under more severe limitation (HL and LL), we observed for the first time a decrease in these ratios. These results may have important biogeochemical implications on the understanding and the modelling of the oceanic biogeochemical cycles, e.g. carbon and silica export. [Bucciarelli, E.] Univ Europeenne Bretagne, Rennes, France; Univ Brest, CNRS, IUEM, IRD,LEMAR,UMR 6539, F-29280 Plouzane, France Bucciarelli, E, Univ Europeenne Bretagne, Rennes, France eva.bucciarelli@univ-brest.fr European Commission [EVK2-1999-00227]; BOA [ANR-05-BLAN-0153] The authors would like to thank two anonymous reviewers and D. Hutchins for their insightful comments which improved this manuscript. This work was funded by the support from the European Commission's Marine Science and Technology Programme under Contract EVK2-1999-00227 (IRONAGES, "Iron Resources and Oceanic Nutrients - Advancements of Global Environment Simulations") and the BOA project (ANR-05-BLAN-0153). The iron concentration in the background medium was measured by M. Gallinari. A. Masson performed carbon and nitrogen measurements. J. Helias is thanked for his help with the Fe limited cultures of T. oceanica.