Effects of an iron-light co-limitation on the elemental composition (Si, C, N) of the marine diatoms IThalassiosira oceanica/I and IDitylum brightwellii/I

摘要

We examined the effect of iron (Fe) and Fe-light (Fe-L) co-limitation oncellular 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 andN per cell tend to decrease with increasing Fe limitation (i.e. decreasinggrowth rate), both under high light (HL) and low light (LL). We observed anincrease (T. oceanica, LL), no change (T. oceanica, HL) and a decrease (D. brightwellii, HL and LL) in BSi percell with increasing degree of limitation. The comparison with literaturedata showed that the trend in C and N per cell for other Fe limited diatomswas similar to ours. Interspecific differences in C and N quotas of Felimited diatoms observed in the literature seem thus to be mostly due tovariations in cell volume. On the contrary, there was no global trend in BSiper cell or per cell volume, which suggests that other interspecificdifferences than Fe-induced variations in cell volume influence the degreeof silicification. The relative variations in C:N, Si:C and Si:N versus therelative variation in specific growth rate (i.e. μ:μ_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 theliterature for other diatoms are contrasted, which may thus be more relatedto growth conditions than to interspecific differences. As observed in otherstudies, Si:C and Si:N ratios increased by more than 2-fold between 100%and 40% of μ_max. Under more severe limitation (HL and LL), weobserved for the first time a decrease in these ratios. These results mayhave important biogeochemical implications on the understanding and themodelling of the oceanic biogeochemical cycles, e.g. carbon and silicaexport.