Effects of an iron-light co-limitation on the elemental composition(Si, C, N) of the marine diatoms Thalassiosira oceanica and Ditylumbrightwellii

摘要

We examined the effect of iron (Fe) and Fe-light(Fe-L) co-limitation on cellular silica (BSi), carbon (C) andnitrogen (N) in two marine diatoms, the small oceanic diatomThalassiosira oceanica and the large coastal species Ditylumbrightwellii. We showed that C and N per cell tend to de-crease with increasing Fe limitation (i.e. decreasing growthrate), both under high light (HL) and low light (LL). We ob-served an increase (T oceanica, LL), no change (T ocean-ica, HL) and a decrease (D. brightwellii, HL and LL) in BSiper cell with increasing degree of limitation. The compari-son with literature data showed that the trend in C and N percell for other Fe limited diatoms was similar to ours. Inter-specific differences in C and N quotas of Fe limited diatomsobserved in the literature seem thus to be mostly due to vari-ations in cell volume. On the contrary, there was no globaltrend in BSi per cell or per cell volume, which suggests thatother interspecific differences than Fe-induced variations incell volume influence the degree of silicification. The relativevariations in C:N, Si:C and Si:N versus the relative variation in specific growth rate (i.e. p,:p,max) followed the same pat-terns for T oceanica and D. brightwellii, whatever the irradi-ance level. However, the variations of C:N under Fe limita-tion reported in the literature for other diatoms are contrasted,which may thus be more related to growth conditions than tointerspecific differences. As observed in other studies, Si:Cand Si:N ratios increased by more than 2-fold between 100%and 40% of Amax. Under more severe limitation (HL andLL), we observed for the first time a decrease in these ratios.