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Silicon and carbon uptake rates were studied over a 24 h light/dark cycle in a synchronised culture of the marine diatom Cylindrotheca fusiformis (Reimann et Lewin) using [.sup.32.Si] and [.sup.14.C]. The silicic acid uptake rate per cell ([rho][.sup.c.Si]) varied between 1.2 and 20.0 fmol Si [cell.sup.-1] [h.sup.-1] and was closely correlated to the G2 + M phase of the cell cycle. A linear and significant relationship was determined between the percentage of cells present in G2 + M and [rho][.sup.c.Si]. Evolution of the soluble free-silicon pool was studied simultaneously. The concentration of the total soluble free pool of silicon ([Q.sup.p]Si) varied from 1% to 7% of the total silicon content. A significant difference of 1.5 fmol Si [cell.sup.-1] between [Q.sup.p]Si and the labelled free pool ([Q.sup.np]Si) was measured, indicating the presence of an unlabelled fraction of the pool. The concentration of [Q.sup.np]Si was around 1.0 fmol Si [cell.sup.-1] prior to cell division and did not change as a function of [rho][.sup.c.Si], which indicated a feedback mechanism coupling uptake into the free pool and incorporation into the frustule. In parallel, [.sup.14.C] uptake variation ([rho][.sup.c.C]) was measured during the division of the population. The value of [rho][.sup.c.C] varied between 0.44 and 0.78 pmol C [cell.sup.-1] [h.sup.-1] and appeared to be maximal when cells were in the G1 phase. This variation of [rho][.sup.c.C] marginally affected the total carbon content of the cells ([Q.sup.T]C) in comparison with the light/dark cycle. The variations in the Si/C ratio, from 0.021 to 0.046, demonstrated the different control mechanisms of Si and C metabolisms during the course of the cell- and photocycle., Introduction Diatoms are unicellular algae that have the capacity to form a silicified cell wall called a frustule, which essentially consists of hydrated amorphous silica (Round et al. 1990). Silicon [...]

Silicon and carbon uptake rates were studied over a 24 h light/dark cycle in a synchronised culture of the marine diatom Cvlindrotheca fusiformis (Reimann et Lewin) using [sup.32]Si and [sup.14]C. The silicic acid uptake rate per cell ([[rho].sup.c]Si) varied between 1.2 and 20.0 fmol Si [cell.sup.-1] [h.sup.-1] and was closely correlated to the G2 + M phase of the cell cycle. A linear and significant relationship was determined between the percentage of cells present in G2 + M and [[rho].sup.c]Si. Evolution of the soluble free-silicon pool was studied simultaneously. The concentration of the total soluble free pool of silicon ([Q.sup.P]Si) varied from 1% to 7% of the total silicon content. A significant difference of 1.5 fmol Si [cell.sup.-1] between [Q.sup.P]Si and the labelled free pool ([Q.sup.np]si) was measured, indicating the presence of an unlabelled fraction of the pool. The concentration of [Q.sup.np]Si was around 1.0 fmol Si [cell.sup.-1] prior to cell division and did not change as a function of [[rho].sup.c]Si, which indicated a feedback mechanism coupling uptake into the free pool and incorporation into the frustule. In parallel, [sup.14C] uptake variation ([[rho].sup.c]C) was measured during the division of the population. The value of [[rho].sup.c]C varied between 0.44 and 0.78 pmol C [cell.sup.-1] [h.sup.-1] and appeared to be maximal when cells were in the G1 phase. This variation of [[rho].sup.c]C marginally affected the total carbon content of the cells ([Q.sup.T]C) in comparison with the light/dark cycle. The variations in the Si/C ratio, from 0.021 to 0.046, demonstrated the different control mechanisms of Si and C metabolisms during the course of the cell- and photocycle.

We studied the silicic acid uptake kinetics of the pennate diatom Cylindrotheca fusiformis grown under a wide range of iron concentrations (from Fe-limiting to Fe-sufficient conditions) to assess the effect of iron availability on diatom cell size, silicon content, and silicic acid uptake kinetic parameters. As the iron stress increased, the growth rate slowed, cell size decreased, and silicification increased. A series of Si kinetic uptake experiments (from Si-limiting to Si-sufficient conditions) performed at different iron concentrations demonstrated the extent of the colimitation domain, where the specific Si uptake rate ([V.sub.Si]) varied as a function of both silicic acid and Fe availability. A decrease in maximal specific uptake rate of silicic acid ([V.sub.Si-max]) under iron limitation was observed along with a decrease in half-saturation constant for silicic acid uptake ([K.sub.Si]. Because [V.sub.Si-max] and [K.sub.Si] vary in the same direction, the specific affinity for silicic acid does not change under iron stress. The variation in cell size is an acclimation to low nutrient concentrations. Our study shows that Si uptake kinetics parameters, especially [V.sub.Si-max] are strongly related to cell size, which is itself constrained by the degree of iron limitation. Thus, the surface to volume ratio should be related to silicic acid flows in size-based biogeochemical models of planktonic ecosystems. Another way to describe the observed variations of Si uptake over the full range of Si and Fe concentrations tested in the present study would be to implicitly take into account the variations in cell size through a scaling of the maximal specific uptake rate ([V.sub.max]) by the iron-limitation term in a multiplicative manner.