Regulation of the Si and C uptake and of the soluble free-silicon pool in a synchronised culture of Cylindrotheca fusiformis (Bacillariophyceae): effects on the Si/C ratio

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 32Si and 14C. The silicic acid uptake rate per cell (?cSi) varied between 1.2 and 20.0 fmol Si cell-1 h-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 ?cSi. Evolution of the soluble free-silicon pool was studied simultaneously. The concentration of the total soluble free pool of silicon (QPSi) varied from 1% to 7% of the total silicon content. A significant difference of 1.5 fmol Si cell-1 between QPSi and the labelled free pool (QnpSi) was measured, indicating the presence of an unlabelled fraction of the pool. The concentration of QnpSi was around 1.0 fmol Si cell-1 prior to cell division and did not change as a function of ?cSi, which indicated a feedback mechanism coupling uptake into the free pool and incorporation into the frustule. In parallel, 14C uptake variation (?cC) was measured during the division of the population. The value of ?cC varied between 0.44 and 0.78 pmol C cell-1 h-1 and appeared to be maximal when cells were in the G1 phase. This variation of ?cC marginally affected the total carbon content of the cells (QTC) 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.