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 [.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 [...]