The cellular silicon, nitrogen, and carbon content and the kinetics of silicic acid use were determined for Thalassiosira weissflogii grown under nutrient-replete, iron-deficient, and zinc-deficient conditions to assess the effect of metal deficiency on dlatom silicon metabolism. Iron- and zinc-deficient T weissflogi~ce lls contained 40 and 66% more silicon, respectively, than their nutrient replete counterparts. Low Zn and low Fe also increased cellular C and N content. Low Zn increased cellular carbon by 55 % and cellular N by 41 %. Low Fe increased cellular C and N by 68 and 45 %, respectively. Fe stress did not alter cellular Si/N ratios significantly, but Si/C ratios declined by 17%. In contrast, Zn stress increased SVC and Si/N ratios by 41 and 53%, respectively. Both Zn and Fe stress dramatically altered the kinetics of silica production by T weissflogii. Zn deficiency increased the half saturation constant (K,) 64 ",, and decreased the maximum specific uptake rate (V,,,) by 60%. In contrast, Fe stress did not affect the value of K,, but decreased V,,, by 66%, similar to the decrease observed under low Zn. The decrease in V,,,,, in Zn-deficient cells was almost entirely due to the higher biogenic silica content of the metal-deficient cells. The decline in V,,, under Fe stress resulted from both the increase in cellular silica content and a 50% decline in the cellular uptake rates for silicic acid. The results indicate that Fe and Zn availabihty can significantly alter silicification in diatoms and affect the number and efficiency of silicon transport molecules in the cell membrane.