1. Evidence is presented that silicon uptake in the diatom Navicula pelliculosa is linked with aerobic respiration.

2. Cyanide, fluoride, iodoacetate, arsenite, azide, and fluoroacetate, at concentrations inhibitory to respiration, were also inhibitory to silicon uptake.

3. 2,4-Dinitrophenol (1 to 2 x 10–5 M) stimulated respiration by 100 per cent, but almost completely inhibited silicon uptake.

4. The respiratory quotient of non-Si-deficient cells decreased from 0.93 to 0.75 after 4 days of starvation in darkness. Glucose (1 per cent) raised the respiratory quotient of such starved cells to 1.05.

5. Silicate (20 mg. Si/liter) stimulated respiration of unstarved Si-deficient cells by about 40 per cent. The effect of silicate on the respiration of Si-deficient cells which had been starved in darkness for 4 days was less marked.

6. The respiratory quotient of Si-deficient cells decreased from 0.8–0.9 to 0.3 after 4 days of starvation in darkness. The addition of silicate to starved cells raised the quotient to 0.5. This represented a 25 per cent stimulation of oxygen uptake concomitant with a 90 per cent stimulation of carbon dioxide evolution.

7. Glucose (1 per cent) caused an increase of respiratory quotient in starved cells from 0.3 to 0.7–0.8. The addition of silicate had no effect on the R.Q. during the oxidation of exogenous glucose.

8. Substrates (glucose, fructose, galactose, lactate, succinate, citrate, glycerol), which caused a stimulation of respiration in starved cells, also stimulated silicon uptake by those cells. However, the stimulation of silicon uptake (50 to 100 per cent) was not proportional to the respiratory stimulation by these substrates (30 to 300 per cent).

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