1. Cells of the fresh water diatom Navicula pelliculosa may be grown in a mineral medium containing a low concentration of silicon. When transferred to a fresh silicate solution and incubated under non-growing conditions such deficient cells rapidly take up silicon from the medium.

2. The utilization of silicon is an aerobic process.

3. When deficient cells are washed with distilled water or saline, their ability to utilize silicon is impaired whereas respiration is unaffected.

4. The ability of washed cells to take up silicon can be partially restored with sulfate or ascorbic acid, and is completely restored by Na2S, Na2S2O3, glutathione, l-cysteine, dl-methionine, or ascorbic acid plus sulfate.

5. The sulfhydryl reagent, CdCl2, inhibits silicon utilization of unwashed cells at concentrations which do not affect respiration. This inhibition similarly is reversed by glutathione or cysteine.

6. However, sodium iodoacetate or sodium arsenite inhibits respiration and silicon utilization at the same concentrations.

7. The silicon taken up by deficient cells is deposited at the cell surface as a thickening of the existing silica frustules.

8. Sulfhydryl groups in the cell membrane may be involved in silicon uptake by diatoms.

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