AUXOTROPHY AND HETEROTROPHY IN MARINE LITTORAL DIATOMS

Forty-four pure cultures of marine littoral diatoms (43 pennate and 1 centric) have been studied to determine (a) whether any accessory organic factors (vitamins) are essential for growth in light, and (b) whether the cells can grow in darkness using glucose, acetate, or lactate as substrates.Six isolates were shown to require thiamine as sole growth factor; eleven require cobalamin only; and three require both thiamine and cobalamin. Sixteen are capable of heterotrophic growth using glucose but not acetate or lactate; eight can use glucose or lactate; one uses glucose or acetate; one uses glucose, acetate, or lactate; and two use only lactate.By the classical criteria of valve structure, the clones fall into about 25 different species, belonging to nine genera. However, it is evident that light microscopy is inadequate for the specific descriptions of many small diatoms. Electron micrographs of acid-cleaned frustules of each isolate served as an aid to specific identifications. Nutritional studies have demonstrated the existence of a range of distinct physiological races within the "species" Amphora coffaeiformis, Nitzschia closterium, and N. frustulum.

Silicon metabolism in diatoms. IV. Growth and frustule formation in Navicula pelliculosa

The cells of Navicula pelliculosa (Bréb.) Hilse, when grown in media which contained 3.5, 8.3, and 34.0 mg. Si per liter, reproduced with generation (doubling) times of 20.4, 13.7, and 13.1 hours respectively. The degree of silicification of the cells in each culture initially fell with increasing population. After exponential growth had ceased, this value varied between 0.8 × 10−9 and 2.2 × 10−9 mg. Si per cell, depending on the amount of silicon originally present in the medium and on the age of the culture. When placed in a fresh silicate solution, the cells took up additional silicate with increases in silicon content of 50%, 90%, and 15%, respectively. Silica (SiO2) may represent from 4% to 22% of the dry weight of the cells. The average thickness of the frustules has been calculated to range between 200 and 600 Å.

Silicon metabolism in diatoms. III. Respiration and silicon uptake in Navicula pelliculosa

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).

Silicon metabolism in diatoms. I. Evidence for the role of reduced sulfur compounds in silicon utilization

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 Na(2)S, Na(2)S(2)O(3), glutathione, l-cysteine, dl-methionine, or ascorbic acid plus sulfate. 5. The sulfhydryl reagent, CdCl(2), 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.