The Uptake and Metabolism of Sulphate in Scenedesmus as Influenced by Citrate, Carbon Dioxide, and Metabolic Inhibitors.

The incorporation of sulphur from external sulphate into different fractions was studied in cells with a P content of 0.6 to 1 mg/g fresh weight as against 3 to 5 mg/g fresh weight in normal cells. — There is a flat and broad optimum for the action of pH in the region between 5.7 and 7. — In most cases, citrate inhibits the uptake of sulphur. — Five per cent CO₂ in the air enhances the incorporation of S. It is suggested that the effects of CO₂ may well be exerted by way of the carboxylation and decarboxylation mechanisms in the cells. — Selenate inhibits the formation of DNA-S and protein S more than that of lipid S, but the main effect in both cases seems to be in an early step of the assimilation of sulphate. In the absence of external phosphate, the inhibition is counteracted by CO₂, especially in darkness. Phosphate enhances the action of selenate on the organic S fractions, and in its presence CO₂ and darkness make the inhibition more pronounced. — Without P in the medium, the effects of selenate on uptake into the inorganic sulphate fraction are smaller than when the organic fractions are concerned. In the presence of CO₂ even stimulations due to selenate have been observed. External phosphate brings the inhibition of uptake into cellular SO₄^2- to the same level as found in the case of organic S. More than one pathway for the uptake as SO₄^2- seem possible. — Anaerobiosis and menadione affect the organic S fractions more than the sulphate; 2,4-dinitrophenol has a more uniform action all over the field. [ABSTRACT FROM AUTHOR]