Sulphate uptake and metabolism in the chrysomonad, Monochrysis lutheri

The intracellular concentration of inorganic ³⁵SO₄ in Monochrysis lutheri cells exposed to 0.513 mM Na₂ ³⁵SO₄ for up to 6-hr remained constant at about 0.038 mM. The exchange rate of this ³⁵SO₄ with the external unlabelled sulphate was negligible compared to the rate of influx across the plasmalemma (0.032 μmoles/g cells/hr). The flux of free ³⁵SO₄ to organic ³⁵S was 0.029 μmoles/g cells/hr. Assuming an internal electrical potential in the cells of-70 mV, this intracellular concentration of inorganic ³⁵SO₄ was well in excess of that obtainable by passive diffusion as calculated from the Nernst equation. These results indicate that sulphate is accumulated by an active mechanism rather than by facilitated diffusion. Sulphate uptake appears to occur via a carrier-mediated membrane transport system which conforms to Michaelis-Menten type saturation kinetics with a K_mof 3.2×10^-5 M and a V_max of 7.9×10^-5 μmoles sulphate/hr/10⁵ cells. Uptake was dependent on a source of energy since the metabolic inhibitor CCCP almost completely inhibited uptake under both light and dark conditions and DCMU caused a 50% decrease in uptake under light conditions. Under dark conditions, uptake remained at about 80% of that observed under light conditions and was little affected by DCMU, indicating that the energy for uptake could be supplied by either photosynthesis or respiration. A charge and size recognition site in the cell is implied by the finding that sulphate uptake was inhibited by chromate and selenate but not by tungstate, molybdate, nitrate or phosphate. Chromate did not inhibit photosynthesis. Cysteine and methionine added to the culture medium were apparently capable of exerting inhibition of sulphate uptake in both unstarved and sulphate-starved cells. Cycloheximide slightly inhibited sulphate uptake over an 8-hr period indicating, either a slow rate of entry of the inhibitor into the cells or a slow turnover of the proteins(s) associated with sulphate transport.