CHARACTERIZATION OF SULFATE TRANSPORT IN THE GREEN-ALGA CHLORELLA-ELLIPSOIDEA

A rapid induction of sulfate transport was observed in the green alga Chlorella ellipsoidea during sulfur-limited growth. Both affinity and V-max increased about five-fold within 6 h of transferring cells from Bold's basal medium with 350 mu M MgSO4 to sulfur-deficient Bold's medium. High affinity sulfate transport was induced within 15 min and reached maximum rate within 3 h of transferring cells to sulfur-deficient condition, indicating that a new, high-affinity-sulfate transport system is induced by sulfur starvation in C. ellipsoidea. Eadie-Hofstee plots of initial rates of sulfate uptake indicated that the K-1/2 of sulfur-starved cells was about 17 mu M. Both sulfur-starved and unstarved cells grown in air had a V-max of 1.5 times higher than that of high-CO2 grown cells. Sulfate transport was completely inhibited by 30 mu M CCCP or 800 mu M KCN both in the light and the dark but transport in the light was not inhibited by 20 mu M DCMU. Treatment with 50 mu M or 500 mu M vanadate caused 50% inhibition of uptake. The rate of sulfate uptake in the dark was twice that in the light and was stimulated by low pH. These results suggest that the sulfate transport system in C. ellipsoidea is operated by proton symport across the plasmamembrane which is partially mediated by P-type ATPase and that these systems depend exclusively on energy derived from oxidative phosphorylation in the mitochondria.