AMMONIA ENHANCED DARK RESPIRATION IN CHLORELLA-VULGARIS IS RELATED TOCOLLAPSE OF A TRANSMEMBRANE PH GRADIENT

Dark consumption of oxygen by Chlorella vulgaris was shown to be stimu lated by increasing the external ammonium concentration. The ammonium effect was pH-dependent suggesting that an increased influx of free am monia at higher pH values was cause to the effect. Breakdown of a tran smembrane pH gradient between the cytoplasm and the interior of a cell ular compartment was found to be related with enhanced oxygen consumpt ion. Acid vacuoles are the most likely target for a collapse of the DE LTApH since they form the only acid compartment where ammonia could ac cumulate as ammonium in dark adapted C vulgaris cells. An alternative hypothesis involving the combined operation of a ATP-requiring proton extrusion pump and a Na+/H+ antiporter in the cytoplasmic membrane has been discussed.