Ammonia enhanced dark respiration inChlorella vulgarisis related to collapse of a transmembrane pH gradient

Abstract

Dark consumption of oxygen by Chlorella vulgaris was shown to be stimulated by increasing the external ammonium concentration. The ammonium effect was pH-dependent suggesting that an increased influx of free ammonia at higher pH values was cause to the effect. Breakdown of a transmembrane pH gradient between the cytoplasm and the interior of a cellular compartment was found to be related with enhanced oxygen consumption. Acid vacuoles are the most likely target for a collapse of the ΔpH since they form the only acid compartment where ammonia could accumulate 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.