Phosphate Relations ofAcetabularia: Phosphate Pools, Adenylate Phosphates and32P Influx Kinetics

Abstract

Steady state phosphate relations are determined for the marine alga Acetabularia mediterranea with respect to cellular phosphate pools and phosphate transport. About 20% of the cellular acid-labile phosphate is found in the cell wall fraction (low speed sediment). By the use of cytoplasm-depleted cell segments, it is established that only about 10% of the total intracellular phosphate is localized in the vacuole when cells are bathed in normal phosphate concentration (<30 //M). Measurements of ATP, ADP, AMP and inorganic phosphate (Pi) in the entire cytoplasm and in isolated chloroplasts has enabled the calculation of the cytosolic adenylate energy charge (0-7-0-9) and phosphate potential (110-170 mV). Influx of 32Pi displays complex kinetics. Four components for uptake (approximate time constants: (A) 1 s, (B) 20 s, (C) 300 s and (D) 3000 s) are tentatively identified. Focusing on the faster components, B and C, and possibly even A, appear to be metabolically-linked, as judged by their sensitivity to temperature and to the inhibitor 2,4-dinitrophenol. These three components of influx are proportional to the external Pi concentration for values <30 fiM, but B and C tend to saturate at higher concentrations. The results are discussed with respect to the energetics of transport at the plasmalemma of Acetabularia, especially the activity of the electrogenic CI -ATPase.