Rapid Cytoplasmic Alkalization and Dynamics of Intracellular Compartmentation of Inorganic Phosphate during Adaptation against Salt Stress in a Halotolerant Unicellular Green Alga Dunaliella tertiolecta: 31P-Nuclear Magnetic Resonance Study

By 31P-in vivo nuclear magnetic resonance spectroscopy, two intracellular compartments were detected, and their pH values were estimated in intact cells of a halotolerant unicellular green alga Dunaliella tertiolecta. They were identified as the cytoplasm (pH 7.1) and vacuoles (pH 6.0). Vacuoles were also visualized with a fluorescence-differential interference microscope. During the adaptation to the salt stress (NaCl concentration from 0.17 to 1.0 M) where cells rapidly synthesize glycerol as osmoticum, both cytoplasmic and vacuolar pH showed transient increases to about 8 and 6.5, respectively. Subsequently, cytoplasmic inorganic phosphate level, as well as sugar phosphates and terminal phosphate group levels increased. These drastic changes in chemical environment in the cytoplasm including pH and inorganic phosphate concentration are discussed to be key factors for osmoregulation that activate the synthesis and inhibit the breakdown of glycerol.