The physiological significance of mannitol accumulation in brown algae: the role of mannitol as a compatible cytoplasmic solute

Abstract

A study of the effects of the major intracellular solutes of brown algae on in vitro activities of the enzymes glutamine synthetase, glyceraldehyde-3-phosphate dehydrogenase (NADH specific), ribulose-1,5-bisphosphate carboxylase and phosphoenolpyruvate carboxykinase isolated from Fucus vesiculosus and Laminaria saccharina has indicated that the major inorganic cellular solutes (K+, Na+, Cl− and NO3−) were potent inhibitors of enzyme activity at measured intracellular concentrations, whereas mannitol was relatively non-inhibitory at concentrations up to 0.85 M. This provides evidence in support of the hypothesis that, in brown algae, the majority of the intracellular pools of inorganic ions are contained within the vacuole, with mannitol acting as a compatible solute in the cytoplasm. Further confirmation of this view was provided by an analysis of the relative solute contents of tissue isolated from the cortex and meristoderm of stipes of Laminaria digitata; these tissues consist of cells with low and high cytoplasm: vacuole ratios, respectively. It was found that the mannitol content of meristoderm tissue was more than double that of the cortex, while the cortical tissue contained significantly more K+, Na+ and Cl−.