The osmotic role of mannitol in the Phaeophyta: an appraisal

Abstract

Natural abundance 13C nuclear magnetic resonance spectroscopy (NMR) has shown mannitol to be the only major low molecular weight organic compound present in osmotically significant amounts within cells of the following marine brown algae: Alaria esculenta, Ascophyllum nodosum, Ectocarpus siliculosus, Fucus serratus, F. spiralis, F. vesiculosus, Halidrys siliquosa, Laminaria digitata, L. hyperborea, L. saccharina and Pilayella littoralis. Samples of the top-shore alga Pelvetia canaliculata were found to contain volemitol in addition to mannitol. Quantitative gas-liquid chromatographic analyses have confirmed the presence of mannitol, at concentrations ranging from 83.3 to 314.0 mmol kg−1 (expressed in terms of intracellular water content) for plants maintained in a full-strength (100%) sea water medium. A study of the changes in intracellular mannitol concentration of six marine brown macroalgae immersed in hyposaline and hypersaline media (20–150% EA 1 sea water medium) showed that mannitol concentration varied as a direct function of salinity in all cases, providing support for the hypothesis that mannitol is intimately involved in osmotic adjustment in response to changes in external water status. Plants of the filamentous form P. littoralis maintained in darkness also showed a marked sensitivity of intracellular mannitol concentration to alterations in the external salt concentration. Overall, the data support an osmotic, rather than a respiratory role for the large internal pools of mannitol in such algal cells.