Chara buckellii, a euryhaline charophyte from an unusual saline environment. I. Osmotic relations at steady state

Chara buckellii is a euryhaline alga that provides a model for the comparative study of osmotic relations under different salinity regimes within the same species. Chara buckellii is native to a saline lake (Waldsea Lake) in which the major ionic constituents are Mg2+ and [Formula: see text] as well as Na+ and Cl− (approximately 110 mM each). Plants were cultured in artificial Waldsea water, ordinary seawater diluted to match the osmotic pressure of Waldsea water (375 mosmol/kg), and freshwater. The major osmotica in the vacuole of Waldsea-water and seawater cells are K+, Cl−, and Na+, although seawater cells have significantly more Na+ than Waldsea-water cells. The vacuole of freshwater cells contains primarily K+, Cl−, and Mg2+. Turgor is regulated in a range from 323 ± 5 to 212 ± 7 mosmol/kg when the external osmotic pressure ranges from 5 (freshwater) to 567 (1.5 × Waldsea water) mosmol/kg. Waldsea-water cells regulate turgor primarily with K+ and Cl−, whereas seawater cells regulate with K+, Cl−, and Na+. The possible role of Mg2+ in the early stages of turgor regulation is discussed. Cytoplasmic K+ concentrations are also reported and are comparable with those found in the literature for related charophyte genera. The results presented here are compared with modes of turgor and osmotic regulation in other euryhaline and freshwater algae.