Turgor Pressure, Volumetric Elastic Modulus, Osmotic Volume and Ultrastructure of Chlorella emersonii Grown at High and Low External NaCl

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Abstract
Chlorella emersonii (211/11n) was grown at external NaCl concentrations ranging between 1.0 and 335 mM (0.08–1.64 MPa). Previous studies showed that there was no significant change in the internal concentrations of Na+ or Cl− over this range, the concentrations remaining below 35 mM. Relative growth rates of C. emersonii were 30–45% lower in 335 mM NaCl than in 1.0 mM NaCl. Turgor pressure varied with the osmotic pressure of the growth medium. Plots of cell volume versus (external osmotic pressure)−1 indicated that cells grown in 1.0 mM NaCl (0.08 MPa) had turgor pressures ranging from 0.5 to 0.8 MPa, while cells in 335 mM NaCl (1.64 MPa) had turgor pressures of 0.0–0.14 MPa. Estimates of turgor pressure derived from the osmotic pressure of cell sap had a mean value of 0.6 MPa for cells in 1.0 mM NaCl, and 0.3 MPa for cells in 335 mM NaCl. The volumetric elastic modulus (ɛ) depended on the osmotic pressure of the growth medium: ɛ was 8.5 ± 1.7 MPa for cells grown in 1.0 mM NaCl, and 0.9 ± 0.6 for cells in 335 mM NaCl. ε was measured by changing turgor pressures over the range 0.0–0.5 MPa, and was found to be independent of turgor. Electron micrographs showed that the walls of cells grown in 335 mM NaCl were 70% thicker than those grown in 1.0 mM NaCl. Other changes in cellular structure were small, however, the area occupied by vacuoles increased from 7% in cells grown in 1.0 mM NaCl to 14% in cells in 335 mM. The percent osmotic volume of cells grown in 1.0–335 mM NaCl (61 ± 17%, v/v) was similar to the percent water content (59 ± 13%, w/w).