Inhibition of Cell Division by High External NaCl Concentrations in Synchronized Cultures of Chlorella emersonii

Effects of high external NaCl concentrations on growth were examined in the unicellular freshwater alga Cldorella emevsonii during different phases of cell development, using synchronized cultures obtained by alternating light-dark cycles. Growth of cultures synchronized at 1 mM NaCl [external osmotic pressure (next=) 0.08 MPa] was compared with (i) cultures synchronized at 200 mM NaCl (n,,, = 1.01 MPa) and (ii) cultures synchronized at 1 mM NaCl from which the daughter cells were suddenly transferred to 100, 150 or 200 mM NaCl. The effects of these two treatments on synthesis of protein, RNA and DNA during cell cycles were similar, and are attributed to the high nexta nd not to specific effects of Na+ and C1-. Growth inhibitions in cells at 200 mM NaCl relative to 1 mM NaCl occurred mainly via effects on cell division; this was confirmed by electron microscopy. There was a lag before net DNA synthesis commenced, and there were reductions in rates of net DNA synthesis in cells at 200 mM NaCl relative to 1 mM NaC1. Rates of increase in cell volume and in protein and RNA content per cell were little affected by high external NaCl concentrations. Consequently, daughter cells at 200 mM NaCl were approximately twice the volume and contained twice as much protein and RNA as daughter cells at 1 mM NaCl, while DNA content was equal in daughter cells at 1 and 200 mM NaCl.