Ionic Osmoregulation during Salt Adaptation of the Cyanobacterium Synechococcus 6311

Abstract

The mechanisms of salt adaptation were studied in the cyanobacterium Synechococcus 6311. Intracellular volumes and ion concentrations were measured before and after abrupt increases of external NaCl concentrations up to 0.6 molar NaCl. Equilibrium volumes, measured with a rapid and accurate electron spin resonance spin probe method, showed that at low NaCl concentrations the cells did not shrink as expected for an impermeable solute. However, when the NaCl concentration exceeded a critical value, volume losses occurred. These losses were not fully reversed by hypoosmotic treatment, suggesting membrane damage. The critical value of irreversible volume loss paralleled the increase in salinity during cell growth. Rapid mixing experiments showed that exposure of Synechococcus 6311 to non-damaging NaCl concentrations caused water extrusion from the cells; the volume decreases were time resolved to about 200 milliseconds. Subsequently, volumes increased rapidly as NaCl moved into the cells. Controls recovered their volumes within 15 seconds, while salt-adapted cells grown at 0.6 molar NaCl required 1 minute for volume equilibration. This decrease in the rate of cell volume recovery indicates that salt adaptation is accompanied by changes in cell membrane properties. Subsequent to these initial rapid volume changes, a more gradual sequence of ion movement and sugar accumulation was observed. Under conditions for photoautotrophic growth, significant Na⁺ extrusion was observed 30 min after salt shock. Sucrose accumulation reached a maximum value after 16 hours and K⁺ accumulation reached equilibrium after 40 hours. The final concentrations of K⁺ and Na⁺ and sucrose and glucose inside the 0.6 molar NaCl-grown cells indicate that the inorganic ions and organic `compatible' solutes are the major osmotic species which account for the adaptation of Synechococcus 6311 to salt.