Physiological response of the unicellular green alga Chlorococcum submarinum to rapid changes in salinity

Abstract

Cell volumes and intracellular concentrations of major solutes of Chlorococcum submarinum were determined before and after salinity shocks. Cells were found to shrink in size by about 30% following changes from 0.1 to 0.5 M NaCl, there was a transitory increase in sodium concentration and more permanent increases in concentrations of potassium, proline and glycerol (the major osmolyte). Conversely, cells doubled in size after the reciprocal downshock, there was rapid loss of about 70% of the cells' glycerol to the medium, a much smaller loss of cellular potassium and a steady disappearance of proline from the cells. The respiratory and photosynthetic responses to salinity fluctuations were also studied. Salinity downshocks stimulated respiration by 30% and inhibited photosynthesis by 16% within 5 min, but within 2 h these rates were identical to control rates. Upshocks caused a slight inhibition of respiration, but decreased photosynthesis by 40% within 5 min and recovery took 2 h. Downshocks had little effect on chlorophyll fluorescence, however, Fo strongly increased and both Fm and Fv/Fm declined within 5 min of salinity increases. This is consistent with a decrease in efficiency of PS2. Ecological and metabolic implications of the results are discussed.