Effects of chronic salt stress on the ultrastructure of Dunaliella bioculata (Chlorophyta, Volvocales): mechanisms of response and recovery

The ultrastructural changes taking place in Dunaliella bioculata after chronic exposure to a sodium chloride-induced stress were examined. Hyperosmotic shock was induced by raising the sodium chloride concentration of the culture medium from 0·3 to 1·3 M, which affected a number of cellular organelles during the initial stages of the stress period, i.e. 24, 48 and 72 h. Changes in whole-cell volume were recorded, as well as alterations in the size of the following components: starch grains and sheath, lipid and plastoglobuli, chloroplast, pyrenoid, nucleus, mitochondria, cytoplasm, Golgi apparatus and endoplasmic reticulum. Cells were examined using transmission electron microscopy and changes to their fine structure quantified via image analysis of the electron micrographs. The image analysis program was designed to measure various geometric parameters for all the cell components within individual algal cells. Quantitative image analysis of cells subjected to a chronic salt stress revealed marked increases in the cross-sectional areas of the Golgi apparatus and the endoplasmic reticulum. The enhanced production of the Golgi apparatus within the algal cells was thought to be the direct result of a salt-stress-induced endoplasmic reticulum production within the cells. The increase in the endoplasmic reticulum was manifested as extensive networks of cortical endoplasmic reticulum. It is suggested that the endoplasmic reticulum serves both physiological and structural roles during chronic salt stress by providing the driving force behind increased synthetic/Golgi apparatus activities of the cells, and by providing a type of ‘cellular scaffolding’ to limit the degree of cell contraction in the face of long-term salt stress.