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

The ultrastructural changes taking place in Dunalielia bioculata after chro nic exposure to a sodium chloride-induced stress were examined. Hyperosmoti c shock was induced by raising the sodium chloride concentration of the cul ture medium from 0.3 to 1.3 M, which affected a number of cellular organell es during the initial stages of the stress period, i.e. 24, 48 and 72 h. Ch anges in whole-cell volume were recorded, as well as alterations in the siz e of the following components: starch grains and sheath, lipid and plastogl obuli, chloroplast, pyrenoid, nucleus, mitochondria, cytoplasm, Golgi appar atus and endoplasmic reticulum. Cells were examined using transmission elec tron microscopy and changes to their fine structure quantified via image an alysis 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 area s of the Golgi apparatus and the endoplasmic reticulum. The enhanced produc tion of the Golgi apparatus within the algal cells was thought to be the di rect result of a salt-stress-induced endoplasmic reticulum production withi n the cells. The increase in the endoplasmic reticulum was manifested as ex tensive networks of cortical endoplasmic reticulum. It is suggested that th e endoplasmic reticulum serves both physiological and structural roles duri ng chronic salt stress by providing the driving force behind increased synt hetic/Golgi apparatus activities of the cells, and by providing a type of ' cellular scaffolding' to limit the degree of cell contraction in the face o f long-term salt stress.