A mechanism of bicarbonate uptake with a high sensitivity to the putative anion-exchange inhibitor 4,4′-diisothiocyanostilbene-2,2′-disulphonic acid (DIDS) has been previously reported in green algae. In this study, DIDS inhibited net oxygen evolution by Dunaliella tertiolecta by up to 22 %, but internal pH regulation, intracellular CO2 accumulation, carbon fixation and affinity for dissolved inorganic carbon (DIC) in Dunaliella tertiolecta showed low or insignificant sensitivity to DIDS. However, in cells grown and tested at pH 9·5, treatment with DIDS elevated the k0·5(HCO3-), suggesting there may be a minor role for a DIDS-sensitive anion-exchange-type HCO3- transporter in DIC acquisition by D. tertiolecta at high pH. In contrast, significant external carbonic anhydrase (CAext) activity and up to 70 % inhibition of DIC-dependent O2 evolution by acetozolamide (AZ) suggest that CAext has an important role in DIC acquisition in D. tertiolecta, in normal seawater conditions and at elevated pH. Furthermore, the rate of DIC-dependent photosynthesis at high pH, in the presence of AZ, was 12 times higher than the calculated uncatalysed rate of CO2 supply from HCO3-. This requires some system for direct HCO3- uptake by D. tertiolecta, which may include a DIDS-insensitive mechanism. The effects of DIDS upon indirect measures of DIC acquisition should be interpreted cautiously as DIDS may have non-specific effects upon whole cell function, and affect ion transport processes not directly related to HCO3- uptake.

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.