The ultrastructure of Dunaliella tertiolecta, a naked, halotolerant unicellular green alga, is described and compared with ultrastructural features reported previously for three other species: D. bioculata, D. primolecta and D. salina. Most internal cell features are chlamy domonad in structure and arrangement. Conspicuous changes occur in D. tertiolecta between logarithmic and stationary growth phases. Vacuoles are more abundant and vac uolar inclusions have a different appearance in stationary as opposed to logarithmic phase cells. Thylakoids of the chloroplast are densely stacked in eight to ten layers in stationary phase cells, but are in stacks of two to four in logarithmic phase. Cytoplasmic and chloroplastic lipids, including eyespot granules, appear to increase with age of the cell. During logarith mic phase the endoplasmic reticulum shows greater proliferation. In cells subjected to increasing osmotic stress, large masses of dense-staining, tubular material is seen protrud ing and sometimes separating from the plasmalemma. Thylakoid stacking is more pro nounced in cells introduced into high salt concentration than in normal cells.
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