The metabolic regulation of membrane lipid composition has been examined using the cell wall-less, unicellular green alga Dunaliella salina (UTEX 1644) as a model system. Low temperature stress was employed to initiate and study the regulatory response.When cultures growing logarithmically at 30 degrees C were chilled to 12 degrees C, cell division ceased for approximately 100 hours, and then the cells resumed logarithmic growth at a slower rate. The phospholipid, glycolipid and protein content, on a per cell basis, was, in each case, approximately 20% higher in cells grown at 12 degrees C. The volume of the 12 degrees C-acclimated cells was 2.8 times that of 30 degrees C-grown cells. The quantity of chloroplast membrane, as determined by morphometric analysis, was 20% greater, whereas the content of microsomal membrane material was more elevated, being approximately 2.8 times that of 30 degrees C-grown cells.Lipid compositional analyses were carried out on purified chloroplasts and microsomes isolated from Dunaliella grown at 30 and 12 degrees C and also from cells 12 and 60 hours following a shift from 30 to 12 degrees C. In both chloroplast and microsomal phospholipids fatty acid unsaturation increased during acclimation to low temperature. Generally, microsomal phospholipids responded more quickly and to a greater extent than did chloroplast phospholipids. Despite these alterations, little change in the relative proportions of phospholipid classes was observed in either cell fraction.In sharp contrast to the pattern of phospholipid change, chloroplast glycolipids responded to low temperature by significantly increasing the proportion of one specific class, digalactosyl diglycerides, relative to monogalactosyl diglycerides, while showing minimal change in fatty acid distribution within any given glycolipid class.The ease and rapidity with which Dunaliella cells can be manipulated with respect to environmental stress and isolation of intact cell organelles makes it particularly well suited for research on intermembrane lipid dynamics within the plant cell.