Changes in Membrane Lipid Composition during Saline Growth of the Fresh Water Cyanobacterium Synechococcus 6311

Abstract

Growth of Synechococcus 6311 in the presence of 0.5 molar NaCI is accompanied by significant changes in membrane lipid composition. Upon transfer of the cells from a 'low salt' (0.015 molar NaCI) to 'high salt' (0.5 molar NaCI) growth medium at different stages of growth, a rapid decrease in palmitoleic acid (C16:1A9) content was accompanied by a concomitant increase in the amount of the two C18:1 acids (C18:1A9, C18:IAI1), with the higher increase in oleic acid C18:1A9 content. These changes began to occur within the first hour after the sudden elevation of NaCI and progressed for about 72 hours. The percentage of palmitic acid (C16:0) and stearic acid (C18:0) remained almost unchanged in the same conditions. High salt-dependent changes within ratios of polar lipid classes also occurred within the first 72 hours of growth. The amount of monogalactosyl diacylglycerol (bilayer-destabilizing lipid) decreased and that of the digalactosyl diacylglycerol (bilayer-stabilizing lipid) increased. Consequently, in the three day old cells, the ratio of monogalactosyl diacylglycerol to digalactosyl diacylglycerol in the membranes of high saltgrown cells was about half of that in the membranes of low saltgrown cells. The total content of anionic lipids (phosphatidylglycerol and sulfoquinovosyl diacylglycerol) was always higher in the isolated membranes and the whole cells from high salt-grown cultures compared to that in the cells and membranes from low salt-grown cultures. All the observed rearrangements in the lipid environment occurred in both thylakoid and cytoplasmic membranes. Similar lipid composition changes, however, to a much lesser extent, were also observed in the aging, low salt-grown cultures. The observed changes in membrane fatty acids and lipids composition correlate with the alterations in electron and ion transport activities, and it is concluded that the rearrangement of the membrane lipid environment is an essential part of the process by which cells control membrane function and stability. up to 0.6 M NaCl. Transfer from low salt (0.015 M NaCi) to high salt (0.5 M NaCI) medium initiates multiple changes in the composition of the cytoplasmic membrane and in activities of membrane-bound enzymes. These changes include alterations in the distribution of integral membrane proteins as seen by freeze-fracture electron microscopy ( 13), increased sodium/proton exchange activity (3) and an increase in the content and activity of Cyt c oxidase (8,19).It has been well recognized that membrane lipid composition modulates the physiological properties of membranes such as barrier function, transport, and signaling. As most protein-lipid interactions are achieved through direct contact ofthe given fatty acid, lipid, or lipid domains with the protein, changes in lipid environment will affect protein function. While changes in the lipid head group composition may alter electrostatic interactions between charged domains on the protein surface and the lipid head group, alterations in the hydrocarbon region will af...

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