Abstract
Linolenic acid is the major fatty acid found in galactolipids of light-grown cells. During cell greening, on resting medium, the C 18 : 2 and C 16 : 2 content of galactolipids increased, while linolenic acid remained a minor component. Labelling experiments provided evidence that the main step in the biosynthesis of linolenic acid is the desaturation of linoleic to linolenic acid. The desaturase in Euglena seems to be regulated by various environmental conditions. The activity of the desaturase was stimulated by addition of NH 4 + to the resting medium or by saturation of the cell suspension with O 2- or CO 2-enriched air. The synthesis of linolenic acid in Euglena also depends on the intensity and quality of light. The temperature used for the greening experiments appeared to be the factor that most affected the activity of the desaturase; at 20°C, 33% of the fatty acids in MGDG is linolenic acid whereas at 28°C galactolipids contain only 15% linolenic acid. In contrast to higher plants, Euglena cells can form fully photosynthetic active membranes without simultaneously synthesizing linolenic acid.
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