Increase in the synthesis of membrane-fluidizing (such as unsaturated) fatty acids in cold-adapted bacteria is well documented. More recently, a polar carotenoid was found to rigidify synthetic membranes in an in vitro study. Enhanced biosynthesis of some fatty acids that increase membrane fluidity, and also of polar carotenoids has been evidenced in one Gram-positive and one Gram-negative psychrotrophic bacterium, isolated from Antarctic soil. A role of carotenoids in homeoviscous adaptation of membrane fluidity has been postulated.