Senescence-Related Changes in the Lipid Transition Temperature of Microsomal Membranes from Algae

The phase behaviour of smooth microsomal membranes from senescing cultures of Scenedesmus quadricauda has been examined by wide-angle x-ray diffraction. The algae were grown in Bristol's medium at 22°C under continuous illumination. The transition temperature, taken to be the highest temperature at which crystalline (gel) phase lipid can be detected, increased with culture age from a low of 0°C for young cultures to a high of about 70°C for 140-day-old cultures. This indicates that for young cultures the membrane lipid is entirely liquid-crystalline (fluid) at physiological temperatures, but as the cultures age portions of the lipid become crystalline. The increase in transition temperature showed a close temporal correlation with loss of chlorophyll and loss of protein per g dry weight, and can thus be construed as an index of senescence. The unsaturated to saturated fatty acid ratio of the membrane lipid, while fluctuating with culture age, did not show any consistent trend that could be related to the change in transition temperature. Thus the formation of gel phase lipid does not appear to be due to a change in fatty acid saturation.