The effect of fluidity of membrane lipids on freeze-thaw survival of yeast.

One approach to studying the importance of membranes in freeze-thaw damage is to modify their composition and study the effect of this modification on survival after freeze-thaw damage. Fatty acid desaturase auxotrophs of yeast cells were enriched with two fatty acids having substantially different physical properties thus resulting in cells whose membranes had very different physical properties. The fatty acids were stearolic acid (mp = +45 °C) and linolenic acid (mp = ?10 °C). Electron-spin resonance studies showed that membranes containing the latter fatty acid were more fluid than those containing stearolic acid. The yeast were grown under either anaerobic or aerobic conditions. In the former case, the mitochondria appear as membraneous shells with little, if any, internal membrane structure; thus, the plasma and tonoplast membranes are the primary membranes. Yeast cells grown under these conditions survived freezethaw damage (?196 °C) significantly better when the fatty acid composition was mainly stearolic acid rather than linolenic acid. The absolute survival depended on the freezing rate and the differences in survival became small at fast rates. With yeast cells grown under aerobic conditions, when functional mitochondria are formed, the pattern in freeze-thaw survival reversed; cells with γ-linolenic acid in their membranes survived significantly better than cells containing stearolic acid.