The degree of unsaturation of membrane lipids of at 30 °C decreased with batch-culture age. These changes were primarily attributable to a decline in microsomal Δ12-desaturase activity and in the relative proportion of linoleate. No change in the fatty acid composition, Δ12-desaturase activity or any increased incorporation of [1-C]acetate into polyunsaturated fatty acids was observed following chilling of early- and mid-exponential-phase cultures to 15 °C. In contrast, chilling of late-exponential and stationary-phase cultures resulted in a rapid and marked increase in the synthesis of polyunsaturated fatty acids. Thus, after 12 h incubation at 15 °C, the relative proportions of oleate and linoleate in these older cultures were similar to those of early- and mid-exponential-phase cultures at 30 °C. Despite these differences, an approximately 9.5 h lag in cell division was evident following chilling of both mid-exponential and late-exponential/early-stationary-phase cultures, and the subsequent pattern of cell division over 60 h incubation was similar in both cases. Oxygen uptake rates in cultures of either age were also decreased approximately equally at 15 °C. In contrast, chilling of mid-exponential-phase cells resulted in only an approximately 44% reduction in the rate of phagocytosis of fluorescently-labelled latex beads, whereas an approximately 98% inhibition of phagocytosis by late-exponential/early-stationary-phase cells resulted at 15 °C. Furthermore, a gradual subsequent increase in the rate of phagocytosis at 15 °C was only observed in the older cultures and was correlated with increases in the fatty acid unsaturation index of these cells. Thus, 12 h after chilling, the cells from late-exponential/early-stationary-phase cultures had achieved rates of phagocytosis similar to those of chilled cells from mid-exponential growth and their fatty acid compositions were now similar. The results suggest a role of membrane lipid unsaturation in the control of phagocytotic activity of


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