Summary: The thermal responses of membrane lipid composition and function in two bacterial species isolated recently from Heywood Lake sediment in Antarctica have been investigated. Both isolates are Gram-negative psychrotolerant (psychrotrophic) species growing well at 0 and 25 °C, but having very different optimum growth temperatures, of 9·7 °C for strain CR3/F/w/1/15 and 20·9 °C for strain CR3/F/w/2/10. The acyl lipids in strain 1/15 contain predominantly branched-chain fatty acids, mainly anteiso-15:0. The fatty acid composition and its response to temperature depends on the culture medium used. In nutrient-poor medium a wider range is found, including unsaturated fatty acids, which are absent when a nutrient-rich medium is used. A decrease in growth temperature produced a shortening of the average fatty acid chain length in rich medium, whereas in poor medium there was a decrease in straight-chain, saturated and unsaturated relative to branched-chain fatty acids. The acyl lipids of strain 2/10 contain 16:0 and 16:1 as the major fatty acids, particularly in rich medium, when they comprise > 90% of the total. Increasing proportions of 16:1 were converted to cyclopropane-17:0 at higher temperatures during growth in poor medium. A decrease in growth temperature in rich or poor medium resulted in an increase in the unsaturation index. When the bacteria were grown in rich or poor medium at 5 °C the strain with more ‘psychrophilic’ characteristics (1/15) took up glucose at a faster rate than did the strain with more ‘psychrotrophic’ characteristics (2/10), whereas when they were grown at 20 °C in either medium the rate of glucose uptake by strain 2/10 was generally faster than that of strain 1/15. Thus, when the bacteria were grown at a low temperature the glucose uptake system of the more ‘psychrophilic’ strain was better adapted to function at zero, compared with that of the more ‘psychrotrophic’ strain. The basis of this difference is not due to the lack of temperature-dependent fatty acid changes, as reported previously for some other cold-adapted bacteria.


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