SUMMARY

Ten obligately psychrophilic bacteria were isolated from North Sea water using conventional enrichment culture techniques at low temperatures. Whereas Arrhenius plots of the growth rate of obligate psychrophiles were linear down to the freezing point of the 0.25% lactate‐artificial sea water medium used, Arrhenius plots of facultative strains showed a deviation from linearity at temperatures a few degrees above 0°. It thus appears that the growth rate of obligate psychrophiles is higher at low temperatures, indicating that in competitive processes in cold natural environments, these organisms are at an advantage. The optimum temperature for growth of one of the isolates, a Pseudomonas sp., was 14.5°; the maximum temperature was between 19 and 20°. Respiration was optimal at 23°, and no inactivation of respiratory activity was found within 5 h at 25°. Cells incubated at supermaximal temperatures showed a release of UV absorbing material. RNA synthesis could not be detected at these temperatures. This is considered to be one of the major factors defining the maximum growth temperature of this organism.

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