TEMPERATURE RESPONSE OF AN ASSOCIATIVE N2-FIXING PSEUDOMONAS SPECIES IN PURE CULTURE

The growth rate and nitrogenase (acetylene reduction) activity of a pseudomonad isolated from the rhizosphere of the grass species Deschampsia caespitosa were investigated in the 0-35-degrees-C temperature range. Growth was detected from 5 to 30-degrees-C with or without provision of fixed N and its optimum growth temperature was about 25-degrees-C, establishing its psychrotrophic response to temperature. Maximum nitrogenase activity was also found at about 25-degrees-C. Low but significant nitrogenase activity was detected below 15-degrees-C. This organism was confirmed to be a microaerobic diazotroph and it was sensitive to a dissolved O2 concentration of ca. 12-mu-M. It is probably dependent on the appropriate environment in the rhizosphere of the host plant in order to persist with sustained nitrogenase activity during cold and dormant periods.