Influence of Oxygen Concentration on the Colony Type of Derxia gumrnosa Grown on Nitrogen-free Media

Summary: At oxygen concentrations of less than 0·2 atm. Derxia gummosa formed only the ‘massive’ colony type on nitrogen-free agar media; the ‘thin’ colony type which predominates in air was absent. Only the ‘massive’ colonies reduced acetylene. The acetylene-reducing activity of samples from a nitrogen-fixing continuous culture was inhibited when the oxygen supply was increased, which suggests that the oxygen sensitivity previously reported during growth in nitrogen-free media is associated with the nitrogen-fixing process. Colonial dimorphism probably arises because only when the local oxygen concentrations on the agar surface are depressed can nitrogen fixation and subsequent growth to the ‘massive’ colony type take place.