Oxygen controls the development of Frankia vesicles in continuous culture

summary Daily addition of sterile media at a dilution rate of 0.100–0.125 d−1 allowed Frankia to be maintained in continuous, derepressed (N2-fixing) culture for periods of more than 30 d. Continuous cultures yielded Frankia populations with stable levels of growth and nitrogenase activity as well as constant vesicle morphology. These were then used to investigate the influence of oxygen on vesicle development. Timing of vesicle induction is independent of po2 although thickening of the vesicle envelope and stalk, a ‘normal’ developmental process, is specifically controlled by oxygen. At higher po2 levels vesicles are thicker-walled and therefore offer greater resistance to oxygen diffusion. Oxygen also influences induction of nitrogenase. Greater delay in the onset of acetylene reduction activity at highpo2 is apparently due to the time taken for vesicles to reach a critical minimum wall thickness. Recovery of nitrogenase underwent a similar time lag when Frankia cultures were exposed to a sudden increase in po2.