The Respiratory System of Azotobacter vinelandii

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Abstract
The growth and respiration of Azotobacter vinelandii, together with the composition and activity of its membrance‐bound respiratory system are compared during nitrogen‐fixing batch growth at high and low aeration. The aeration rate determines the length of the lag period, the extent of logarithmic growth and the respiratory activity of the culture; logarithmic growth ceases when the dissolved oxygen concentration drops to approximately 32 μM. Aconsiderable increase in whole cell respiratory activity is observed on changing from low to high aeration conditions. During logarithmic, non‐oxygen‐limited growth, oxidase activities and respiratory carrier concentrations, with the exception of cytochrome o, remain low; the concentration of cytochrome o starts to rise when the ambient oxygen concentration falls to approximately 120 μM. Phosphorylation efficiencies, particularly at site I, are very low at high ambient oxygen concentrations but increase as the oxygen concentration decreases. Site I P/2e ratio is maximal (approx. 0.5) only when the oxygen concentration becomes growth limiting. At the turnover point into non‐logarithmic, oxygen‐limited growth, respiratory carrier concentrations increase considerably; primary dehydrogenase activities increase by 2‐fold whereas the activity of the minor phosphorylating terminal pathway increases by 9–13 fold. A working hypothesis is proposed to account for the ability of the ambient oxygen concentration to control both the activity and the energy conservation efficiency of A. vinelandii respiration.