SUMMARY: The heterocyst envelope of the N-fixing cyanobacterium thickened as exogenous O partial pressure (pO) was increased from 5 to 40 kPa. The majority of the thickening occurred in the glycolipid layer area of the envelope. Such thickening appears to be an O-induced mechanism for providing a greater O diffusion barrier against O inhibition of nitrogenase. Nitrogenase activity decreased at pO levels above ambient (20 kPa), indicating that a thicker envelope is not completely effective as a barrier to O diffusion. However, when cultures grown at 10 kPa and 40 kPa pO were transferred to ambient pO, the 40 kPa pO cells showed higher nitrogenase activity 24 h after transfer compared to those grown at 10 kPa or ambient pO, indicating O-protection of nitrogenase by thicker heterocyst envelopes. Heterocyst frequency was lowest at 20 kPa O.


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