Heterocyst envelope thickness, heterocyst frequency and nitrogenase activity in Anabaena flos-aquae: influence of exogenous oxygen tension

SUMMARY: The heterocyst envelope of the N2-fixing cyanobacterium Anabaena flos-aquae thickened as exogenous O2 partial pressure (pO2) 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 O2-induced mechanism for providing a greater O2 diffusion barrier against O2 inhibition of nitrogenase. Nitrogenase activity decreased at pO2 levels above ambient (20 kPa), indicating that a thicker envelope is not completely effective as a barrier to O2 diffusion. However, when cultures grown at 10 kPa and 40 kPa pO2 were transferred to ambient pO2, the 40 kPa pO2 cells showed higher nitrogenase activity 24 h after transfer compared to those grown at 10 kPa or ambient pO2, indicating O2-protection of nitrogenase by thicker heterocyst envelopes. Heterocyst frequency was lowest at 20 kPa O2.