Reconciling the incompatible: N₂fixation And O₂

summary N 2 fixation is an extremely O 2 ‐sensitive process. N 2 ‐fixing bacteria (diazotrophys) have therefore evolved a variety of strategies by which they maintain an active nitrogenase in the presence of atmospheric O 2 and, in some cases, of O 2 generated photosynthetically. In this review, the effects of O 2 on nitrogenase activity and synthesis are described, as are the mechanisms by which diazotrophs limit O 2 damage to nitrogenase. These mechanisms are classified as behavioural strategies, physical barriers or physiological and biochemical strategies. Individual diazotrophs frequently empoly a combination of these. In addition, the particular problems faced by the O 2 ‐evolving cyanobacteria are discussed. CONTENTS Summary 571 I. Introduction 571 II. N 2 fixation 571 III. O 2 toxicity 574 IV. Interrelations between O 2 and N 2 fixation 577 V. Strategies for limiting O 2 damage to nitrogenase 581 Acknowledgements 601 References 602