Reconciling the incompatible: N2 fixation And O2

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Abstract
Summary: N2 fixation is an extremely O2‐sensitive process. N2‐fixing bacteria (diazotrophys) have therefore evolved a variety of strategies by which they maintain an active nitrogenase in the presence of atmospheric O2 and, in some cases, of O2 generated photosynthetically. In this review, the effects of O2 on nitrogenase activity and synthesis are described, as are the mechanisms by which diazotrophs limit O2damage 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 O2‐evolving cyanobacteria are discussed. CONTENTS Summary 571 I. Introduction 571 II. N2 fixation 571 III. O2 toxicity 574 IV. Interrelations between O2 and N2 fixation 577 V. Strategies for limiting O2 damage to nitrogenase 581 Acknowledgements 601 References 602