Reconciling the incompatible: N2fixation And O2

summary N2fixation 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 O2and, in some cases, of O2generated photosynthetically. In this review, the effects of O2on 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. N2fixation 571 III. O2toxicity 574 IV. Interrelations between O2and N2fixation 577 V. Strategies for limiting O2damage to nitrogenase 581 Acknowledgements 601 References 602