Changing perspectives in marine nitrogen fixation

Changing views and a changing ocean As a component of many biomolecules, nitrogen is a crucial element for life, especially in nutrient-poor environs such as the open ocean. Atmospheric dinitrogen gas (N 2 ) is abundant but must be fixed by reduction to ammonia, a process limited to certain organisms and environments. Zehr and Capone review changes in our understanding of what marine microorganisms are fixing N 2 , where they live, and what environmental features influence their activity. N 2 fixation is more widely distributed than previously thought, and we still have much to learn about the physiology and regulation involved. We now have better estimates of global- and basin-scale inputs and outputs, but questions remain as to whether the oceanic N cycle is balanced. New tools are enabling better understanding of ocean N 2 fixation despite disruptive consequences from human activities, including ocean acidification and warming. Science , this issue p. eaay9514