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Cyanobacterial diazotrophs are considered to be the most important source of fixed N 2 in the open ocean. Biological N 2 fixation is catalyzed by the extremely O 2 -sensitive nitrogenase enzyme. In cyanobacteria without specialized N 2 -fixing cells (heterocysts), mechanisms such as decoupling photosynthesis from N 2 fixation in space or time are involved in protecting nitrogenase from the intracellular O 2 evolved by photosynthesis. However, it is not known how cyanobacterial cells limit O 2 diffusion across their membranes to protect nitrogenase in ambient O 2 -saturated surface ocean waters. Here, we explored all known genomes of the major marine cyanobacterial lineages for the presence of hopanoid synthesis genes, since hopanoids are a class of lipids that might act as an O 2 diffusion barrier. We found that, whereas all non-heterocyst-forming cyanobacterial diazotrophs had hopanoid synthesis genes, none of the marine Synechococcus , Prochlorococcus (non-N 2 -fixing), and marine heterocyst-forming (N 2 -fixing) cyanobacteria did. Finally, we conclude that hopanoid-enriched membranes are a conserved trait in non-heterocyst-forming cyanobacterial diazotrophs that might lower the permeability to extracellular O 2 This membrane property coupled with high respiration rates to decrease intracellular O 2 concentration may therefore explain how non-heterocyst-forming cyanobacterial diazotrophs can fix N 2 in the fully oxic surface ocean.

Hopanoid lipids may facilitate aerobic nitrogen fixation in the ocean