Hopanoid lipids may facilitate aerobic nitrogen fixation in the ocean

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.