UC Santa Cruz

Cyanobacterial diazotrophs are considered to be the most important source of fixed N₂ in the open ocean. Biological N₂ fixation is catalyzed by the extremely O₂-sensitive nitrogenase enzyme. In cyanobacteria without specialized N₂-fixing cells (heterocysts), mechanisms such as decoupling photosynthesis from N₂ fixation in space or time are involved in protecting nitrogenase from the intracellular O₂ evolved by photosynthesis. However, it is not known how cyanobacterial cells limit O₂ diffusion across their membranes to protect nitrogenase in ambient O₂-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 O₂ diffusion barrier. We found that, whereas all non−heterocyst-forming cyanobacterial diazotrophs had hopanoid synthesis genes, none of the marine Synechococcus, Prochlorococcus (non−N₂-fixing), and marine heterocyst-forming (N₂-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 O₂. This membrane property coupled with high respiration rates to decrease intracellular O₂ concentration may therefore explain how non−heterocyst-forming cyanobacterial diazotrophs can fix N₂ in the fully oxic surface ocean.