资源环境科技发展态势分析平台

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 majormarine 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.