Hopanoid lipids may facilitate aerobic nitrogen fixation in the ocean

Abstract

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.

Keywords: hopanoid lipids; marine cyanobacteria; nitrogen fixation; oxygen diffusion barrier.

Fig. 1.

Hopanids in marine cyanobacteria. (Upper) A schematic representation of the hopanoid biosynthesis and modification pathways, including enzymes and products. (Lower) Summary of the presence/absence of the genes involved in the synthesis and modification of hopanoids across a selection of the major marine cyanobacterial lineages. All of the available marine cyanobacterial genomes in NCBI (May 2019) were screened for this analysis, yet only 21 are shown, for simplification. Asterisk (*), experimentally tested in refs. and (n.t., not tested). Enzymes participating in hopanoid pathways: squalene−hopene cyclase (SHC), hopanoid biosynthesis-associated radical SAM protein (HpnH), hopanoid-associated phosphorylase (HpnG), hopanoid biosynthesis-associated glycosyltransferase protein (HpnI), hopanoid biosynthesis-associated protein (HpnK), hopanoid biosynthesis-associated radical SAM protein (HpnJ), aminotransferase (HpnO), hopanoid 2-methyltransferase (HpnP), and hopanoid C3 methylase (HpnR); 3-methylhopanoid production has never been found in marine cyanobacteria (28); hpnO was absent in all of the screened strains. Dashed arrows indicate that enzymes driving intermediate steps are unknown. See ref. for further details on hopanoid biosynthesis.