Dynamic diel proteome and daytime nitrogenase activity supports buoyancy in the cyanobacterium Trichodesmium.

Cyanobacteria of the genus Trichodesmium provide about 80 Tg of fixed nitrogen to the surface ocean per year and contribute to marine biogeochemistry, including the sequestration of carbon dioxide. Trichodesmium fixes nitrogen in the daylight, despite the incompatibility of the nitrogenase enzyme with oxygen produced during photosynthesis. While the mechanisms protecting nitrogenase remain unclear, all proposed strategies require considerable resource investment.

Iron conservation by reduction of metalloenzyme inventories in the marine diazotroph Crocosphaera watsonii.

The marine nitrogen fixing microorganisms (diazotrophs) are a major source of nitrogen to open ocean ecosystems and are predicted to be limited by iron in most marine environments. Here we use global and targeted proteomic analyses on a key unicellular marine diazotroph Crocosphaera watsonii to reveal large scale diel changes in its proteome, including substantial variations in concentrations of iron metalloproteins involved in nitrogen fixation and photosynthesis, as well as nocturnal flavodoxin production. The daily synthesis and degradation of enzymes in coordination with their utilization results in a lowered cellular metalloenzyme inventory that requires ∼40% less iron than if these enzymes were maintained throughout the diel cycle.

Phenotypic and genotypic characterization of multiple strains of the diazotrophic cyanobacterium, Crocosphaera watsonii, isolated from the open ocean.

Diazotrophic cyanobacteria have long been recognized as important sources of reduced nitrogen (N) and therefore are important ecosystem components. Until recently, species of the filamentous cyanobacterium Trichodesmium were thought to be the primary sources of fixed N to the open ocean euphotic zone. It is now recognized that unicellular cyanobacteria are also important contributors, with members of the oligotrophic genus Crocosphaera being the only cultured examples.