Nitrogen fixation by marine cyanobacteria

Discrepancies between estimates of oceanic N2 fixation and nitrogen (N) losses through denitrification have focused research on identifying N2-fixing cyanobacteria and quantifying cyanobacterial N2 fixation. Previously unrecognized cultivated and uncultivated unicellular cyanobacteria have been discovered that are widely distributed, and some have very unusual properties. Uncultivated unicellular N2-fixing cyanobacteria (UCYN-A) lack major metabolic pathways including the tricarboxylic acid cycle and oxygen-evolving photosystem II. Genomes of the oceanic N2-fixing cyanobacteria are highly conserved at the DNA level, and genetic diversity is maintained by genome rearrangements. The major cyanobacterial groups have different physiological and ecological constraints that result in highly variable geographic distributions, with implications for the marine N-cycle budget.