The symbiotic N
2
-fixing cyanobacterium UCYN-A, which is closely related to
Braarudosphaera bigelowii
, and its eukaryotic algal host have been shown to be globally distributed and important in open-ocean N
2
fixation. These unique cyanobacteria have reduced metabolic capabilities, even lacking genes for oxygenic photosynthesis and carbon fixation. Cyanobacteria generally use energy from photosynthesis for nitrogen fixation but require mechanisms for avoiding inactivation of the oxygen-sensitive nitrogenase enzyme by ambient oxygen (O
2
) or the O
2
evolved through photosynthesis. This study showed that symbiosis between the N
2
-fixing cyanobacterium UCYN-A and its eukaryotic algal host has led to adaptation of its daily gene expression pattern in order to enable daytime aerobic N
2
fixation, which is likely more energetically efficient than fixing N
2
at night, as found in other unicellular marine cyanobacteria.