Low genomic diversity in tropical oceanic N^sub 2^-fixing cyanobacteria

High levels of genomic and allelic microvariation have been found in major marine planktonic microbial species, including the ubiquitous open ocean cyanobacterium, Prochlorococcus marinus. Crocosphaera watsonii is a unicellular cyanobacterium that has recently been shown to be important in oceanic N^sub 2^ fixation and has been reported from the Atlantic and Pacific oceans in both hemispheres, and the Arabian Sea. In direct contrast to the current observations of genomic variability in marine non-N^sub 2^-fixing planktonic cyanobacteria, which can range up to >15% nucleotide sequence divergence, we discovered that the marine planktonic nitrogen-fixing cyanobacterial genus Crocosphaera has remarkably low genomic diversity, with