A global census of nitrogenase diversity

Summary The global diversity of nitrogen‐fixing microorganisms was assessed through construction and analysis of an aligned database of 16 989 nifH sequences. We conclude that the diversity of diazotrophs is still poorly described and that many organisms remain to be discovered. Our analyses indicate that diversity is not distributed evenly across phylogenetic groups or across environments and that some of the most diverse assemblages and environments remain the most poorly characterized. The majority of OTUs were rare, falling in the long tail of the frequency distribution. The most dominant OTUs fell into either the Cyanobacteria or the α , β , and γ Proteobacteria , and five of these dominant OTUs do not have any representatives cultivated in isolation. Soils contained the greatest diversity of nifH sequences of all of the environments surveyed. Cluster III, which is dominated by nifH