Widespread distribution and expression of Gamma A (UMB), an uncultured, diazotrophic, γ-proteobacterial nifH phylotype

Marine dinitrogen (N₂) fixation studies have focused nearly exclusively on cyanobacterial diazotrophs; however γ-proteobacteria are an abundant component of the marine community and have been largely overlooked until recently. Here we present a phylogenetic analysis of all nifH γ-proteobacterial sequences available in public databases and qPCR data of a γ-proteobacterial phylotype, Gamma A (UMB), obtained during several research cruises. Our analysis revealed a complex diversity of diazotrophic γ-proteobacteria. One phylotype in particular, Gamma A, was described in several traditional and quantitative PCR studies. Though several γ-proteobacterial nifH sequences have been described as laboratory contaminants, Gamma A is part of a large cluster of sequences isolated from marine environments and distantly related to the clade of contaminants. Using a TaqMan probe and primer set, Gamma A nifH DNA abundance and expression were analyzed in nearly 1000 samples collected during 15 cruises to the Atlantic and Pacific Oceans. The data showed that Gamma A is an active, cosmopolitan diazotroph found throughout oxygenated, oligotrophic waters reaching maximum abundances of 8 × 10⁴ nifH DNA copies l^-1 and 5 × 10⁵ nifH transcript copies l^-1. Gamma A nifH transcript abundances were on average 3 fold higher than nifH DNA abundances. The widespread distribution and activity of Gamma A indicate that it has potential to be a globally important N₂ fixing organism.