Assessment of Nitrogenase Diversity in the Environment

Diazotrophic microorganisms are taxonomically and metabolically diverse and they populate a wide variety of habitats. Diazotroph diversity is typically quantified through the analysis of nitrogenase nifH gene sequences recovered from environmental samples. In this chapter we review the global census of diazotrophic microorganisms as assessed by the diversity of nifH gene sequences in public sequence databases. We assess the diversity of diazotrophs found in different environments and taxonomic groupings and we also evaluate PCR primers and probes used to assess diazotroph diversity. Furthermore, we assess the range of habitats and geographic regions in which diazotrophic diversity has been assessed. We find that diazotrophs in many habitats remain under sampled. We also find that nifH cluster III, which includes obligate anaerobes, contains the greatest diversity of nifH sequences and that anoxic environments both contain the greatest diversity of diazotrophs and are the environments in which diazotrophic diversity is most poorly sampled. Our knowledge of diazotroph diversity is likely to expand dramatically as next-generation sequencing technologies are targeted towards surveys of the nifH gene.