Nitrogen fixation by methanogenic Archaea, literature review and DNA database-based analysis; significance in face of climate change

Archaea represents a significant population of up to 10% in soil microbial communities. The role of Archaea in soil is often overlooked mainly due to its unculturability. Among the three domains of life biological nitrogen fixation (BNF) is mainly a trait of Eubacteria and some Archaea. Archaea mediated processes like BNF may become even more important in the face of global Climate change. Although there are reports on nitrogen fixation by Archaea, to best of our knowledge there is no comprehensive report on BNF by Archaea under environmental stresses typical to climate change. Here we report a survey of literature and DNA database to study N₂-fixation among Archaea. A total of 37 Archaea belonging to Methanogens of the phylum Euryarchaeota within the class Methanococcus, Methanomicrobia Methanobacteria, and Methanotrophic ANME2 lineages either contain genes for BNF or are known to fix atmospheric N₂. Archaea were found to have their nif genes arranged as clusters of 6-8 genes in a single operon. The genes code for commonly found Mo-nitrogenase while in some archaea the genes for alternative metal nitrogenases like vnf were also found. The nifHDK gene similarity matrices show that Archaea shared the highest similarity with the nifHDK gene of anaerobic Clostridium beijerinckii. Although there are various theories about the origin of N₂-fixation in Archaea, the most acceptable is the origin of N₂-fixation first in bacteria and its subsequent transfer to Archaea. Since Archaea can survive under extreme environmental conditions their role in BNF should be studied especially in soil under environmental stress.