On early Earth, abiotic processes formed the initial supply of fixed (bioavailable) nitrogen necessary to maintain basic metabolic functions. As life began to flourish, this supply probably couldn't meet demand, and evolutionary pressures selected for a means of biological nitrogen fixation. How did the requisite enzymatic machinery—nitrogenase proteins that catalyze the reduction of N2 into bioavailable ammonium—develop, and which microorganisms possessed it? By examining phylogenetic relationships between nitrogenases and other related proteins from modern bacteria and archaea (such as the one at left), Boyd et al. show that molybdenum (Mo)–dependent nitrogenases, which are currently the most common form of nitrogen-fixing enzymes, originated after vanadium- and purely iron-based variants. The first Mo-nitrogenase probably appeared 1.5 to 2.2 billion years ago in an ancestor of methanogenic archaea, and later transferred to bacteria through lateral gene transfer. At that time, the ocean was still adjusting to widespread oxygenation from oxygenic photosynthesis. Because the N2 reduction is an anaerobic process, however, it must have occurred in anoxic layers of a stratified ocean, where fixed nitrogen was limited and Mo was available from oxidative weathering of the continents.
Geobiology 9, 221 (2011).