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Cycling Nitrogen in Ancient Oceans
The nitrogen synthase complex. Nitrogenase is shown in the center, with two chains in cyan and two chains in grey. Image Credit: University of Arizona
The nitrogen synthase complex. Nitrogenase is shown in the center, with two chains in cyan and two chains in grey. Image Credit: University of Arizona

In recent years, scientists have found evidence that a 'near complete' biological nitrogen cycle existed in the oceans during the late Archean to early Proterozoic (from 2.5 to 2 billion years ago). Modern bacteria use an enzyme called nitrogenase to cycle nitrogen from one form to another. This enzyme is dependent on the presence of metallic elements like iron (Fe), vanadium (V) and, most often, molybdenum (Mo). However, ancient oceans didn't contain much molybdenum. Could Fe-nitrogenase or V-nitrogenase have played a larger role in the archaean oceans than they do today? To answer this question, a team of researchers supported by the NASA Astrobiology Institute studied the phylogenetic relationships between the proteins that allow nitrogenase to interact with each of the three elements. Their results suggest that the protein (known as Nif protein) actually developed in methanogenic microorganisms, and was then incorporated into bacteria by lateral gene transfer around 1.5-2.2 billion years ago. Ultimately, if Mo-nitrogenase originated under anoxic conditions in the Archaean, it would have likely happened in an environment where both methanogens and bacteria coexisted, and where molybdenum was present for at least part of the time.

The emergence of enzymes like Mo-nitrogenase was a significant step in the evolution of life, and had powerful repercussions for planet Earth and its biosphere as a whole. This research can help answer important questions about the environmental conditions that were present on the early Earth, and the interactions that occurred between life and the ancient planet.

The results were published in the May edition of the journal Geobiology (E.S. Boyd et al. 2011. A late methanogen origin for molybdenum-dependent nitrogenase. Geobiology. Vol. 9, No. 3)

Last Updated: 22 July 2011

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Last Updated: 22 Jul 2011