A newly discovered nitrogen-fixation reaction may have played a role in the emergence of life, according to German researchers.
In order for amino acids and nucleic acids to have emerged in the bubbling cauldron that was the early earth and so produce the first signs of life, the element nitrogen was essential. Molecular nitrogen in the atmosphere, however, is too unreactive to have easily been incorporated into the molecules that would build themselves into the first organisms. Ammonia on on the other hand is a much more likely agent. But how was it first generated from atmospheric nitrogen? Günter Kreisel and Wolfgang Weigand and their colleagues at the University of Jena and the Max Planck Institute for Biogeochemistry in Jena have a new theory.
Because the bond between the two atoms in a nitrogen molecule is so difficult to break, the manufacture of ammonia is carried out with catalysts and high temperatures and pressures. However, nitrogen-fixing bacteria can convert atmospheric nitrogen into ammonia under much milder conditions. They utilize the enzyme nitrogenase, which requires a cofactor containing several iron- and sulfur units. These two elements were plentiful on the young earth perhaps a pre-biotic reaction exploited them to mildly fix nitrogen. The researchers have now demonstrated that just such a nitrogen-fixation reaction can work without enzyme but at not too far above room temperature (70-80 Celsius).
Nitrogen bubbled into an aqueous suspension of iron sulfide reacts with dissolved hydrogen sulfide to form ammonia, the researchers say, driven by the formation of iron disulfide. The reaction works only if the iron sulfide used is freshly produced and the researchers suggest that there are unknown iron-sulfur species present on the surface of the particles that carry out the tough job of splitting the nitrogen molecule.
“Our synthesis of ammonia may serve as a model for a primordial nitrogen fixing system and conforms well with theories of the chemoautotrophic origin of life,” say the researchers.
Dörr, M., Käßbohrer, J., Grunert, R., Kreisel, G., Brand, W., Werner, R., Geilmann, H., Apfel, C., Robl, C., & Weigand, W. (2003). A Possible Prebiotic Formation of Ammonia from Dinitrogen on Iron Sulfide Surfaces Angewandte Chemie International Edition, 42 (13), 1540-1543 DOI: 10.1002/anie.200250371