23 SEPTEMBER 1998
Jay Brandes, Bob Hazen, and their Geophysical Lab colleagues* in Washington, D.C. report that one of the necessary first steps for life to begin the conversion of nitrogen to ammonia may have occurred readily in deep ocean vents. Nitrogen is an essential ingredient of amino acids and nucleic acids. But molecular nitrogen, N2, is relatively inert and unlikely to have given rise to the first vital signs of life. Most scientists believe instead that nitrogen in a more reduced, reactive form, i.e., ammonia, or NH3, was required. How did nitrogen, then, become ammonia?
Brandes and colleagues suggest that the most likely sites for ammonia production to occur were in the early Earth's crust and in hydrothermal vents, where iron-bearing minerals act as catalysts. The group conducted experiments investigating what happens when nitrogen, both as N2 and as its oxidized forms, NO2 and NO3 (all forms presumed to be present in the ancient oceans), react with iron oxides, iron sulphides, and basalt at high temperatures and pressures. Basalt is the predominant rock type at ocean ridge systems, and iron sulfides and oxides are ubiquitous in hydrothermal vents.
Using iron sulfides, they found that at 500C, up to 89% of the nitrogen converted to NH3 within 15 minutes! Iron oxide converted up to 46% of the nitrogen to ammonia, while powdered basalt converted up to 20% under these conditions. Reduction of N2, the largest reservoir of nitrogen on Earth, was slower but still resulted in up to 17% conversion to ammonia within 24 hours. The ammonia is stable, they found, to temperatures up to 800C, but only nitrogen gas survives temperatures above that temperature. Thus, N2 or its oxidized forms would have provided raw materials for an important and plentiful source of ammonia in the ancient oceans, perhaps providing oases of NH3 for the production of amino acids and nucleic acids in early life forms.
Brandes and coauthors point to two important conclusions, in addition to the unexpectedly high production of ammonia. The observation that ammonia does not survive at temperatures above 800C indicates that nitrogen on the Earth's surface would have been present only as nitrogen gas (N2) during the early phase of the Earth's development, when asteroid bombardments raised the surface temperature well above 800C. Thus, the ammonia necessary for the origins of life must have been generated after the initial formation of the Earth.
A second important conclusion of the work relates to a long-standing problem regarding the early atmosphere, known as the early faint-sun paradox. The radiation output of the Sun has increased gradually over the past several billion years. During the Earth's formation, the Sun's radiant energy does not appear to have been sufficient to maintain liquid oceans unless, that is, a significant amount of a greenhouse gas was present to trap the Sun's energy and keep the planet warm. Ammonia is one of the most efficient greenhouse gases. If the oceans maintained a steady production of ammonia from hot vents, that ammonia would have also enriched the atmosphere through water-gas exchange at the ocean's surface, perhaps resolving the paradox.
Surprisingly, this research does little to settle an ongoing debate regarding the most probable site of life's origins. The prevailing paradigm for almost half a century has been that life began near the ocean's surface, bathed in sunlight. The rival hypothesis, that life arose near deep hydrothermal vents, is now being investigated at the Geophysical Laboratory and several other labs around the world. The present work demonstrates that high concentrations of ammonia might occur near vents, possibly making them sites for some interesting pre-life chemistry. But this ammonia will spread throughout the ocean and enter the atmosphere as well, thus providing a source of reduced nitrogen for almost any conceivable origin environment.
The Carnegie Institution, through its Geophysical Laboratory and Department of Terrestrial Magnetism co-located in upper northwestern Washington, D.C., is a member of NASA's new Astrobiology Institute, an effort to blend astronomy, biology, chemistry, and physics in the search to identify and understand the origins of life in the universe. NASA funded the research reported here.
The experiments were conducted at the Geophysical Laboratory using a technique developed years ago by Lab director emeritus Hatten S. Yoder, Jr. for the study of the origins of basalt. The nitrogen, water, and minerals are encapsulated in small gold containers and then subjected to high pressures and temperatures in Yoder's high-pressure apparatus. Capsules are opened and ammonia yields are determined by wet chemical techniques.
The Geophysical Lab, led by its new director, Wesley T. Huntress, Jr. (who arrives in October), is one of five departments of the Carnegie Institution of Washington, a nonprofit organization devoted to advanced research and education in the physical and biological sciences. The institution's president is the biologist Maxine F. Singer.
* Brandes is a postdoctoral fellow at the Geophysical Laboratory. Hazen is a staff member. The other authors are visiting investigator Nabil Z. Boctor, staff member George D. Cody, student intern Benjamin Cooper, and director emeritus Hatten S. Yoder, Jr. Benjamin Cooper was killed in an auto accident during the completion of research on the project. The Nature paper is dedicated to his memory.