Deep-sea hydrothermal vents give rise to some of the most bizarre forms of life on the planet, such as blind albino crabs. Now a study in tomorrow's issue of Nature suggests that billions of years ago the vents may have helped set the stage for life itself by spewing out large amounts of ammonia, a compound thought to play a critical role in the formation of amino acids, the building blocks of proteins.
Among the many puzzles surrounding life's origins is the origin of nitrogen in amino acids and other biomolecules. Nitrogen gas (N2) is thought to have been abundant in the early atmosphere, but it's largely nonreactive and therefore an unlikely source. Ammonia (NH3) is far more reactive, but its initial source in the early Earth is mysterious.
Robert Hazen and Jay Brandes and their colleagues at the Carnegie Institution of Washington's Geophysical Laboratory wondered whether the high pressures and temperatures found at deep-sea vents could have helped churn it out. Such vents line the midocean ridges, where magma wells up to form new ocean crust. They also spew a continuous stream of superheated water past iron-rich--and possibly catalytic--vent minerals, such as pyrrhotite and magnetite.
Hazen and his colleagues devised a laboratory test by combining a vent mineral, a nitrogen source such as N2 or nitrate (NO3-), and water, then cooking the mixture at varying temperatures and pressures. The results were clear. In most ventlike conditions, the minerals turned into little ammonia factories. At 500 degrees Celsius and 500 atmospheres of pressure, for example, pyrrhotite converted up to 90% of the nitrate to ammonia in just 15 minutes. At lower temperatures of about 300° to 350°C, Hazen says, the ammonia conversion was still as high as 70%.
"It could be that this is the dominant mechanism" for forming ammonia on the early Earth, says Chris Chyba, an early Earth expert at the Search for Extraterrestrial Intelligence Institute in Mountain View, California, and Stanford University. Hazen says that he and his colleagues are currently extending their studies to see if similar conditions help give rise to amino acids and other more complex molecular precursors of life.