Origins of life research: Geothermal ammonia

Life requires nitrogen. Amino acids are – as many of us first learned in the third grade – the “building blocks of life”; proteins and nucleic acids are the machine tools and factory parts of the cell, and to build those you have to use fixed N. Atmospheric N2 is not very useful for building amino acids, as it happens. The N2 triple bond is very strong and very stable, and requires environmental heat sources such as lightning or fire to break, opening up N radicals to recombination with water or oxygen gas to produce nitrogen oxides. Fixed N is required for life to exist, but it’s also in short supply.

On the primordial Earth there had to be enough fixed N lying around to get life going. Meteorites and comets brought in a bounty of pre-made amino acids, but there had to be a continuous fresh supply somewhere on Earth itself, otherwise life would have been impossible. In particular life needs ammonia, NH3, because it’s easier chemically to produce new amino acids using ammonia instead of oxidized N compounds like nitrate. On the early Earth there would have been a steady trickle of nitrate produced from atmospheric N2 by lightning, but ammonia would have been a lot more useful to originate and promote life.

A new study by Jay Brandes at the Skidaway Institute of Oceanography, and colleagues, provides a possible method by which ammonia could have been produced in Earth’s oceans during the Hadean: hydrothermal vents. Near sea floor volcanoes such as those found at today’s mid-ocean ridges, hot water is constantly cycled through geothermal pipeworks. The water spouting from those sea floor hot springs is charged with reactive sulfide and hot metals, which plate out to form chimney rock as the geysering blasts spew into cold, deep seawater. That chimney rock is made of iron sulfide, and the new study by Brandes and company shows that hot iron-sulfide rock can be very useful in converting nitrate into ammonia.

Lightning can turn N2 into nitrate – this works even without oxygen in the air, and there wouldn’t have been any O2 back then – and nitrate rains out quickly to dissolve into ocean water. Once in the ocean, nitrate today is used up readily by plankton, but before the onset of life nitrate was free to build up more in ocean water. On the early Earth sea floor geothermal vents were very common, because the Earth was younger and hotter, so nitrate in seawater would have constantly cycled through these vent systems. The new Brandes study shows that under conditions like those, nitrate reacts with iron-sulfide rock to form ammonia with fairly high efficiency rates. Plenty of ammonia on hand, in just the locations where life was getting its first foothold.

More work needs to be done, exploring how efficiently this process could have aided the production of more complex amino constructs like nucleic acids, but even so it’s a strong step forward in origins of life research.

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~ by Planetologist on February 9, 2009.

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