As a graduate student at the University of Chicago in the early 1950s, Stanley Miller attempted to recreate the atmosphere of early Earth in a round flask. He mixed water vapor, methane, hydrogen, and ammonia, and zapped it with electricity to simulate lightning. The resulting chemical reactions yielded a collection of amino acids, showing that a handful of gases could provide the building blocks to create proteins. Yet in the decades since Miller’s pioneering experiment, scientists have pointed out that his gas composition likely did not match the primitive atmosphere and cast doubt on the possibility that the amino acids in his dilute "primordial soup" could concentrate and evolve into more complex compounds. But, no one since had found abiotic amino acids created independently of life from terrestrial geochemical processes, preserved in Earth’s geological record.
Now, DCO Deep Energy Community members have discovered the “geological mirror” of Miller’s atmospheric experiments and found amino acids that formed in mantle-derived rocks, called serpentinites, from the subseafloor. Bénédicte Ménez (Institut de Physique du Globe de Paris and Université Paris Diderot, France), Céline Pisapia, (Institut de Physique du Globe de Paris, Université Paris Diderot, France, and Synchrotron SOLEIL, France), Muriel Andreani (Laboratoire de Géologie de Lyon, France), Frédéric Jamme, Matthieu Réfrégiers (both at Synchrotron SOLEIL, France), and Laurent Richard (Nazarbayev University, Kazakhstan) worked with colleagues Alain Brunelle, Quentin Vanbellingen (Institut de Chimie des Substances Naturelles, France) and Paul Dumas (Synchrotron SOLEIL, France). The researchers merged several high-resolution microscopy techniques to identify amino acids associated with clays in serpentinites. Their presence lends support to the theory that Earth’s first life may have arisen within hydrothermal systems, beneath vents on the seafloor. They describe their discovery in a new paper in Nature .
“People have predicted the existence of abiotic amino acids from experiments and thermodynamic calculations, but no one has seen them occurring in terrestrial settings, neither in ponds nor in rocks,” said Ménez. “In small pores in the rock, organic compounds can concentrate and react, which is a huge difference from Stanley Miller’s ‘soup,’ where all the compounds are likely too dilute to evolve when formed.”
Read the accompanying News and Views article by John Baross here.
The rock samples came from about 175 meters deep within the Atlantis Massif, a dome of mantle material sitting on the seafloor near the Mid-Atlantic Ridge and hosting the Lost City hydrothermal field. The International Ocean Drilling Program Expedition 304 collected the rocks while drilling into the Atlantis massif in 2004. Within the massif, seawater reacts with mantle-rock forming minerals in a process called serpentinization, to create hydrogen-rich fluids that react with inorganic carbon species to form organic compounds. Such rocks were abundant in early Earth’s crust, and some scientists have estimated that they hosted the ideal conditions for the emergence of life.
At first glance, the researchers thought they were seeing the remnants of microbes in the rocks. “At the beginning, we expected a wide diversity of organic compounds,” said Pisapia. “Using various techniques we found only small organic molecules including amino acids, which was surprising.” Further investigation revealed that the amino acids were free, instead of bound up in proteins, as they exist within cells. Also, they found no biomarkers from biological cells, suggesting that the amino acids formed abiotically, inside tiny pores in the clay. The pores, which were too small to host microbes, served as “nanoreactors” and the iron-rich clays likely catalyzed the formation of the amino acid tryptophan. The researchers suggest that the tryptophan formed through a chemical process called Friedel-Crafts-type reactions, which can add branches to a carbon ring.
These amino acids may be a previously unknown food source for deep microbes in hydrothermal systems, in addition to other abiotic organic molecules that microbes consume, such as methane and formate, at Lost City.
This new discovery builds on the group’s recent paper  in Scientific Reports, where in collaboration with Ludovic Duponchel (Laboratoire de Spectrochimie Infrarouge et Raman, France) they used advanced spectroscopy and a data analysis technique called chemometrics to map organic compounds within rocks and to elucidate their spatial and genetic relationships with minerals. Prior to investigating their nature, the researchers demonstrated a close link between organic compounds and secondary clays formed during serpentinization.
Now, the researchers are working on characterizing the diversity of the amino acids created in serpentinized rocks, and the temperature and chemical conditions under which they form. They also want to sample deeper within the massif to see what kinds of organic compounds form at hotter temperatures. What they find out may help fill in gaps in our knowledge of how Earth shifted from a mineral world to a living one.
“We have to enlarge our way of thinking because up to now, there was no proof of the abiotic synthesis of amino acids on Earth, even less within rocks, and everyone thought it was provided by meteorites,” said Ménez and Pisapia. “We provided pictures showing that Earth can do it deep in its crust.”
Main image: Using a technique called deep UV-microfluorescence, performed at the DISCO beamline of the French synchrotron SOLEIL, researchers show the presence of the amino acid tryptophan inside a clay. The tryptophan, which glows yellow, red, and pink, formed abiotically during the aqueous alteration of the oceanic crust. Credit: IPGP