An origin-of-life reactor

The origin of life is one of the most exciting unanswered questions in science. The discovery of Lost City, an alkaline hydrothermal vent system off the mid-Atlantic ridge, gives striking insights into how life might have started on Earth. This project sets out to test some of the possible chemistry and self-organisation that might have arisen in such vent systems four billion years ago, using an origin-of-life reactor that simulates their key properties in the lab.

Living cells are highly ordered, and this order is maintained by a constant flux of energy. Even with modern enzymes, growth requires an extraordinary amount of energy: to grow a single milligram of bacteria takes a flow of electrons equivalent to that in a bolt of lightning! Before the origin of efficient biological catalysts, this flow of energy and matter must have been even greater, yet the laws of thermodynamics mean it still had to be highly structured (low entropy, high enthalpy).

Unlike the famous black smokers, alkaline vents are not volcanic, but are formed from the reaction of water with a mineral common in the oceanic crust, olivine. This reaction metamorphoses olivine into serpentinite, producing large quantities of hydrogen, dissolved in alkaline hydrothermal fluids, as well as heat. The heat drives these fluids up to the sea floor, where they precipitate into microporous honeycomb vent structures. Lost City was formed in this way; but the chemistry of such vents would have been very different 4 billion years ago, when there was little oxygen, and the oceans were much richer in dissolved iron and CO2.

Alkaline vents are natural electrochemical reactors. Four billion years ago their walls would have been composed of minerals including catalytic iron, nickel and molybdenum sulfides. The fluids entering were warm (70-90°C), rich in hydrogen, and strongly alkaline (pH 11), whereas the oceans were cool and rich in CO2, making them mildly acidic (pH 5-6). This means that alkaline vents were riddled with thermal gradients that can concentrate organic molecules; with proton (pH) gradients over mineral walls, and with electrical differences between electron donors (mostly H2) and acceptors (mostly CO2).

         

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An alkaline hydrothermal vent at Lost City, courtesy of Deborah Kelley, University of Washington (scale bar = 1m).

What is remarkable and exciting about these vents is that they offer simple geochemical equivalents to the biological processes still operating in bacteria living there today. These cells gain both energy and organic carbon from the reaction of H2 with CO2. The enzymes responsible contain clusters of iron, nickel and molybdenum sulfide that are almost identical in structure to minerals found in vents. Most intriguingly of all, these bacteria can only grow by using proton gradients across membranes, exactly like those found in the vents. Recent theoretical work indicates why these properties are so important; but the theory has never been tested experimentally.

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Thermal image showing ‘hydrothermal’ fluids entering the reactor via a titanium flow distributor. The reactor volume will be plugged with a microporous ceramic foam (not seen here), which maintains thermal gradients of 60°C across the reactor vessel.

         

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Electron micrograph of ceramic foam, which mimics the microporosity of alkaline hydrothermal vents, while enabling control over the chemistry of the matrix.

We plan to draw on these remarkable parallels to address the origin of life experimentally in a reactor. We hope to show that the thermodynamic driving forces operating in our reactor are sufficient to form key biological molecules such as amino acids, sugars and nucleotides at the concentrations required for the self-assembly of larger structures, and ultimately, we hope, proto-cells.

Dr Nick Lane
University College London

Nick was awarded a Research Project Grant in March 2012; providing £248,883.