Serpentinization of Ocean Crust, Life’s Mother Engine?

Shelf-like “flange” structures jut from the wall of one of the spires in the Lost City hydrothermal field. Image Credit: IFE URI-IAO, Lost City Science Party, and NOAA

In a new study published in Philosophical Transactions of the Royal Society B, scientists funded by the NASA Astrobiology Institute (NAI) advance a theory about life’s origins based on the idea of “reservoir-mediated energy.” This paradigm—in cells—involves constantly filling up and depleting a kind of chemical reservoir that is created by pushing a lot more protons onto one side of a membrane than the other—just like pumping water uphill to fill a lake behind a dam.

Then, mimicking how hydroelectric turbines are driven by water flowing downhill, these protons are only allowed to flow back “downhill” through the membrane by passing through a turbine-like molecular “generator” which creates, instead of high-voltage electricity, a chemical fuel called ATP, the cell’s “gasoline.” All cells then “burn” ATP in order to power their vital processes.

The study looks at how a geochemical process known as serpentinization pioneered this system before life even began, giving it a “free gift!” At the time life arose, the world was almost entirely covered in a weakly-acidic ocean, the atmosphere was rich in CO2, and tectonic processes constantly replenished and destroyed the crusts of the ocean floor, as they still do today. It is the exposure of newly made ocean crust to the ocean, such as what happens at hydrothermal vents, that gives rise to the geochemical magic of serpentinization.

The study could aid in the search for signs of life on worlds like Europa. Image Credit: NASA/JPL-Caltech/University of Arizona

As areas of new ocean crust cool, the still-stressed rock becomes brittle and develops cracks. Cold seawater gravitates down the cracks where it is heated and reacts chemically with rock minerals to form a highly-alkaline solution. This transformed water, or vent fluid, is then driven back to the surface, where, in Hadean times, it reacted with cooler, mildly acidic ocean water. These reactions create precipitates that form massive chimney-like towers similar to chemical gardens.

These highly-structured precipitate-chimneys are comprised of numerous micro-compartments bounded by semi-permeable “mineral membranes.” Across these membranes, a proton (pH) gradient arises between the extremely alkaline emerging vent fluids and the surrounding, relatively acidic ocean.

This pH gradient is almost exactly the same as the gradient that all living cells constantly recreate with the same strength and the same direction: acidic on the outside and alkaline on the inside.

“It is at least highly suggestive that every living thing is constantly and indeed furiously recreating something equivalent to this ancient ‘ocean effluent’ membrane-based proton gradient that serpentinization handed life to start with on the rocky floor of the ancient Hadean ocean,” said co-author Elbert Branscomb of the University of Illinois. “It was, in part, by exploiting that naturally-given, geochemical proton gradient that the engines required to produce the molecular ‘starter kit’ of life got going. So suddenly it’s obvious why we pump protons and use this silly method—we became dependent on this ‘free lunch’ energy system when life was born, developed a lot of fancy machinery for using it, and have never severed that umbilicus since.”

The scientists have developed an experimental system to study serpentinization and look at chemical reactions that pave the way for life in simulated vents. They observe that hydrothermal vent fluids lead to the production of a simple chemical called acetate (similar to vinegar). Acetate can then be transformed into biological molecules.

Their findings could help define chemical signatures to look for when searching for life on icy worlds like Europa and Enceladus.