Life’s Fuel Cells

Life’s Fuel Cells: How Worlds Without Air Could Power Marine Colonies

Hydrothermal-vent ecosystems vs plant photosynthesis. Hydrothermal-vent ecosystems derive their energy from chemicals in a process called "chemosynthesis." In chemosynthesis, the hydrothermal vent acts an energy source (1) instead of the sun. Both processes use carbon dioxide (2) and water to produce sugars (3). As an end product, chemosynthesis produces sulfur (4) while photosynthesis generates oxygen. Credit: Image courtesy Woods Hole Oceanographic Institution and NASA Science News

What is life? To prove that life exists, or did at one time, what signs should we be looking for?

Possible signs of life include a wide range of features on which we can focus. For example, cells walls or membranes made of simple carbohydrates encapsulate the most successful life-forms on our planet: bacteria. Fatty acids-chains of carbons, hydrogens and oxygens-are diagnostic of biosynthetic processes here on Earth. Certain complex molecules, such as proteins and DNA, can only arise from active biological processes. All of these processes and structures have something in common, though: they use energy to renew themselves.

The authors of a paper released today in Astrobiology titled, “The Fuel Cell Model of Abiogenesis: A New Approach to Origin-of-Life Simulations,” suggest that rather than focusing exclusively on the structure of life, we should also be looking for the primary process of life: energy transport. To prove their point, the authors built miniature metabolic fuel cells out of elements that would have been available to life as it labored to evolve on Earth.

At their most basic, fuel cells are systems that convert hydrogen and oxygen into water, and in the process, release energy.

“You can use electrochemical techniques to simulate planetary seafloor systems and some reactions at the emergence of life, since many geological and biological systems function similarly to fuel cells,” said Dr. Laurie Barge, lead author and scientist with the NASA Astrobiology Institute (NAI) Icy Worlds team at JPL. “A hydrothermal vent is a ‘geochemical fuel cell’ because it can drive the transfer of electrons from hydrothermal fuels to seawater oxidants, generating an electrical current in a precipitated mineral chimney wall.”

Shrimps, crab, and sea anenome flourishing near a vent in the Indian Ocean. Credit: Image courtesy Woods Hole Oceanographic Institution and NASA Science News

All life-plant, animal and bacteria-utilizes energy systems based on passing around electrons. One molecule loses an electron, and another receives it. These reactions are classified as “redox”, which is short for, “oxidation-reduction.”

Where there is abundant air and fresh water, life forms hand electrons to molecules eager to accept them, like oxygen and carbon dioxide. Animal cells oxidize sugar and oxygen receives the stripped electron. Plants oxidize water into oxygen in a neat series of reactions we know as photosynthesis.

Unlike animals and plants, bacteria around hydrothermal vents are surrounded by reducers-elements that love to lose electrons-such as iron and nickel. One third of the heat from underwater volcanos and vents is channeled into bacterial communities. The communities then use the heat and those metals to survive using a process called chemosynthesis. Barge’s experiments are trying to simulate whether the geochemistry of these systems can also even encourage the emergence of life by providing enough energy to drive pre-life metabolic reactions, even in the absence of oxygen and light.

“It’s possible that other worlds, such as Europa or Enceladus, could drive similar chemical reactions between their oceans and rocky seafloors to produce electrical potentials and pH gradients,” said Barge.

When you consider what we know about tidal heating in places like Europa, the results of Barge’s experiments are electrifying. Chemosynthesis on Earth has been shown to support not only the growth of bacteria, but shrimp, tubeworms and other marine creatures. Now that Barge’s and her team’s preliminary experiments have been successful with early-Earth-abundant minerals, they can be modified to simulate the geo-electrochemical systems on other worlds that might also have oceans and a rocky seafloor: like Europa, Enceladus, or early Mars.

“If we knew the composition of Europa’s ocean, ocean crust, and the hydrothermal fluid that would result from interaction of these,” said Barge, “then we could build a fuel cell to simulate how much energy would be generated in a hydrothermal system on Europa.”