Barge, L.M., Kee, T.P., Doloboff, I.J., Hampton, J.M.P., Ismail, M., Pourkashanian, M., Zeytounian, J., Baum, M.M., Moss, J.A., Lin, C.-K., Kidd, R.D. & Kanik, I.
Astrobiology, 14, pp 254-270.
Fuel cells are similar to biological cells, as well as certain geological environments, in that electrons are transferred from fuels to oxidants to produce electrical current. It has been proposed that the electrical energy in hydrothermal systems could have powered some processes that could give rise to life. We argue that fuel cells could be used to simulate some prebiotic reactions and the origin of life because of the conspicuous fuel-cell-like properties of metabolic and geo-electrochemical systems. These modular experiments could be used to investigate a variety of prebiotic catalysts and reactions, and could also be adapted to simulate seafloor systems on other planets, to see if these worlds provided enough energy to kick-start life.
The processes of microbial life today are analogous to those of a fuel cell, in that life maintains gradients across membranes, and harnesses these gradients to drive metabolism. Life does this using metal-containing enzyme ‘electrodes’ to transfer electrons as well as tiny molecular engines that convert proton potentials to chemical energy. The first life on Earth also likely contained some version of these fundamental components that make the biological fuel cell function (ATP-synthase, ion-selective membranes maintaining pH/electrical gradients, the electron transport chain). Certain geological environments, such as hydrothermal vents, also have fuel-cell-like properties: they can generate pH gradients and electrical current through abiotic chemical reactions, and the electrochemical energy provided in these geological settings can drive redox reactions in some ways similar to metabolism. It has been suggested that the first metabolism could have arisen in hydrothermal systems, and so we aimed to develop laboratory methods of understanding the energetic processes that bridged the gap between geological processes of the early Earth and the emergence of life on this planet.
In a paper recently published in the journal Astrobiology, we set out to examine whether origin-of-life chemistry in fuel-cell-like geochemical environments (such as hydrothermal vents) could be simulated in the laboratory as an actual fuel cell. This is a new method for combining fuel cell technology with planetary science / prebiotic chemistry simulations. We first modeled the hydrothermal system as a fuel cell: defining the surfaces of a hydrothermal chimney as the electrodes, the chimney wall as the proton exchange membrane, the hydrothermal fluid as the fuel, and the seawater as the oxidant. We grew little simulated hydrothermal chimneys in the laboratory under early Earth conditions, and created fuel cell electrodes using this chimney material as the electro-catalyst. We then tested if this material can help transfer electrons, as hydrothermal chimneys today are sometimes capable of, as are similar biological enzymes. By testing different types of materials in the simulated chimneys, these fuel cell experiments allow us to observe the particular redox / proto-metabolic reactions that might occur within a hydrothermal chimney, driven by the geological electrochemical gradients – and thus narrow down on the chemistry that might have taken place when life first arose on Earth.
One particular advantage of using fuel cell experiments to simulate geological environments is that fuel cells are modular – meaning one can easily swap out components. So, we can test different compositions of the oceans and hydrothermal fluid, as well as different minerals that might exist with hydrothermal chimneys and act as catalysts. We can investigate the effects of adding organic molecules, or even cells, and experimentally test the transition from geo-energetics to bio-energetics. Finally, there is the exciting possibility of building Mars or Europa fuel cells to simulate the water-rock interfaces on these worlds, and thus help in understanding whether life could have emerged in other planetary environments.