Underwater thermal vents may have given rise to the first molecular precursors of life

To test the hypothesis, researchers from Brazil, the United States, and Japan built bench-scale reactors that simulate the interaction between hydrothermal fluids and primitive ocean water.

By José Tadeu Arantes  |  Agência FAPESP – A study published in the Journal of the American Chemical Society recreated in the laboratory chemical reactions that may have occurred on Earth about 4 billion years ago, producing the first molecular precursors for the emergence of life. The experiment showed that, without the presence of enzymes, natural gradients of pH, redox potential, and temperature present in underwater hydrothermal vents could have promoted the reduction of carbon dioxide (CO₂) to formic acid (CH₂O₂) and the subsequent formation of acetic acid (C₂H₄O₂). Redox potential is a measure of the tendency of a substance to gain or lose electrons in an oxidation-reduction reaction. The results confirmed the hypothesis that underwater hydrothermal vents played a key role in the process.

“The hypothesis is that these physical-chemical contrasts present in the vicinity of the thermal vents generate a natural voltage, as occurs between the inside and outside of the mitochondria. It’s this voltage that sustains the chemical reactions,” said the first author of the study, Thiago Altair Ferreira. Ferreira holds a PhD in science from the Department of Physical Chemistry at the São Carlos Institute of Chemistry at the University of São Paulo (IQSC-USP) in Brazil and is currently a researcher at the Institute of Physical and Chemical Research (RIKEN) in Wako, Japan.

Alkaline hydrothermal vents release hot fluids (typically around 70 °C) that are basic (with a pH between 9 and 12) and rich in molecular hydrogen (H₂). These fluids mix with the colder water (around 5 °C) from the primitive ocean, which is slightly acidic (pH around 5.5). In these environments, mineral walls rich in micropores and capable of conducting electrons form from iron and nickel sulfides. The contrast generates natural gradients analogous to those that sustain cellular metabolism today.

“In the Hadean, there would have been a colder, more acidic ocean and, emanating from hydrothermal vents, a hot, alkaline fluid. That alone would have produced a certain voltage, comparable to what we know exists in cellular processes today. Our experiment sought to determine whether this voltage alone could trigger a carbon fixation reaction. And we found that it could,” Ferreira summarizes.

The Hadean is the oldest eon in Earth’s history. A geological eon is the largest unit of time on the geological scale. It can last from hundreds of millions to billions of years and is subdivided into geological eras. The Hadean corresponds to the period from approximately 4.6 billion years ago, when the planet formed, to about 4 billion years ago, when the next eon, the Archean, began.

To test the hypothesis, the researchers built bench-scale reactors that simulate the interaction between hydrothermal fluids and primitive ocean water. These reactors have independent controls for temperature, mineral composition, and the passage of electrical currents, whether spontaneous or induced. Iron-sulfur (Fe-S) minerals and their nickel-containing variants (Fe-Ni-S) were used as mineralogical mediators of the process. “Iron-sulfur and iron-nickel-sulfur minerals are very similar to the metal centers we see today in various enzymes. This allows us to consider protometabolism – a metabolism without enzymes – as the trigger for the process,” Ferreira says.

In the experiments, micromolar concentrations of formic acid and acetic acid were detected on the “oceanic” side of the reactor under pH gradients and in the presence of Fe-S or Fe-Ni-S. This indicates coupling between H₂ oxidation on the “hydrothermal” side and CO₂ reduction on the “oceanic” side through the conductive mineral barrier. These are the first two steps of the Wood-Ljungdahl pathway.

Named after American biochemist Harland Wood (1907-1991) and Swedish biochemist Lars Ljungdahl (1926-2023), this pathway is a metabolic route for carbon fixation that uses hydrogen as an electron donor. In this pathway, methanogenic and acetogenic bacteria convert CO₂ into acetyl coenzyme A (acetyl-CoA), which has phosphate bonds that can store considerable amounts of energy, similar to those in adenosine triphosphate (ATP). ATP is the main molecule responsible for energy storage and transport in all living cells. The Wood-Ljungdahl pathway is considered one of the oldest biochemical pathways on Earth and was possibly active as early as the Hadean eon.

“We focused on two products: formic acid and acetic acid. The first step – converting CO₂ into formic acid and then into acetic acid – is the limiting factor in the process, the most difficult part in terms of energy. We solved it using only minerals,” Ferreira explains.

The study also examined the role of electric currents and found that tiny currents, on the order of nanoamperes (10⁻⁹ A), were enough to efficiently reduce CO₂. “This suggests that very small but constant electric currents at the bottom of the primitive sea would be enough to sustain a protometabolism,” Ferreira comments.

The results of the study reinforce the role of alkaline hydrothermal vents on primitive Earth, showing that two protometabolic stages can emerge from natural gradients and mineral surfaces without the need for complex biological machinery. “The initial condition for life is not a ‘soup’ of organic molecules, but order in the right place and at the right time, maintained by exchanges of energy and entropy. We worked on the logic of physical-chemical gradients triggering reactions in the presence of mineral surfaces that resemble the active sites of enzymes,” Ferreira summarizes.

Although the study focused on basic science with possible astrobiological applications (proposing scenarios for oceanic environments on Jupiter’s moon Europa and Saturn’s moon Enceladus), the approach also inspires technological applications. “Given the importance of metal sites analogous to those of enzymes, we can conceive of more stable and effective materials and conditions for electrocatalysis and hydrogen production, which is currently a major focus as a sustainable energy alternative, as well as for reducing atmospheric CO₂, which is a fundamental problem in the context of climate change,” Ferreira suggests.

The study brought together researchers from Brazil, Japan, the United Kingdom, and the United States. Among them was Professor Hamilton Varela, Ferreira’s doctoral advisor.

“The work, developed by Ferreira during his doctoral studies and then refined during his postdoctoral studies, provided experimental evidence of the role of temperature, pH, and potential gradients in CO₂ reduction and opened up important perspectives in the field. This study was developed as part of a Thematic Project of the Electrochemistry Group at IQSC-USP and corroborates the transdisciplinary aspect of electrocatalysis and the importance of basic research,” Varela says.

The study also received support from FAPESP through a research internship abroad.

The article “Carbon reduction powered by natural electrochemical gradients under submarine hydrothermal vent conditions” can be read at pubs.acs.org/doi/10.1021/jacs.5c01948.