Research into the emergence of life on Earth has often focused on the role of deep-sea hydrothermal vents, these imposing structures located at the bottom of the oceans which constantly pump out a mixture of organic and inorganic materials.
Inside these plumes are minerals called iron sulfides, which scientists believe may have helped trigger the first chemical reactions that gave rise to life.
These same minerals are also found in current hot springs, such as the Grand Prismatic Spring in Yellowstone National Park, in the United States. Hot springs are bodies of underground water heated by volcanic activity beneath the Earth’s surface.
New research adds to a small but growing body of evidence that ancient versions of these hot springs could have played a key role in the emergence of life on Earth. This helps bridge the gap between competing hypotheses about where life may have arisen.
The importance of carbon sequestration for life
Carbon fixation is the process by which living organisms transform carbon dioxide present in the air and dissolved in water into organic molecules. Many life forms, including plants, bacteria, and microorganisms called archaea (archaea), use different pathways to achieve this. Photosynthesis is an example. Each of these pathways includes a cascade of enzymes and proteins, some of which contain cores made of iron and sulfur.
Proteins containing these iron-sulfur cores are found in all forms of life. Indeed, researchers suggest that they go back to the Last Universal Common Ancestor (LUCA), an ancestral cell from which life as we know it would have evolved and diversified.
Iron sulfides are minerals that form when dissolved iron reacts with hydrogen sulfide, the volcanic gas that makes hot springs smell like rotten eggs.
If we look closely at the structure of these iron sulfides, we find that some of them are incredibly similar to iron-sulfur clusters. This link between iron sulphides and carbon fixation led some researchers have proposed that these minerals played a crucial role in the transition from early Earth geochemistry to biology.
This research deepens this knowledge by studying the chemical activity of iron sulfides in ancient terrestrial hot springs whose geochemistry is similar to that of deep water springs.
The simulation gave clear results
For this study, the researchers designed a small custom chamber that allowed them to simulate the hot spring environments of early Earth. They then dispersed samples of iron sulphide synthesized by this chamber.
The researchers simulated the conditions of primitive hot springs to answer several questions.
Some were pure. Others were dosed with other metals normally found in hot springs. A lamp placed above these samples simulated sunlight on the surface of the early Earth. Different lamps were used to simulate lighting with different amounts of ultraviolet rays.
Carbon dioxide and hydrogen are constantly pumped into the chamber. These gases have been shown to be important for carbon fixation in deep-water chimney experiments. It was found that all synthesized iron sulfide samples were capable of producing methanol, a product of carbon fixation, to varying degrees.
These results show that iron sulfides can facilitate carbon fixation not only in deep-sea hydrothermal vents, but also in terrestrial hot springs.
Methanol production also increased with visible light irradiation and at higher temperatures. Experiments with varying temperatures, lighting, and water vapor contents showed that iron sulfides likely facilitated carbon fixation in terrestrial hot springs on the early Earth.
The conclusions are satisfactory
Additional experiments and theoretical calculations revealed that methanol production occurs via a mechanism called the reverse water-gas shift reaction. A similar reaction has been observed in the pathway used by some bacteria and archaea to transform carbon dioxide into nutrients.
This pathway, called the “acetyl-CoA” pathway or the “Wood-Ljungdahl” pathway, is considered as the oldest form of carbon fixation that appeared in the early stages of life.
This similarity between the two processes is interesting because the first takes place on dry land, at the edge of the hot springs, while the second takes place in the humid environment inside the cells.
This study demonstrates the production of methanol under a wide range of conditions that might have been encountered in hot springs on the early Earth.
These results expand the range of conditions under which iron sulfides can facilitate carbon fixation. They show that this can happen both in the deep sea and on land, although it is due to different mechanisms.
The scientists believe these results support the current scientific consensus that suggests that iron-sulfur clusters and the acetyl-CoA pathway are ancient and likely played an important role in the emergence of life, whether it took place on land or at the bottom of the sea.
Article reference:
Iron sulfide-catalyzed gaseous CO2 reduction and prebiotic carbon fixation in terrestrial hot springs. Nature Communications (2024). Nan, J., Luo, S., Tran, Q.P. et al.
