Further research on the origin of life of the Institute of Molecular Evolution

In modern hydrothermal vents as well in those of the early Earth, molecular hydrogen (H₂) is the key to redox reactions, catalyst synthesis, and ion gradient formation. The hydrogen is created by the spontaneous geochemical process of serpentinization. Water reacts with ferrous minerals deep in the Earth‘s crust. During serpentinization, the mineral catalysts awaruit (Ni₃Fe) and magnetite (Fe₃O₄) are formed in the hydrothermal vents. Organic synthesis in hydrothermal vents is relevant to the origin of life because the reactions involve sustained energy release based on the imbalance between CO₂ and liquid amounts of molecular hydrogen H₂. Hydrogen has been a source of electrons and energy since liquid water existed on the early Earth, and it fueled early anaerobic ecosystems in the Earth‘s crust. One of the oldest and the only energy-releasing pathway of CO₂ fixation in living cells is the acetyl-CoA pathway. It accounts for most of the energy metabolism of acetogens and methanogens, ancient anaerobic autotrophic microbes that live on H₂ and CO₂ via the acetyl-CoA pathway and that still inhabit the Earth’s crust today. As in hydrothermal vents, the used electrons and the generated energy come from H₂. While minerals catalyze the reactions in hydrothermal vents, it is enzymes (proteins) that catalyze modern microbial reactions. The acetyl-CoA pathway provides three key requirements for life: reduced carbon, electrons, and ionic gradients for energy storage. The pathway is linear, not cyclic, it releases energy rather than requiring it, and its enzymes are full of native metal cofactors. The acetyl-CoA pathway traces to the last universal common ancestor, LUCA, and abiotic, geochemical organic syntheses resembling segments of the pathway occur in modern hydrothermal vents.

Although the enzymes that catalyze these modern microbial reactions have been extensively studied, little is known about the catalysts that promote abiotic reactions in modern-day hydrothermal vents and that may have been involved in the origin of life. A fully abiotic analogue of the acetyl-CoA pathway from H₂ and CO₂ as found in life has not been described so far. In order to study the mechanisms of hydrothermal metabolic reactions that mimic original metabolic pathways, we examined three different iron minerals that occur naturally in hydrothermal systems for their ability to catalyze metabolic reactions: greigite (Fe₃S₄), magnetite (Fe₃O₄) and the nickel-iron alloy awaruite (Ni₃Fe).

The geochemical reactions in hydrothermal vents are very similar to the metabolic reactions of the most primitive microbes on Earth. At the origin of metabolism and thus of life, CO₂ fixation by hydrothermal H₂ in serpentinizing systems may have preceded biotic pathways. Can minerals from hydrothermal vents therefore also catalyze biological metabolic reactions and the reaction between CO₂ and H₂? We at the Institute of Molecular Evolution investigated this question.

In experiments in stainless steel reactors, conditions such as those prevailing in alkaline hydrothermal vents are simulated. High pressures and temperatures can be set and controlled. The substances that were available on the still inanimate Earth are put into the reactor: water, minerals made of nickel, iron and sulfur and hydrogen and carbon dioxide. These gases are introduced via hoses after closing the lid. This creates high pressure inside the reactor. Water and minerals are contained within the stainless-steel reactor in small glass bottles that stand in a Teflon cylinder. This prevents the samples from coming into contact with the stainless steel of the reactor, which contains iron, among other things. After the gas has been fed in, the high-pressure reactor is continuously heated to 100 °C using a hot plate and left to stand for a few hours or days. The minerals used were greigite, magnetite and awaruite, which were ground into a very, very fine powder using a ball mill, or which were produced synthetically. Although very different in structure and composition, greigite, magnetite and awaruite are synthesized geochemically in hydrothermal systems from pre-existing ferrous iron and nickel. At the end of the selected incubation times, the reactor was opened after it had cooled and the pressure released, and the samples were examined for their contents.

The American Stanley L. Miller (1930–2007) was the first scientist to experimentally simulate the origin of life. In 1953, he discharged numerous bolts of lightning into a mixture that matched the condition of young Earth. Miller produced simple biomolecules such as amino acids, but lacked important components for designing more complex molecules. William F. Martin and his team want to close this gap with their experiments. Such stainless-steel reactors as can be seen here are used in the experiments. The reactors are the key to the door back to the beginnings of life on Earth. In the reactors, conditions from alkaline hydrothermal vents are simulated. High pressures and temperatures can be set and controlled. The substances that were available on the still inanimate Earth are put into the reactor: water, minerals made of nickel and iron and hydrogen and carbon dioxide are fed in via hoses after the lid is closed. This creates high pressure. Then the high-pressure reactor is heated over a hot plate. The high-pressure reactor then stands still for a few hours or days. The reaction products are then analyzed. They should provide the answer to the question of how and where the first living beings lived and how life came into being. The results of the experiments in the high-pressure reactors and the conclusions that can be drawn from them can be seen in the further course.

Building on evidence for the catalytic reactivity of minerals found in the Earth‘s crust at hydrothermal vents, we investigated the ability of the minerals greigite, magnetite and awaruite to support the reductionof CO₂ with H₂ in water. And thus, to synthesize the first building blocks of life: carbon compounds. Very mild hydrothermal conditions were simulated in the laboratory in a stainless-steel high-pressure reactor: 100 °C and a pressure of 24 bar of a mixture of H₂ and CO₂ (80:20). Water and various minerals (greigite, magnetite and awaruite) were incubated in it. After the incubation, the analysis of the synthesis products showed that in the presence of greigite (Fe₃S₄), a synthesis of the short-chain carbon compounds formate (formic acid) and acetate from H₂ and CO₂ in an almost neutral and alkaline aqueous solution had taken place. Formate, acetate, pyruvate, and methanol were also detected in the samples containing magnetite (Fe₃O₄) as well as in the awaruite (Fe₃S₄) samples. No products were obtained without the minerals. What we see is that the ancient core of microbial carbon and energy metabolism spontaneously unfolds in front of our eyes, overnight at 100 °C in a simulated hydrothermal vent on the laboratory bench. Maybe the reactions of central metabolism are more natural than one might think.

Each mineral has a specific property that leads to a different interaction between the starting materials and the formed materials. For example, nickel is a good catalyst for the formation of formate, while awaruites, which are composed of nickel and iron, are a better catalyst for the synthesis of pyruvate. Therefore, every metal reacts differently on its surface. Thus, an environment with a mixture of various native metals and lots of molecular hydrogen – the alkaline hydrothermal vents – is a good place for the synthesis of some of the oldest prebiotic molecules and thus the building blocks of life.

These products detected in the experiments are identical to the intermediates and products of the acetyl-CoA pathway, the oldest CO₂ fixation pathway and the backbone of carbon metabolism in H₂ dependent autotrophic microbes (acetogens and methanogens). These results strongly suggest that the minerals can catalyze the formation of the building blocks of life without organic catalysts: the carbon compounds. Which agrees well with theories of an autotrophic origin of microbial metabolism under hydrothermal conditions. This implies acetogens and methanogens as very primitive microbial lineages, in agreement with early predictions from physiology and with predictions based on similarities between geochemical and biochemical responses. Life may have originated from alkaline hydrothermal vents.

Our experiments at the Institute of Molecular Evolution confirmed that the metabolism seems to have emerged from geological reactions at hydrothermal vents. If life on Earth started in geochemical environments like hydrothermal vents, it started with gases like CO₂ and H₂. Anaerobic autotrophs still live on these gases today, and they still inhabit the Earth‘s crust. When searching for connections between abiotic processes in ancient geological systems and biotic processes in biological systems, it becomes clear that the chemical activation (catalysis) of these gases and a constant energy source are crucial. The geochemical catalysts identified by the experiments of the Institute of Molecular Evolution, which activate these gases on the way to organic compounds and small autocatalytic networks, is an important step in understanding prebiotic chemistry, which works only on the basis of chemical energy, without the entry from solar radiation. So, if life originated in the dark depths of hydrothermal vents, then understanding reactions and catalysts that occur under such conditions is critical to understanding its origins.

All life, all cells consist of the same matter. Cells function because they have a network of about 1,000 reactions that supply and organize the building blocks of life. Cells form autocatalytic webs of molecules that make copies of themselves. How the first self-sustaining metabolic networks emerged at the origin of life is a big open question. Autocatalytic groups that are smaller than metabolic networks have been proposed as transient intermediates at the origin of life. The Institute of Molecular Evolution was able to demonstrate self-sustaining networks in microbial metabolism that collectively catalyze all their reactions. These autocatalytic networks can be found in the metabolism of primordial anaerobic autotrophs. They uncover intermediate stages in the formation of metabolic networks and close the gaps between early Earth chemistry and life.

Research on alkaline hydrothermal vents and the origin of life has shown that life must have originated in a place that was without sunlight, without oxygen, not too hot, rich in minerals, alkaline and full of gases (H₂, CO₂, N₂). The last common ancestor was anaerobic, CO₂-fixing, H₂-dependent, N₂-fixing, and thermophilic. The metabolism of the cells still contains evidence today that reflects the process by which they arose. The chemolithoautotrophic way of life—the conversion of inorganic carbon into cell mass with inorganic electron donors using chemical energy instead of light—is common among modern microbes that inhabit environments similar to those of the early Earth. This implies acetogens and methanogens as very primitive microbial lineages, consistent with early predictions from physiology and with predictions based on similarities between geochemical and biochemical responses. It is also consistent with the identification of overlapping autocatalytic networks in the metabolism of acetogens and methanogens, implying a role for small molecule reaction systems prior to the advent of protein and RNA.

All life forms known to us use proteins made from amino acids, nucleic acids made from purines, pyrimidines, sugars and phosphate. This means that the first life forms from which all modern forms descended had this core chemistry in addition to the universal genetic code. But how many reactions are needed to synthesize the building blocks of cells and the cofactors needed to make them? What was the nature of the early cells? This gives an idea of how challenging it would be to produce the main components of life at source, with or without enzymes. A bioinformatic analysis by the Institute of Molecular Evolution answered the question. There are 402 reactions found in the original metabolism that converts H₂, CO₂, and NH₃ into amino acids, bases, and cofactors. The core represents a collection of reactions that support the synthesis of RNA and proteins. It was present in the first cells, but it can hardly have arisen all at once. The high-pressure reactor experiments indicate that the core itself probably originated from H₂ and CO₂ and grew outward from pyruvate while nitrogen was incorporated from NH₃. How complex the nucleus might have become prior to the emergence of enzymes is a question for future studies.