Metal cells may have held the chemicals of life’s origin captive
In the black depths of the ocean, tectonic plate shifting causes fissures to open in the Earth’s crust. Ocean water is sucked into these cracks, becomes superheated by molten rocks, and then is forcibly expelled. As the scalding water rushes back up through the crust, it becomes infused with chemicals like hydrogen, sulfur, and iron.
|Due to the antiquity of hyperthermophiles, some scientists believe that hydrothermal vents, like the one shown above, could be the birthplace for life on Earth.
Image Credit: NASA
Despite the harsh conditions of these hydrothermal vent sites, they are home to various organisms, including some of Earth’s most primitive life forms: archaeans known as hyperthermophiles. These single celled organisms feed on vent chemicals and grow best in temperatures ranging between 80 and 113 degrees C (176 and 235 degrees F). Due to the primitive nature of hyperthermophiles, some scientists believe that hydrothermal vents could be the birthplace for life on Earth.
Others argue that the hyperthermophiles were not the earliest life forms, but instead are the sole survivors of an episode in Earth’s history when temperatures rose dramatically. Such an event, they say, killed off all life forms except those that could survive the hot temperatures. Thus, the birthplace of life remains a controversial and much-debated topic.
New support for the hydrothermal origin theory was recently provided by William Martin of Heinrich-Heine University in Dusseldorf, Germany, and Michael Russell of the Scottish Environmental Research Centre in Glasgow. Publishing in the journal Philosophical Transactions, the authors suggest life originated in iron sulfide deposits formed at hydrothermal vents.
The iron sulfide at the vents piles up over time, creating unprepossessing gray lumps with a complex honeycomb structure inside. The holes are typically a few hundredths of a millimeter across, and the scientists suggest that the first life may have been contained within these microscopic cavities.
"It is very worthwhile for biologists and biochemists to have a look at such (deposits), because most biologists and chemists would imagine an iron sulfide precipitate as a mere clump of mineral with no internal structure," they write.
|Dr. Michael Russell of the Scottish Environmental Research Centre in Glasgow believes that life originated in iron sulfide deposits formed at hydrothermal vents.
Image Credit: Scottish Environmental Reseach Centre
The scientists say the iron sulfide compartments acted as semi-permeable cell walls, allowing hydrothermal fluids rich in hydrogen, cyanide, sulfides, and carbon monoxide to enter, yet preventing these chemicals from diffusing away into the ocean. Thus, the metal compartments kept the building blocks of life concentrated, providing an enclosed environment for chemical reactions to take place.
According to the scientists, the iron sulfide compartments also provided three-dimensional molds for the first lipid cell walls. Once a lipid membrane could be manufactured, the cells became free-living and floated out into the ocean, free at last after millions of years of evolution. But once freed from their compartments, the cells had to be able to sustain themselves away from the continuous input of vent chemicals. The cells that survived were the ones who developed the proteins associated with redox chemistry, allowing them to produce energy by feeding on inorganic chemicals (chemoautotrophy).
The cells were "alive" before their release from the iron sulfide chambers, say the scientists, because they had developed the precursors to RNA and DNA. The cells could replicate themselves and pass their genetic information on to their offspring. Before this type of replication, the cells had reproduced through inflation by the vent seepage waters.
"We say that the last common ancestor of all cells was alive in the sense that it had replication, translation, protein synthesis, redox chemistry and the like, but it was confined to its inorganic, iron monosulfide incubator," says Martin. "Within those confines, we argue, some differentiation of lineages belonging to last common ancestor population was also possible and did occur. Later, free living cells arose."
According to Martin, this scenario explains why all cells have the same genetic code, genetic material, and homochirality (left-handed amino acids and right-handed sugars), and on the other hand why bacteria and archaea differ in so many fundamental biochemical attributes beyond the basic genetic system.
|The first person to suggest an iron sulfide origin of life was a German patent lawyer named Gunter Wachtershauser (shown above).
Image Credit: Wachtershauser & Hartz
For instance, the scientists say that the prokaryotic branches of bacteria and archaea split while still contained within the iron sulfide birthplace. The ancestors of today’s bacteria made one kind of lipid membrane, while the ancestors of the archaea generated lipids in a completely different way.
"Both the fatty acid lipids for bacteria and the isoprenoid lipids for archaea start from acetyl-coenzyme A, a truly universal intermediate," says Martin. "The function of the fatty acid and isoprenoid lipids is the same, but the route to get there differs. As a modern example, think of wings in insects and birds; those wings arose completely independently."
Martin says the different lipids might just be a case of nature experimenting with different routes and yet arriving at the same conclusion. Russell also suggests that temperature variations in the iron sulfide mound could account for two compartments receiving the same inputs, yet developing completely different methods to engineer a lipid membrane.
David Deamer, a professor of chemistry at the University of California, Santa Cruz, studies the origin and evolution of biological membranes. Deamer says Martin and Russell’s new hypothesis is a thoughtful attempt to link the origin of cellular life to the porous spaces within iron sulfide mineral deposits. However, he also thinks the hypothesis needs to be tested experimentally.
"This is a reasonable idea, but the authors should try to demonstrate that iron sulfide membranes represent a permeability barrier to the free diffusion of ionic solutes," says Deamer. "Biological membranes are diffusion barriers because they contain a lipid bilayer, and the hydrophobic ‘oily’ portion of the bilayer prevents ionic solutes from readily entering or leaving the compartment. It seems unlikely to me that a mineral membrane could perform a similar feat."
The first person to suggest an iron sulfide origin of life was a German patent lawyer named Gunter Wachtershauser. In 1988, he suggested that a mineral in the Earth’s crust called pyrite (iron disulfide, or FeS2) may have acted as a catalyst for the precursor chemicals of living cells. In this "iron sulfide world," chemicals bound to the positively charged pyrite were able to derive energy from chemical reactions.
Rather than the first life creating its own lipid membranes in a 3-D metal chamber, as Martin and Russell propose, Wachtershauser’s model has prebiotic chemistry starting on a 2-D metal surface. In this view, one of the catalytic chemical processes on the pyrite started to generate lipids, which then formed a bubble around the prebiotic system. This 3-D cell then drifted free of the surface on which it was generated.
Russell also published a study in 1988 implicating iron sulfide as the site of life’s origin, but he looked at botryoids (grape-like clusters) that could have acted as compartments. Martin and Russell say that surface catalysis models like Wachtershauser’s have a fundamental drawback: "Once two molecules have reacted on a surface, they diffuse away into the Hadean ocean, never to react again," they write.
|Iron monosulphide precipitates shown in electron micrograph of a 360 million year old pyrite precipitate found at the Tynagh ore deposit, Ireland
Image Credit: Banks, Phil. Trans.
If life did arise from iron sulfide, it may have gone something like this: heated water laden with the carbon- and hydrogen-rich gases around the hydrothermal vents combined with the help of metallic ions in iron sulfide to form acetic acid. With the addition of another carbon monoxide molecule, pyruvic acid formed. When pyruvic acid met with ammonia, it formed amino acids, which then linked up into proteins. Formaldehyde formed sugars, and cyanide formed bases. From that point, the genetic precursors to RNA and DNA could arise.
In 2000, George Cody and his colleagues at the Carnegie Institution of Washington conducted an experiment that seemed to support the iron sulfide hypothesis. To replicate primitive vent conditions, Cody used a recipe of iron sulfide, formic acid (another element at the vents), and alkyl thiol (a sulfurous compound similar to alcohol that is produced by the combination of iron sulfide and carbon monoxide). These chemicals were enclosed in a small gold capsule and then subjected to elevated temperatures and pressures similar to the conditions at hydrothermal vents. The researchers measured an increased yield of pyruvic acid with pressure.
Cody’s finding addresses one of the biggest arguments against life’s origin at hydrothermal vents: the incompatibility of high temperatures with life’s precursor chemicals. Pyruvic acid is extremely heat-sensitive, decomposing at 165 C (329 F). Amino acids, RNA, and other molecules necessary for life also break down when exposed to high temperatures. Vent temperatures typically range between 350 C to 70 C (662 F – 158 F), but the Cody study suggests that high-pressure conditions may have kept complex organic compounds stable.
But according to Norm Sleep, professor of geophysics at Stanford University, we don’t really know what the pressure conditions were like during Earth’s early years, because we don’t know how deep the oceans were back then. But Sleep does say that the temperature of the vents may have been lower. Even today, the temperature of the vents is largely controlled by the physics of the water.
"Hydrothermal systems can be quite cold – a few degrees Celsius in the high Arctic and Antarctic," says Sleep. "The iron sulfide deposits near a modern vent can be cool also."
According to Sleep, the Earth was a hot, molten place after a Mars-sized body collided with Earth 4.5 billion years ago and produced the moon. Eventually, the Earth cooled to 100 C (212 F), but did not linger at that temperature for very long.
"There is only enough internal heat in the Earth to affect ocean temperatures and climate for the first few million years after the moon-forming impact," says Sleep. "The sun was dim then and the impact ejecta kept carbon dioxide low in the air. The ocean was probably clement to cold during most of the Hadean."
Life is believed to have originated sometime between 4.3 and 3.8 billion years ago, and Martin and Russell suggest that the vent seepage at that time could have been only 50 C (122 F). Under these cooler conditions, the molecules of life could have survived.
If life did arise from iron sulfide at hydrothermal vents, then life might just as easily arise on other rocky worlds with volcanic activity and liquid water. Mars, for instance, has a wide range of volcanic features and it also may have once had a water ocean.
Hydrothermal vents on Earth are still generating iron sulfide mounds, but Martin says it’s unlikely that any new life forms could arise because of the type of iron ions available.
"In the ancient ocean, Fe(II) was the predominant form," says Martin. "Today, almost all iron is Fe(III), the more oxidized form. Since there is no Fe(II) left in the oceans today, the model for the origin of life we envisage would not work here and now in nature."
Analyzing modern iron sulfide deposits therefore will not add any insight to the composition of the last common ancestor, but it may be possible to look at ancient deposits for clues to life’s origin.
"Deposits that were laid down during anoxic times – more than 2.5 billion years ago, when Fe (II) was abundant – would be very interesting indeed, provided that they are well preserved," says Martin.
In addition, Russell is studying the formation of inorganic membranes, and setting up laboratory recreations of the hydrothermal vent conditions.
"We have already shown that iron sulfide membranes generated in conditions simulating those of the early Earth can hold a tension of 500 millivolts," says Russell. "(That’s) quite enough to drive a primitive metabolist."