What fool's gold reveals about early life – berry-like pyrite crystals only arise from biogenic precursors

Signing early in life? If spherical pyrite crystals appear in an ancient rock, this may be an indication of traces of ancient life. Such berry-shaped pyrites only form when biologically produced magnetite reacts with sulfur-rich hot spring water, an experiment has now shown. Pyrite spheres are thus a biosignature that can help distinguish between true traces of early life and geochemically formed minerals.

When and where did life first appear on Earth? This is not yet clear because there are almost no visible fossils, and the first cells have not been preserved. Paleontologists therefore rely on indirect evidence, including tiny, cellular-shaped fossils or chemical compounds that typically make up biological organisms. But the problem is that many of these molecules and minerals can also originate abiotically, thus misleading researchers.

From magnetite to pyrite

The mineral pyrite (FeS), found in deep-sea hot springs, is a potential biosignature of early life.₂) – also known as “fool's gold”. This iron-sulfur complex can originate geochemically, but also secondarily from biogenic magnetite (Fe₃Hey₄). This iron oxide is made by certain archaea and bacteria that reduce iron as part of the metabolism.

Magnetite biosynthesis remains widespread today among single-celled organisms in seafloor hydrothermal vents – hot springs considered a possible cradle of life. But the problem is that magnetite does not last long in the area around marine vents because it reacts with sulfur-containing fluids in hydrothermal fluids to form pyrite. When it comes to pyrite, it is still difficult to determine whether this mineral was formed biotically or abiotically.

Hydrothermal spring in the laboratory

But now Eric Runge of the University of Tübingen and his colleagues have discovered a feature that points to the prehistoric origin of pyrite. For their study, the team recreated in the laboratory how magnetite interacts with sulfur-rich fluids in hot springs. To do this, they created a mixture of seawater that had a temperature of about 80 degrees and was low in oxygen and whose composition was similar to that of hydrothermal vents on prehistoric Earth.

The researchers then added two different types of magnetite to this mixture: a magnet that is produced purely abiotically through a chemical reaction and a variant of magnetite that is produced biologically by the iron-reducing bacteria Geobacter sulfurreducens. Both approaches were incubated under the same conditions for several days. Runge and his team then examined what happened to the magnetite and what form the pyrite forms.

Cubes, branched branches and gnarled balls

The result: “We observed that both non-biological and biological magnetite were largely dissolved within hours,” says Runge. However, careful analyzes using a scanning electron microscope revealed that the crystal forms of magnetite-forming pyrite differed significantly: pyrite with its abiotic magnetite ancestor developed branched Christmas tree-shaped pyrite crystals or compact cubes.

This was different with pyrite, which is composed of biogenic magnetite: it formed more spherical structures. “The structure of this spherical pyrite is similar to that of a mulberry,” explains Runge's colleague Andreas Kapler. “It only forms this way if the primary magnetite was formed by iron-reducing bacteria.” Typically, the first “raspberries” of pyrite appeared after about three weeks of incubation in warm, sulfur-containing seawater.

Berry-like pyrite as a biosignature

“We demonstrate that the reaction of biogenic magnetite with sulfur-containing hydrothermal water can lead to the formation of berry-like pyrite,” the researchers wrote. Because this distinctive mineral form does not form abiotically, it could serve as fossil evidence of early bacterial life — “particularly in the oldest rocks on our planet that formed from hot springs,” Kapler says.

“Research into biosignatures is not just about deciphering the history of life on Earth,” emphasizes senior author Jan-Peter Duda from the University of Tübingen. These molecular indicators are also relevant in the search for extraterrestrial life. “Hot springs, similar to those found in the deep sea, could exist on Saturn's moon Enceladus, for example,” Duda says. “Studies like ours provide the basis for learning about their effects.” (Communications Earth & Environment, 2024; doi: 10.1038/s43247-024-01400-z)

Source: Eberhard Karls University Tübingen

May 17, 2024 – Nadia Podbrigar