A team of researchers at the Universities of Tübingen and Göttingen has found that certain minerals with characteristic shapes could indicate the activity of bacteria in hydrothermal vents—or black smokers—in the deep ocean several billion years ago.
The geological record shows that hot springs have existed on our planet for at least 3.77 billion years. Researchers consider that, due to their extremely dynamic physical and chemical conditions, hot spring systems may have given rise to organic substances and to the first life on Earth. Similar systems are thought to exist on other planets in our solar system, suggesting life could exist there too.
“In order to understand how life originated, we are following the evolution of microorganisms back billions of years. To do this, we are looking for traces of life, which we call biosignatures, in the oldest rocks on earth,” explains Eric Runge, who conducted the research at the University of Tübingen.
It is not always clear whether structures or minerals in rocks are formed by the action of living organisms such as microorganisms or solely by chemical and physical processes. “We are honing our search for biosignatures, gaining a better understanding of how biologically formed minerals change over long geological periods,” Runge says.
One particularly promising biosignature is the iron-sulfur mineral pyrite (popular as fool’s gold) which is abundant in hydrothermal vents on the ocean floor. Pyrite can either be formed directly or secondarily from the mineral magnetite when it reacts with sulfur-rich fluids found with it. Crucially, it occurs in various forms, with one form—globular aggregates of pyrite crystals called framboids—of particular interest to the authors.
“In our analyses, pyrite in its characteristic spherical form proved to be particularly interesting, with a structure similar to that of a raspberry,” reports Andreas Kappler, study coauthor and a geomicrobiologists at the University of Tübingen. “It only formed in this shape when the starting material—magnetite—was formed by iron-reducing bacteria.”
In the absence of air, certain bacteria can grow and generate energy by transferring electrons from sulfur-compounds to oxidized iron. This is reduced and magnetite (Fe2+Fe3+2O4) can be formed; a process that is widespread in today’s hydrothermal vents on the ocean floor.
In the experiment, the research team has now simulated how magnetite reacts chemically with the sulfur-rich fluids produced there. To do this, they took both non-biologically formed magnetite and magnetite formed biologically in bacterial cultures, and exposed them separately to the conditions that prevail in the extreme habitats of today’s magnetite-forming bacteria around black smokers.
“We observed that both the non-biological and biological magnetite were largely dissolved within hours. However, our investigations using a scanning electron microscope showed that the crystal forms of the transformation products differed significantly after a few weeks,” so Runge. “While pyrite crystals – branched and shaped like fir trees – formed in the experiments with non-biological magnetite, the pyrite in the experiments with biological magnetite was more spherical.”
Therefore, Kappler concludes, such spherical pyrites can serve as fossil evidence for early bacterial life, “especially in the oldest rocks formed by hot springs on our planet.”
“However, research into biosignatures is not relevant solely for deciphering the history of life on Earth,” adds Jan-Peter Duda, another geomicrobiologist involved in the study. “Hot springs, similar to those on the ocean floor, could occur for example on Saturn’s moon Enceladus. If there is life there, it is most likely to be microorganisms. Studies like ours provide the basis for recognizing the traces of such organisms.”
The full research paper “Hydrothermal sulfidation of biogenic magnetite produces framboid-like pyrite” was published in the journal Communications Earth & Environment and can be found online here.
Additional material and interviews provided by the University of Tübingen.
