Certain minerals with distinctive shapes may indicate the activity of bacteria in deep-sea hot springs several billion years ago. They make a crucial contribution to understanding the origins of life. This is what the study by a research team led by Eric Runge and Professor Jan-Peter Duda, who now work at the University of Göttingen, as well as Professor Andreas Kappler and Dr. Muammar Mansor from Geomicrobiology at the University of Tübingen. The study was published in the journal Communications Earth & Environment .
According to geological reports, hot springs have existed on our planet for at least 3.77 billion years. Similar systems are also suspected on other celestial bodies in our solar system on which life could exist. Due to the extremely dynamic physical and chemical conditions, such systems are considered a possible place of origin for the first organic substances on Earth – and also for the first living things.
Tracing evolution
“To understand how life arose, we want to trace the evolution of microorganisms back over billions of years. To do this, we look for traces of life, so-called biosignatures, in the oldest rocks on earth,” explains Eric Runge, who conducted research at the University of Tübingen in an Emmy Noether working group led by Jan-Peter Duda funded by the German Research Foundation both moved to the University of Göttingen. However, according to Runge, it cannot always be clearly clarified whether minerals in rocks were formed through the action of living beings such as microorganisms or solely through chemical and physical processes. “We are sharpening our search for biosignatures and understanding better and better how biologically formed rocks change over long geological periods,” he says.
A particularly promising biosignature is the iron-sulfur mineral pyrite – better known as fool’s gold – found in hot springs in the deep sea. Pyrite can be formed either directly or secondarily from the mineral magnetite when it reacts with the sulfur-rich fluids that occur there. What is crucial is that it occurs in different forms. “In our analyses, pyrite proved to be particularly interesting in its characteristic spherical shape; the structure is similar to that of a raspberry,” reports Andreas Kappler. “It only emerged in this form if the raw material magnetite was formed by iron-reducing bacteria.”
Recreated in the experiment
In the absence of air, certain bacteria can grow and generate energy by transferring the electrons from their food not to oxygen – like humans and animals – but to oxidized iron. This is reduced and magnetite can be formed; a process that is widespread in today’s deep-sea hot springs. In the experiment, the research team has now simulated how magnetite reacts chemically with the sulfur-rich fluids of the hot springs. To do this, it simultaneously exposed non-biologically formed magnetite and biologically formed magnetite in bacterial cultures to conditions such as those found around hot springs in the extreme habitats of today’s magnetite-forming bacteria.
“We observed that both the non-biological and biological magnetite were largely dissolved within hours. However, our investigations using the scanning electron microscope, which were carried out in the Tübingen Structural Microscopy Core Facility (TSM), showed that the crystal shapes of the conversion products differed significantly after a few weeks,” reports Runge. “While in the experiments with non-biological magnetite branched, ‘Christmas tree-shaped’ pyrite crystals formed, the pyrite in the experiments with biological magnetite was more spherical.” Such spherical pyrites can serve as fossil evidence for early bacterial life, summarizes Kappler, ” particularly in the oldest hot spring rocks on our planet.”
“Researching biosignatures is not only relevant for deciphering the history of life on Earth,” says Jan-Peter Duda. “Hot springs, similar to those in our deep sea, could also occur on Saturn’s moon Enceladus, for example. If there is life there, it is most likely microorganisms. Studies like ours provide the basis for identifying their traces.”
‘Black smokers’, hot springs in the deep sea, emit volcanically heated fluids rich in dissolved metals and sulfur. These environments are havens of deep-sea microbial activity.
In the field studies on black smokers, diving robots are used in water depths of up to 3,000 meters, which are equipped with extensive sensors and collect samples of the fluids and rocks.