Trace elements of early microbial life in rocks could help determine when life began on Earth

Using a powerful microscope, University of Alabama researchers have demonstrated a method that can provide powerful data for determining the origin of life on Earth and whether life existed on other planets.

Led by UA geologists specializing in the use of powerful microscopes for biological material, researchers have demonstrated that trace elements of early microbial life can be found in rocks and minerals, which help determine the onset of of life on earth could help Proceedings of the National Academy of Sciences.

Certain bacteria produce crystals, but it can be difficult to decide whether an ancient tiny crystal in a rock was made by bacteria or by inorganic mineral formation. Using atom probe tomography in a study led by Dr. Alberto Perez-Huerta, professor of geological sciences at UA, the researchers showed they could identify the nanoscale chemical imprints left by the bacteria that distinguish the crystals from those formed in a geological process.

The method could be applied to samples from other planets, such as a meteorite or a sample brought back from Mars by spacecraft, to assess whether the results say bacteria were once present in the sample.

“Previous studies that have argued for a link between the origin of life and minerals were based on circumstantial evidence,” said Perez-Huerta. “Our approach provides the striking weapon that allows scientists to visualize direct evidence of this connection. This can unlock a lot of valuable data about how life appeared on Earth millions of years ago and may raise questions about what constitutes life on other planets.”

For the study, the UA researchers used the instruments at the Alabama Analytical Research Center, a research facility dedicated to nanoscale characterization — the study and manipulation of materials that can be 1,000 times smaller than a human hair. It includes atom probes that show a detailed 3D atomic map of a material.

However, studying organic materials at this scale has been difficult, limiting answers to the question of when life arose on Earth. Scientists date the earliest rocks to around 3.8 billion years, with possible evidence of bacteria appearing in rocks around 3.5 billion years ago. However, it is not clear how or when bacteria formed, nor is it clear what type of bacteria formed earliest.

Crystal formation is the most likely place to find these early microbes. A type of life form called magnetotactic bacteria form magnetic crystals within themselves to align with the Earth’s magnetic field, essentially creating a compass for navigation. When they die, the crystals are left behind, but at that size it was hard to tell if a crystal came from the bacteria.

Using samples known to contain organic crystals, the UA-led project showed that the magnetotactic bacteria leave behind nano-sized traces of the elements carbon and nitrogen, necessary for life on Earth.

“We have shown that on this small scale we can find the most important building blocks for life,” said Perez-Huerta. “The ability to find these biosignatures allows us to understand the primitive life forms that started evolution.”