NATIONAL MUSEUM OF NATURAL HISTORY
Clay-Encrusted Microbes Provide Clues to How Early Life Developed on Earth and Potentially Mars
Smithsonian scientists study saline lakes in the Chilean desert to travel back in time to ancient Earth and beyond
Buried deep within Earth’s geologic record lies evidence of at least 3.5 billion years of life. The oldest traces take the form of layered sedimentary rocks called stromatolites, which are built by microorganisms over time. Deciphering how modern microbial communities construct these geologic features could help researchers discover even earlier forms of life on Earth and potentially even Mars.
For Erica Suosaari, a carbonate sedimentologist at the Smithsonian National Museum of Natural History, studying the modern is the best way to understand the past. It was this ambition that led her to Northern Chile in 2017 to conduct research on the Puquios lakes in the Atacama Desert. These lakes act as windows into the ecosystems of early Earth, providing present-day examples of microbe-mineral interactions that preserve structures in the geologic record. “It’s like how a dinosaur leaves a footprint,” said Suosaari. “They leave recognizable patterns within the fabric of the rock, which are evidence that bacteria were once living there.”
The Puquios lakes may sound inviting, but they are extremely salty and shallow. While they wouldn’t make for a refreshing dip on a hot day, the lakes are still teeming with life: each one hosts unique communities of cyanobacteria that form multi-layered microbial mats.Suosaari was looking for more than just microorganisms on her expedition. She became fascinated by how the organic mats were developing inorganic minerals. “We went to see how the minerals are forming as a result of what the microbes are doing,” she said. “And in that we found this stepwise pattern of clays and carbonates.”
Previous research studies had discovered that microbial communities form Magnesium-rich clays. But an additional step in the preservation process where the clays give rise to minerals remained largely unknown. The carbonate minerals that form from the clays are what eventually complete the geologic formations that preserve microbial mats for billions of years.
After taking samples of microbial mats from several saline lakes, Suosaari and her colleagues launched into a lengthy summer of analysis, hoping to answer some of the fundamental questions about this mineralization process. The main method that the researchers used was Focused Ion Beam Scanning Electron Microscopy (FIB-SEM), a technique which essentially vaporizes the samples and allows the imaging of successive slices that are then used to rebuild a 3D model of the internal structures of microbe-mineral associations. “What is remarkable about this technique is that we can capture the shape and arrangement of very tiny crystals,” said Ioan Lascu, a bio-mineralogist in the museum’s Department of Mineral Sciences and a lead researcher on the Chilean study. “We are talking nanometers that are 1000 times smaller than the width of a human hair.”
In December 2022, the research team published their findings in the journal Communications Earth & Environment, pulling back the curtains on a process that was vital to understanding life that existed on Earth billions of years ago.
Microbial communities produce an organic goo that has many functions, one of which is to help buffer the microorganisms from the surrounding environment. The goo is therefore especially important in extreme environments like those observed in the Puquios, and the team noticed that it provided the perfect surface for clays to form. Then, when the organic goo degraded, carbonate mineral began to fill in the remaining pore spaces around the clay, encasing the shapes and patterns left behind by the microbes, etching them into the fossil record. “Researchers in the past have mostly focused either on the organic matter or the minerals, but Erica used a sample preparation technique, developed at the Smithsonian many years ago, that preserves both,” Lascu said. “You can see beautifully both the organic and the inorganic components.”
According to Suosaari, studying modern microorganisms can reveal clues about life in the distant past. The Atacama Desert is the driest and oldest desert on Earth, with extreme aridity and high UV radiation that is reminiscent of the harsh conditions that would have been found on early Earth.
But a near-inhospitable climate is not the only factor linking modern and ancient settings. Before the Cambrian explosion brought countless new complex organisms to light around 540 million years ago, nearly the entire history of Earth had been dominated by microbial life. “Now you have eukaryotes everywhere, except when you get into these very extreme environments that keep the more evolved organisms out,” said Suosaari. “These extreme sites allow microbes to really flourish and do what they might have been doing millions and even billions of years ago.”The new study illustrates how microbes, clay and carbonate minerals join together in the preservation process, exemplifying the deep, and often overlooked, connection between minerals and life. One of the most compelling questions is how life originated, and a major piece of that puzzle is whether minerals aided in the origins of living organisms. “There are different worlds in which life could have appeared, and some of them involve minerals and mineral pathways possibly even in shallow ponds just like these,” Lascu said.
Further research around the globe is revealing that the mineral pathway from microbes to clays to carbonates is more common than anyone could have predicted, giving scientists a massive clue to look for when searching for early life on Earth and even other planets, including Mars. “Now that our eyes are open to it, we are finding it everywhere,” said Suosaari, who notes that these associations of minerals could be a signature of life in rocks from the past.
Understanding the environments that ignite this chain reaction of preservation could help researchers pinpoint specific areas of Earth and Mars that could produce evidence of microbial communities going back billions of years. “When you are looking for life, it’s like looking for a needle in a haystack, and in the sense of Mars your haystack is absolutely huge,” Suosaari said. “But now you can take this association and narrow down the size of your haystack, because you suddenly have very pointed areas that you can target to look for signs of life.”
Recent studies have revealed clays and carbonates on the surface of Mars, offering a promising new avenue for future research. Although Lascu and his co-authors are focusing on expanding their explorations on Earth, he thinks that scientists examining samples from Mars should take a close look for these associations to “figure out if there are signatures of life on Mars.” According to Lascu: “You never know what they could find.”
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