As you read this story, you will learn the following:
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Mysterious microscopic caves in desert marble and limestone may have been formed by microorganisms that lived millions of years ago.
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Exactly what kind of microorganisms were burrowing into the rocks remains unknown, nor is it known whether they still exist or have become extinct.
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Whatever formed the cave must be alive, as the researchers were able to rule out weathering and abiotic processes.
For most organisms on Earth, rocks are objects, not food. But for one strange microbe, desert limestone appears to be on the menu. However, it is unknown whether this mysterious life form still exists or became extinct eons ago.
Recently, geologist Cees Passchier of Johannes Gutenberg University Mainz discovered tiny caves that look like marble and limestone in the deserts of Namibia, Oman and Saudi Arabia. Erosion exposed the fossil cave, and while nothing was crawling inside anymore, Paschier and his team investigated further and found biological material inside.
“The biological origin of the observed structures assumes the presence of liquid water, without which biological growth would be impossible,” the team said in a study published in the journal Nature. Journal of Geomicrobiology. “The survey area is currently dry but experiences occasional showers and frequent coastal fog, and there have been wet periods in the past.”
So, what type of microorganisms are the caves formed by? Bacteria, fungi and lichens have demonstrated that they can survive in extreme conditions, some of which are intralithicmeaning they live inside rocks. Pasquier wanted to see if the creatures that created the caves might have belonged to any of these groups. Fungi can penetrate rock and leave tubes, and certain types of cyanobacteria can also thrive on limestone or marble.
The mysterious creatures are unlikely to be cyanobacteria – they need sunlight for photosynthesis, so they don’t penetrate as deep into the rock as the caves the team found. Fungi secrete digestive agents not found in rocks, and they also form complex networks of hyphae, or filaments, called mycelium. Mycelial networks tend to be orderly. The burrows were parallel and evenly spaced, which is unusual for fungi, and no other pattern was observed. So, they might not be the culprits either.
Because the caves were found to be so wide that they could only have been formed by one organism at a time, and because they showed growth rings, it is more likely that they were formed by microbial colonies. Calcium carbonate dust found in tunnels is also a common excretion of microorganisms living in such rocks. However, no fossils of the organisms have been found – just evidence of their existence.
However, these setbacks do not rule out the idea of ​​life. Conversely, while weathering or non-biological chemical processes may produce structures that are mistaken for signs of life, thorough microscopic examination shows that this is not the case. The chemical composition of rock samples from the cave suggests that whatever they were made of must be alive.
“Because there are no known chemical or physical weathering mechanisms that could explain this phenomenon with the microstructural and geochemical observations presented here, and the microcavities formed within the host rock, we believe that they are biogenic in origin,” Paschier and colleagues said in the same study.
Although there are questions about whether microorganisms still exist in the tunnels, they are long dead. Maybe it’s still crawling around somewhere, digging new tunnel systems for us to one day discover.
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