635 million-year-old mushroom-like microfossils that saved us from an ice age have been discovered

When you think of mushrooms, what comes to your mind may be a crucial ingredient in a recipe or their incredible ability to break down dead organic matter into vital nutrients. But new research from Shuhai Xiao, professor of geosciences at Virginia Tech College of Science, and Tian Gan, visiting doctoral student. student in the Xiao lab, highlights another important role that fungi have played throughout Earth’s history: helping the planet recover from an ice age.

A team of scientists from Virginia Tech, Chinese Academy of Sciences, Guizhou University of Education, and University of Cincinnati have discovered the remains of a fungus-like microfossil that emerged at the end from an ice age about 635 million years ago. It is the oldest terrestrial fossil ever found. To put it in perspective, this microfossil predates the oldest dinosaurs about three times.

Their results were published in Nature communications Jan 28.

The fossil was found in small cavities within well-studied sedimentary dolomite rocks of the lowest Doushantuo Formation in southern China. Although the Doushantuo Formation has provided a plethora of fossils to date, researchers did not expect to find fossils near the lower base of the dolomites.

But against all odds, Gan found some long threadlike filaments – one of the main characteristics of fungi.

“It was an accidental discovery,” Gan said. “At that point, we realized that this could be the fossil that scientists have been looking for for a long time. If our interpretation is correct, it will be useful in understanding paleoclimatic change and the evolution of early lives.

This discovery is essential for understanding the multiple turning points in Earth’s history: the Ediacaran period and the terrestrialization of fungi.

When the Ediacaran Period began, the planet was recovering from a catastrophic ice age, also known as ‘Snowball Earth’. At that time, the ocean surfaces were frozen to a depth of over a mile and it was an incredibly harsh environment for virtually all living things except microscopic life that managed to thrive. Scientists have long wondered how life returned to normal and how the biosphere could have become larger and more complex than ever.

With this new fossil in hand, Tian and Xiao are convinced that these microscopic and inconspicuous caves played many roles in the reconditioning of the Earth’s environment during the Ediacaran era. One role involved their formidable digestive system.

Mushrooms have a rather unique digestive system that plays an even bigger role in the cycle of vital nutrients. Using enzymes secreted into the environment, terrestrial fungi can chemically break down rocks and other stubborn organic material, which can then be recycled and exported to the ocean.

“Fungi have a mutualistic relationship with the roots of plants, which helps them mobilize minerals, such as phosphorus. Because of their connection to terrestrial plants and important nutritional cycles, terrestrial fungi have a critical influence on biochemical weathering, global biogeochemical cycling and ecological interactions, ”said Gan.

Although previous evidence indicates that terrestrial plants and fungi formed a symbiotic relationship around 400 million years ago, this new discovery has recalibrated the chronology of the colonization of the earth by these two kingdoms.

“The question was, ‘Were there fungi in the terrestrial realm before the rise of terrestrial plants,” said Xiao, an affiliate faculty member of the Fralin Life Sciences Institute and the Global Change Center. “And I think our study suggests so. Our mushroom-like fossil is 240 million years older than the previous record. This is, to date, the oldest record for terrestrial fungi. “

Now, new questions arise. Since the fossilized filaments were accompanied by other fossils, Gan will set out to explore their past relationships.

“One of my goals is to limit the phylogenetic affinities of these other types of fossils associated with fungal fossils,” Gan said.

Xiao is happy to tackle the environmental aspects of these microorganisms. Sixty years ago, few believed that microorganisms, like bacteria and fungi, could be preserved in the form of fossils. Now that Xiao has seen them with his own eyes, he plans to learn more about how they were practically frozen in time.

“It’s always important to understand organisms in the environmental context,” said Xiao. “We have a general idea that they lived in small cavities in dolomite rocks. But little is known about exactly how they lived and how they were preserved. Why something like mushrooms, which did neither bone nor shell, can it be preserved in the fossil record? “

However, it cannot be said for sure whether this fossil is a definitive fungus. While there is a fair amount of evidence behind this, the investigation into these microfossils is ongoing.

“We would like to leave things open to other possibilities, as part of our scientific investigation,” Xiao said. “The best way to put it is that we might not have disapproved that it was mushrooms, but it’s the best interpretation we have yet.”

Three separate Virginia Tech groups and laboratories were crucial in identifying and time-stamping this fossil. The Fralin Life Sciences Institute’s Confocal Laser Scanning and Microscopy Lab assisted Tian and Xiao with an initial scan that led to further investigation at the University of Cincinnati.

The Massey Herbarium of the Department of Biological Sciences, which houses over 115,000 specimens of vascular plants, fungi, bryophytes and lichens, has provided specimens of modern fungi for comparison with fossils.

The team brought in technicians to perform a geochemical analysis using secondary ion mass spectrometry, which ionizes nanomoles of material from small areas that are a fraction of the thickness of a wick of hair, to analyze the isotopic abundance of sulfur-32 and sulfur-34 in order to understand the fossilization environment.

Advanced computerized tomography was crucial in obtaining the 3-D morphology of the filaments, which are only a few microns thick. And a combination of focused ion beam scanning electron microscopy and transmission electron microscopy allowed researchers to cut samples with surgical precision and examine every nanometer of the filaments even more closely.

“It wasn’t a single person or even a single lab that did this work,” Xiao said.

Xiao also stressed the importance of interdisciplinary research in this study and many others.

“It is very important to encourage the next generation of scientists to train with an interdisciplinary perspective, because new discoveries always occur at the interface of different fields,” said Xiao.