Oldest fungus-like microfossils discovered in China

The discovery of terrestrial fungus-like fossils dating back 635 million years ago to the end of the Cryogenian ice ages can provide clues as to how the frozen Earth was able to return to normal and allow life to develop.

An analysis of the find by an international team of scientists appears in the Jan. 28 journal Nature Communications. The researchers are from Virginia Tech College of Science, the Chinese Academy of Sciences, Guizhou Education University and the University of Cincinnati. 

Current Science Daily interviewed Virginia Tech professor of geosciences Shuhai Xiao, who led the research work. Xiao described the discovery of the fossils in Guizhou Province in South China, where they were "encased in minerals that precipitated in ancient karstic crevices."

The site is part of the Ediacaran Doushanto Formation, comprised of phosphorite and dolomite sedimentary rocks, which has yielded many other types of fossils. 

Xiao highlighted the importance of these fossils, possibly representing the oldest known terrestrial fungi.

"The terrestrial ecosystem has been dominated by land plants since at least 410 million years ago, as shown by fossils from the Rhynie chert in Scotland. Living together with land plants were various kinds of fungi," he said. "However, we know very little about the terrestrial ecosystem before 410 million years ago. Scientists believe that before land plants, the terrestrial environments were colonized by microbial organisms, including fungi, but the fossil evidence is scanty."

Fungi are characterized by long, thread-like branching filaments called hyphae. Current Science Daily asked how the researchers identified these fossils as fungi, Xiao explained that "the identification was based on a combination of morphological and geochemical features." 

"We examined the fossils under light microscopes and electron microscopes," Xiao said. "We were able to show that the fossils consist of branching and anastomosing [connecting] filaments, which are characteristic of some lower fungi and certain types of bacteria (for example, actinobacteria). 

"The diameter of these filaments is comparable to fungal hyphae but thicker than actinobacterial filaments. This and other considerations led us to favor the fungal identification of the fossils."

The toolbox for identifying fossils

The research team used a remarkable set of microscope and imaging techniques to help in identifying the microfossils as fungi. 

Xiao described some of these techniques. 

"The bread-and-butter results came from light microscopy," he said.  "Although the filaments can be 1.0 millimeter long, they are less than 0.01 millimeter thick. We cut the fossiliferous rock into paper-thin slices, so thin that light can penetrate the slice, and then examined the slices under light microscopes so that the thin filaments can be visualized and photographed."

The next technique used, Xiao said, was an ion beam. 

"We used an ion beam to remove even thinner slices of the fossils-- only about 0.0001 mm in thickness--and examined the slices under electron microscopes in order to analyze the chemical composition of the fossils," he said. "It turns out that the fossils were replicated by a mineral called pyrite, commonly known as fool’s gold, consisting of iron and sulfur."

A third technique in the fossil analysis was secondary ion mass spectrometry (SIMS). SIMS was used "to analyze the sulfur isotope composition of the fool’s gold in the fossil. This information tells us how the organisms were fossilized," Xiao said.

"The fossils contain a small amount of organic matter, which was analyzed using Fourier transform-infrared spectroscopy (FTIR) and Raman spectroscopy. This information gives us some hint about the original organic matter and how it was modified by geological processes," he said.

Two further techniques helped complete the investigation, Xiao continued. 

"Finally, because light microscopy of a paper-thin slice of the rock gave us only a two-dimensional view of the fossils, we used synchrotron radiation X-ray tomographic microscopy and confocal laser scanning microscopy to obtain three-dimensional morphologies of the fossils," he said. "These last two techniques are somewhat similar to medical X-ray techniques that allow the doctors to visualize fractured bones and teeth, for example, except that they have much better resolution than medical X-ray machines."

In a news release on the find issued by Virginia Tech, Xiao noted that he was leaving things open for other possibilities about the identity of the fossils. 

"The other possibility would be certain types of bacteria, for example, actinobacteria," he said. "But the weight of evidence favors a fungal interpretation."

Xiao also emphasized in the news release the importance of the interdisciplinary research. 

"It's very important to encourage the next generation of scientists to be trained in an interdisciplinary light because new discoveries always happen at the interface of different fields," he said.

Why these microfossils are important

The research paper concludes by noting the importance of the find for the terrestrial ecosystem development after the Great Ice Age (the so-called Snowball Earth) ended 635 million years ago.  

"Together with other terrestrial microbes that likely included cyanobacteria and green algae, these fungus-like microorganisms fostered a relatively simple terrestrial ecosystem in the aftermath of the terminal Cryogenian Snowball Earth glaciation," the paper said. 

"If proven to be ecologically widespread, these terrestrial microbes could accelerate chemical weathering and the delivery of phosphorus into the ocean, thus stimulating marine bio-productivity. They could also facilitate the production of detrital clay minerals, which play a key role in organic carbon sequestration."

The paper added, "Together, elevated marine bio-productivity coupled with greater efficiency of organic carbon sequestration means enhanced organic carbon burial and resultant atmospheric-oceanic oxygenation....  Thus, the Doushanto fungus-like microorganisms, as cryptic as they were, may have played a role in catalyzing atmospheric oxygenation and biospheric evolution in the aftermath of the terminal Cryogenian global glaciation."