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Microbes may help fossilize ancient embryos

Nov. 24, 2008, Courtesy Indiana University and World Science staff.

Bacterial decay was once viewed as the mortal enemy of fossilization. But new research suggests resilient colonies of bacteria, called biofilms, may have actually helped preserve the fossil record's most vulnerable stuff: animal embryos and soft tissues.

Scientists have found that bacteria can invade dying embryo cells and form densely packed biofilms in those cells. These completely replace the cell structure and generate a replica of the embryo, say the researchers, who call this formation of bacteria filling the shape of an embryo a "pseudomorph."

The investigators, led by Indiana University Bloomington biologists Rudolf and Elizabeth Raff, report their findings in this week's early online issue of the research journal Pro-ceedings of the National Academy of Sciences.

"The bacteria consume and replace all the cytoplasm in the cells, generating a little sculp-ture of the embryo," said Elizabeth Raff, the report's lead author.

But "certain conditions must be met if the bacteria are going to aid the preservation pro-cess." For one, she explained, the embryo must have died in a low-oxygen environment, such as the bottom of a deep ocean or buried in lakeside mud. Oxygen would make em-bryos self-destruct as digestive enzymes break free and wreak havoc.

Then, "bacteria able to survive in low-oxygen conditions must then infest the cells of the dying embryo," Raff said. The bacteria form biofilms, crowded assemblies of bacterial cells held together by sticky fibers made of proteins and sugars. As the biofilms fill the embryo cells, the tiny bacteria insinuate themselves between and among the structures within the cells, forming a faithful representation of the cell's innards.

This early-stage embryo is protected by a fertilization envelope, seen here as a white line encircling the embryo cells. (Credit: E.C. Raff and R.A. Raff)

High-resolution imaging of a trove of half-a-billion-year-old embryo fossils from China, offered evidence that bacteria may have been involved in the preservation, research-ers say. (Credit: F.R. Turner, E.C. Raff, and R.A. Raff).

Last, the bacteria must leave a permanent record, she added: in the case of finely pre-served fossil embryos, the bacteria likely excrete tiny crystals of calcium phosphate which eventually replace the bacterial sculptures. These crystals would provide the support for embryo and soft tissue fossilization.

High resolution imaging of a trove of half-billion-year-old animal embryo fossils from Doushantuo, China, offered scientists tantalizing evidence that bacteria may have been involved in the preservation of the delicate cells, she said.

The Raffs studied early-stage embryos of two Australian sea urchin species, Heliocidaris erythrogramma and Heliocidaris tuberculata. Experimental results with modern embryos were compared to the high resolution images of fossil embryos prepared by colleagues.

Although it's impossible to know whether bacteria aided the preservation of the embryo fossils from Doushantuo and elsewhere, the Raffs argue the evidence they gathered strongly favors the view that bacteria are a fundamental force in fossil formation, as rapid biological processes must be available to convert delicate cells into a stable form and trig-ger their mineralization.

"This work is important because it helps us understand fossilization as a biological as well as geological process," Elizabeth Raff said. "It gives us a window onto the evolution of the embryos of the earth's first animals."

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