Bacterial biofilms as fossil makers(p Lastly the bacteria must leave a pe...)

Lastly, the bacteria must leave a permanent record. In the case of finely preserved fossil embryos, the bacteria likely excrete tiny crystals of calcium phosphate (CaPO₄), which eventually replace the bacterial sculptures. It is these crystals, Raff says, that provide the support for embryo and soft tissue fossilization.

"That's a crucial step," said Rudolf Raff. "Calcium deposits can show us even minute details of structure and shape, not only of the bacteria laying down the minerals, but also of the embryo cell structures all around them. In our experiments, we observed bacteria depositing calcium carbonate (CaCO₃), but not calcium phosphate. We'll need to simulate different conditions to fully replicate this step."

High resolution imaging of a trove of half-a-billion-year-old animal embryo fossils from Doushantuo, China, provided scientists with tantalizing evidence that bacteria may have been involved in the preservation of the delicate cells. Scanning electron microscopy shows oblong concavities on the surface of the embryo fossils, suggesting the cells had been infested with bacteria or bacterial biofilms.

The research presented in the PNAS paper reveals how bacteria-aided fossilization could happen.

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

The scientists examined embryos in the presence of high and low oxygen, with or without inoculums of oxygen-poor marine mud, and in the presence or absence of bacteria-killing antibiotics. In the experiments that produced embryo-infesting biofilms, the scientists used DNA sequence comparisons to identify the bacterial species present.

The researchers learned low-oxygen conditi
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