Meet the Winners:

What did the Earth look like billions of years ago? And what were the early life forms that shaped the world and set life on its evolutionary trajectory? As a geobiologist, I have spent years trying to answer these questions. I study ancient microfossils preserved in rocks that are hundreds of millions to billions of years old and preserve a record of some of the earliest life forms on our planet. I was always struck by how incredible it was that we can look at rocks like chert – a rock composed of microcrystalline quartz – and see actual bacterial cells that were alive over a billion years ago. We have this beautiful snapshot of ancient life that inhabited the oceans during the Proterozoic Eon – 2.5 billion years ago to 541 million years ago – but I always wondered how this was possible. How did these cells get preserved? How was it possible that minerals could form so quickly around these delicate structures to freeze them in time? These questions set me on a path to better understand that early life and how it may have not only been fossilized, but maybe played a role in its own fossilization.

During my PhD, I tackled my questions through fossilization experiments. Historically, scientists have thought that the process of fossilization that froze those bacterial cells in time was purely abiotic – that it occurred without the help of biology and that the cells were simply innocent bystanders in their own entombment. But I had a suspicion that this might not be true. I hypothesized that the microbes themselves may have had a hand in their own preservation, that they may in some way have contributed to the formation of minerals that preserved them. I tested my hypothesis by incubating living microbes in seawater similar to Proterozoic oceans. After a month of waiting, I was shocked to find that the cyanobacteria had created a sort of “candy coating” of amorphous silica around their cells, preserving the soft organic cells just like the fossils. Oddly, though, the candy-coating of silica was enriched in magnesium, an element that we had not expected. Eventually we were able to untangle the mechanism of fossilization and found that the cells had fossilized themselves through cation bridging. Cell surfaces are often negatively charged, a detail that may seem trivial but in this case is important. Some of the dissolved silica in the water around the cells is also negatively charged in solutions that are slightly basic (typical in seawater). Chemistry and physics tell us that like repels like, so how to you get a negative silica molecule to bind to a negative cell surface? This is where cation bridging comes in. The magnesium cations act as positively charged bridge to bind the silica to the cell surfaces. Through this process, the cells acted as a template to facilitate silica precipitation and their own fossilization.