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Web Date: March 17, 2017

Geochemical precipitates may fool fossil hunters

Inorganic crystals can mimic biofossils, but they’re formed without biological help
Department: Science & Technology
Keywords: biomorphs, fossils
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Inorganic precipitates from high-pH spring water resemble biominerals.
Credit: Juan Manuel Garcia-Ruiz
A micrograph showing abiotically produced mineral structures that resemble those produced biologically.
 
Inorganic precipitates from high-pH spring water resemble biominerals.
Credit: Juan Manuel Garcia-Ruiz

Geochemists searching for evidence of ancient life often look to collections of round, twisted, and loopy mineral precipitates they find in soil or water samples as possible microbial fossils.

A team led by Juan Manuel Garcia-Ruiz at the Spanish Higher Council of Scientific Research and University of Granada and Oliver Steinbock at Florida State University, however, have now made these types of inorganic structures without the help of biology (Sci. Adv. 2017, DOI: 10.1126/sciadv.1602285).

The discovery, which blurs the line between processes that are purely biological and ones that are geological, may complicate efforts to study ancient terrestrial or extraterrestrial biofossils.

The group collected water from the Ney spring in California’s Siskiyou County that has an unusually high pH of 11.9 and is rich in silica. The researchers added solutions of barium chloride dihydrate and calcium chloride dihydrate to the organism-free water and then watched as a gallery of fantastically shaped precipitates formed.

An inorganic tube precipitates out of a high-pH silica water from Ney spring. (scale bar is 20µm)
Credit: Juan Manuel García-Ruiz
An inorganic tube precipitates out of a high-pH silica water from Ney spring. (scale bar is 20µm)
Credit: Juan Manuel García-Ruiz
 

In addition to barium carbonate structures that resemble microbial fossils, the researchers also found “silica gardens” of metal-silicate-hydrate tubes and calcium carbonate “shells.”

Until recently, structures of this sort were thought to be produced via biological mechanisms. Biogeochemists have considered the chemistry and morphology of these crystals as biomarkers for possible extraterrestrial microbial life.

Then a few years ago, García-Ruiz and his colleagues reported that similar biomineral-like structures could be precipitated by adding carbonates to lab-prepared alkaline, silica-rich solutions. They dubbed the precipitates “biomorphs.”

That these conditions can be found in a natural setting such as the Ney spring makes an even stronger case for a possible reframing of biological and geological chemistry, García-Ruiz and his team say.

Resarchers led by Dominic Papineau at University College London recently reported in Nature that they had found microbial fossils in hydrothermal vent precipitates up to 4.22 billion years old (Nature 2017, DOI: 10.1038/nature21377). The new work from García-Ruiz’s group “is clearly very important to help understand morphological, biological, and nonbiological signatures,” Papineau says.

“It is exactly this kind of science that significantly contributes to the progress of the fields of Precambrian biogeochemistry and exobiology,” he notes.

Biominerals expert Fabien Salport, at Paris Diderot University, says the work “shows the major difficulty to detect unambiguously records of the most ancient terrestrial biological activity. The ability to synthesize crystal structures that mimic biominerals makes it even more difficult to search for the oldest traces of life on Earth.”

 
Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Comments
Laurie Gower (Wed Mar 22 13:15:57 EDT 2017)
Just because scientists can mimic a morphology in the lab does not mean that such structures are not also created by organisms. After all, it is not surprising that scientists could come across conditions similar to those that organisms came across during their evolution, and which were preserved because they provided some benefit. I think there has been enough research to show that unusual mineral morphologies can result from relatively simple physico-chemical mechanisms, and do not necessarily rely on highly specific molecular recognition mechanisms that were once thought to be a part of all biological processes. This research supports that premise, and shows that the biogeochemists and paleontologists need to take more things into consideration than just morphological features.
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