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As rock salt — or “halite”, as mineralogists call it — crystallises out of saline waters, it can entrap tiny amounts of the parent waters as a so-called fluid inclusion. And if the originating waters harboured microscopic life forms, these organisms can end up trapped within inclusions as well, residing in a tiny microenvironment held inside the crystal. In their research, geologist Sara Schreder-Gomes of the West Virginia University and her colleagues analysed bedded halite from the Browne Formation, an 830-million-year-old deposit that lies beneath central Australia.
Samples were taken from some 4,858–4,987 feet beneath Officer Basin via the Empress 1A core, which was drilled up by the Geological Society of Western Australia in 1997.
The halite was very well preserved, the researchers said, and they were able to study beds of the crystal that had been deposited at 10 different depths along the rock core.
Based on the presence of both small “cumulate” crystals that would have formed at the water’s surface and larger “chevron” ones that form at the sediment–water interface, the team believe that the halite was formed in shallow saline surface waters.
Studying the salt crystals under the microscope via both transmitted and ultraviolet light, the team found that fluid inclusions in the halite contain organic material.
Drops of fluid preserved within rock salt that formed 830 million years ago contain microorganisms
The halite in the rock cores (pictured) was very well preserved, the researchers said
The team said: “These objects are consistent in size, shape, and fluorescent response with cells of prokaryotes and eukaryotes and with organic compounds.”
Prokaryotes and eukaryotes are both forms of cell — with the latter distinguished by having both a membrane-bound nucleus and organelles.
According to the researchers, the microorganisms would have been trapped in the inclusions since the rock salt precipitated in the middle of the Tonian Period, and many don’t appear to have suffered from significant decomposition in the whopping 830 million years since.
They added: “The blue fluorescence is consistent with that of modern microorganisms, suggesting unaltered organic material.
“In contrast, the white and gold fluorescence in some cells and lack of fluorescence in other cells may be the result of organic decay.”
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Samples were taken from some 4,858–4,987 feet beneath Officer Basin, Australia
Fluid inclusions in bedded halite from the Browne Formation, central Australia
This is not the first time that prokaryotic and eukaryotic organisms have been found within halite crystals, although it is by far the oldest.
In 2000, for example, researchers led from West Chester University in Pennsylvania succeeded in identifying a previously unknown species of spore-forming bacteria within 250-million-year old salt crystals from Carlsbad, New Mexico.
What is particularly special about that case is that the researchers were able to extract the bacteria from the crystals and culture them — they were still alive.
As Ms Schreder-Gomes and her colleagues explain: “Some halophilic microorganisms, such as Dunaliella algae, shrink and greatly reduce biological activity when host waters become too saline.”
“These algal cells may be revived during later flooding events.”
It is possible that such mechanisms allowed the bacteria in the New Mexico salt crystals to survive for 250 million years, the researchers said — and, by extension, it is “plausible that microorganisms from the Browne Formation are extant.”
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Pictured: clear prokaryotes, yellow algae, and air bubbles and clear daughter crystals
The researchers said: “Possible survival of microorganisms over geologic time scales is not fully understood.”
While some experts have argued that radiation would destroy organic matter over such long periods of time, evidence has been found to suggest that microorganisms buried in halite for a few hundred million years may only be exposed to negligible amounts of radiation.
The team added: “Microorganisms may survive in fluid inclusions by metabolic changes, including starvation survival and cyst stages, and coexistence with organic compounds or dead cells that could serve as nutrient sources.
“One such organic compound, glycerol, produced by the cellular breakdown of some algae, may provide energy for longevity of coexisting prokaryotes.
“Non–spore-forming prokaryotes are continually, but minimally, metabolically active, so they are able to repair DNA should it be necessary.”
Pictured: a chain of yellow algal cells and clear cocci in a fluid inclusion
Unfortunately, it may be quite a while before we learn whether the Browne Formation halite contains living organisms.
Ms Schreder-Gomes told Express.co.uk: “Yes, it’s plausible that microorganisms in the Browne Formation halite are extant, but extracting them is not in our plans at this time.”
The inclusions that contained the microorganisms, she explains, were very small at just some 5–20 microns across — which is 14–3.5 times thinner than human hair is thick.
Alongside this, the inclusions were located at varying depths from the surface of samples.
Ms Schreder-Gomes said: “This together makes it incredibly difficult to do a targeted extraction of their contents.”
In fact, she noted, the previous studies that have extracted prokaryotes from ancient halites did so by either bulk crushing/dissolution or by syringing large inclusions of undetermined origin.
This means that we cannot be absolutely certain that those organisms were the same age as the crystals in which they were trapped, unlike with the present study, which considered the petrographic context.
Ms Schreder-Gomes added: “Visual analyses and further non-destructive optical techniques can still provide a wealth of information about the contents of the fluid inclusions in Browne Formation halite.”
The findings of the new study may have implications for the search for traces of extra-terrestrial life on Mars.
The team said: “The Browne Formation is a possible analogue for some Martian rocks because both contain a similar suite of minerals, sedimentary structures, and diagenetic features.
“Mars once contained saline lakes that precipitated chemical sediments, including halite.
“Microorganisms that may have existed in surface brines on Mars in the ancient past may be trapped as microfossils in chemical sedimentary rocks.”
The full findings of the study were published in the journal Geology.