An incredible discovery has just revealed a potential new source for
understanding life on ancient Earth.
A team of geologists has just discovered tiny remnants of prokaryotic and
algal life – trapped inside crystals of halite dating back to 830 million
years ago.
Halite is sodium chloride, also known as rock salt, and the discovery
suggests that this natural mineral could be a previously untapped resource
for studying ancient saltwater environments.
Moreover, the organisms trapped therein may still be alive.
The extraordinary study also has implications for the search for ancient
life, not just on Earth, but in extraterrestrial environments, such as Mars,
where large salt deposits have been identified as evidence of ancient,
large-scale liquid water reservoirs.
The organisms don't look like you might be expecting. Previous ancient
microfossils have been found pressed into rock formations, such as shale,
dating back billions of years. Salt is not able to preserve organic material
in the same way.
Instead, when the crystals are forming in a saltwater environment, small
amounts of fluid can be trapped inside. These are called fluid inclusions,
and they're remnants of the parent waters from which the halite
crystallized.
This makes them scientifically valuable, since they can contain information
about the water temperature, water chemistry and even atmospheric
temperature at the time the mineral formed.
Scientists have also found microorganisms living in recent and modern
environments where halite forms. These environments are extremely salty;
nevertheless, microorganisms such as bacteria, fungi and algae have all been
found thriving in them.
In addition, microorganisms have been documented in fluid inclusions in
gypsum and halite, mostly modern or recent, with a handful dating back to
ancient times. However, the method of identifying these ancient organisms
has left some doubt as to whether they are the same age as the halite.
"Therefore, a question has persisted amongst geomicrobiologists," wrote a
team led by geologist Sara Schreder-Gomes of West Virginia University. "What
are the oldest chemical sedimentary rocks that contain prokaryotic and
eukaryotic microorganisms from the depositional environment?"
The middle of Australia is desert now, but it was once an ancient salty sea.
The Browne Formation is a well-characterized and dated stratigraphic unit
from central Australia, dating back to the Neoproterozoic. It includes
extensive halite, indicative of an ancient marine environment.
Using a core sample from the Browne Formation extracted by the Geological
Survey of Western Australia in 1997, Schreder-Gomes and her colleagues were
able to conduct investigations of unaltered Neoproterozoic halite using
nothing but non-invasive optical methods. This left the halite intact;
which, importantly, means that anything inside had to have been trapped at
the time the crystals formed.
They used transmitted-light and ultraviolet petrography, first at low
magnification to identify halite crystals, then at up to 2,000x
magnification to study the fluid inclusions therein.
Inside, they found organic solids and liquids, consistent with prokaryotic
and eukaryotic cells, based on their size, shape and ultraviolet
fluorescence.
The range of fluorescence was interesting, too. Some of the samples showed
colors consistent with organic decay, while others demonstrated the same
fluorescence of modern organisms, suggestive, the researchers said, of
unaltered organic material.
It's even possible that some of the organisms are still alive, the
researchers noted. The fluid inclusions could serve as microhabitats where
tiny colonies thrive. And living prokaryotes have been extracted from halite
dating back 250 million years; why not 830 million?
"Possible survival of microorganisms over geologic time scales is not fully
understood," the researchers wrote.
"It has been suggested that radiation would destroy organic matter over long
time periods, yet Nicastro et al. (2002) found that buried 250
million-year-old halite was exposed to only negligible amounts of radiation.
Additionally, 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."
This absolutely has implications for Mars, where deposits can be found that
have similar compositions to the Browne Formation, the researchers said.
Their research shows how such organisms can be identified without destroying
or disrupting the samples, which could give us a new set of tools for
identifying them – and better understanding Earth's own history, too.
"Optical examination should be considered a fundamental step in any study of
biosignatures in ancient rocks. It allows geologic context of microorganisms
to be known prior to further chemical or biological analyses … and it
provides a target for such analyses," the team wrote.
"Ancient chemical sediments, both of terrestrial and extraterrestrial
origin, should be considered potential hosts for ancient microorganisms and
organic compounds."
The research has been published in Geology.
Reference:
Sara I. Schreder-Gomes; Kathleen C. Benison; Jeremiah A.
Bernau, 830-million-year-old microorganisms in primary fluid inclusions
in halite, Geology (2022) DOI: 10.1130/G49957.1