A rich and diverse bacterial community has been found deep in the almost uninhabited surface of the Atacama Desert in Chile. The researchers who discovered these bacterial communities say they are most likely 19,000 years old and may even be related to microbial life on Mars.
The Atacama Desert in northern Chile is the driest non-polar desert in the world, with very few species of flora and fauna. With rain usually falling only once every ten years, the desert is so dry that NASA used it as a stand-in for the Martian landscape. But what lives beneath this parched surface? New research shows that it's very small, very numerous, and very old.
While the drought of the Atacama Desert means that higher organisms are scarce, a wide variety of bacteria are known to predominate the soil here. However, the researchers' goal is to dig deeper to see what kinds of microbes live more than a meter (3.3 feet) below the surface.
A cracked depression in one of the driest places in the Atacama Desert in the Yungay Valley Lucas Horstmann/ETH Potsdam, Germany
The site they chose was in a Playa region of the Yungai Valley, one of the driest places in the desert's ultra-arid core. Praia are depressions or basins that once contained surface water bodies; Elsewhere, mineral gypsum and anhydrous gypsum are usually close to the surface, within the upper 50 cm/20 in, while in the Prea region, they are buried at a depth of about 2 meters/6.6 feet. On the contrary, anhydrous gypsum converts into gypsum when exposed to water.
When they dug 4.2 meters/13.8 feet underground, the researchers found salt deposits such as gypsum, anhydrous gypsum, and glauconite (commonly known as rock salt), as well as cations (sodium, calcium) and anions (sulfates, nitrates, chlorides).
Maps of mineral, cationic, and anionic abundances plotted from subsurface depths by Horstmann et al
"The upper part of the profile at a depth of 184 cm (72.4 inches) is composed mainly of silt sediments, with occasional thin layers of sand," the researchers said. Between 184 cm and 230 cm (90.6 in) depths, the sediment transitions to a coarser texture, including sand and pebbles. Below 230 cm, the profile always contains [sic] pebble-to-pebble-sized particles".
They use invertebrate-derived DNA (iDNA) analysis and compare it to geochemical analysis (X-ray diffraction and ion chromatography) to study microbiology subsurface. Gene sequencing revealed a rich and diverse microbial community in different strata.
Most of the sequences are classified as bacteria, and 0.5% are archaea, which are single-celled microorganisms that are structurally similar to bacteria but distinct from each other in evolution. Archaea are considered to be an ancient group between bacteria and eukaryotes or organisms that contain DNA and cells that contain a unique nucleus. Three groups of bacteria (phyla) predominate, accounting for more than 90% of the genetic sequence: Actinobacteria, Firmicutes, and Proteobacteria.
Microbial composition at different underground depths
In the uppermost sediment at depths of 2 to 5 cm (0.8 to 2 inches), actinomycetes make up 95% of the total number of microorganisms. The percentage of sessile bacteria is high, ranging from 47% at a depth of 40 cm/15.7 inches to 93% at a depth of 30 cm/11.8 inches. Only at 70 cm/27.6 in did a lower relative abundance of sessile bacteria (34%) occur, and a significant decrease below 200 cm/78.7 in. In sediments below 200 cm, the microbial community is still dominated by actinomycetes, reaching depths of up to 4.2 meters.
Ecologically, depression sedimentation is relatively new, and sedimentation began about 19,000 years ago. However, alluvial deposits are much older, with a depth of 4.2 meters dating back 3.8 million years. The researchers believe that the actinomycete communities they found may have been colonized in the soil "early" and then buried under the alluvium. This could mean that the previously unknown deep biosphere will extend infinitely downward in extremely arid desert soils.
Streptomyces is the largest actinomycete genus in the CDC/Dr. David Baird
One of the most striking findings of the study was that microorganisms appeared in sediments below 200 centimeters, where the depression transitioned to an alluvial layer of gravel, sand, silt, or clay deposited on a river channel or alluvial plain. It was thought that the microbial diversity and abundance at these depths would be low, but this is not the case. In the Atacama Desert, gypsum has been shown to support the microbial community. Here, the researchers believe that deeper gypsum deposits play a crucial role in microbial diversity by providing water or increasing the water retention of desert highly arid soils.
"Although gypsum may not be prevalent in the subsurface of all deserts, the presence of such subsurface niches may indicate that the diversity of global deserts has been underestimated to date, and that in specific cases, subsurface communities can persist in the deepest layers of the driest places on Earth," the researchers said. This research is of great significance for the search for extremophiles beyond Earth."
As mentioned at the beginning of the article, NASA uses the Atacama Desert as a representation of Mars. Well, Mars also has gypsum deposits. So, could the gypsum on Mars also be the source of water for microbial life on Mars?
该研究发表在《PNAS Nexus》杂志上。