Marine ecologists used to call them “black pools of death.” Only as far back as a decade ago, teal-colored, deep-sea brine pools had been assumed to be oxygen-sapping dead zones, despite the wealth of deeply weird and alien-like “extremophiles” that biologists have found living along their margins.
But something was alive within these phenomenally salty undersea lakes—and now researchers have found evidence of a once-active brine pool whose remains may explain how some of Earth’s earliest primordial life survived in the eons before oxygen gas became abundant. According to their new study, conducted with scientists onboard the research vessel (R/V) OceanXplorer, these even-more-extreme extremophiles managed to make this now “extinct” brine pool in the Red Sea their home between roughly 2,000 and 16,000 years ago.
Species like these, as the team wrote in its study for AGU Advances, may have been “an early mechanism for energy production in Earth’s pre-Great Oxidation Event.” These microbes, in other words, may have set the stage for the colossal flood of oxygen that erupted into Earth’s atmosphere from its oceans between 2.4 and 2.1 billion years ago.
These extremophiles, including novel forms of Myxococcota phylum “slime bacteria” and the no less scummy genus Nitrospira, also appear to oxidize free-floating manganese and iron into minerals, offering “a rare glimpse into how early life on Earth may have functioned” and a valuable clue for locating more extinct brine pools.
The briny deep
Morgan Chakraborty, a PhD candidate in marine geosciences at the University of Miami’s Rosenstiel School of Marine, Atmospheric, and Earth Science, worked with her advisor Professor Sam Purkis, their academic colleagues, and the nonprofit OceanX on the new study. OceanX’s R/V OceanXplorer—as well as its Mariner XL Argus remotely operated vehicle (ROV)—went out on three expeditions to investigate five brine pools between 2020 and 2023.
Two of these sites proved to be the most useful for this study: One was an active deep-sea brine pool, dubbed “NEOM,” in the Gulf of Aqaba off the northern edge of the Red Sea between Israel, Saudi Arabia, Jordan, and Egypt, about 5,807 feet (1,770 meters) below sea level. The other was the so-called Hume pool down 4,495 feet (1,370 meters) deep in the northern Red Sea rift.
“Like the NEOM pool, this site bears all the hallmarks of an active brine pool,” Chakraborty, Purkis, and their coauthors wrote. The Hume seabed depression included sedimentary layers rich in calcium-carbonate skeletons of tiny multicellular metazoans, as well as a “radically discolored ‘beach,’ directly akin to the brightly colored microbial zones which surround the NEOM pool.”
“The only component lacking from this site is the brine itself […] Thus, we consider our second ‘Hume Deep’ site to be an extinct pool,” the researchers said.
Salty characters
It might be dawning on you now why this was a project led by marine geoscientists and not marine biologists. Chakraborty, Purkis, and their collaborators collected samples of sediment mixed with organic matter from both Hume and the NEOM brine pool. In Hume’s case, the included echinoderm skeletons and other tiny fossils. In NEOM’s case, they deployed metagenomic techniques to learn more about the DNA of the extremophile organisms still living there.
The team turned to a similar technique, metatranscriptomics, to learn more about how these extremophiles’ genes actually express themselves. The results led to the realization that many of these extremophile microbes, like those of the Candidatus Brocadiae grouping, harvest and oxidize minerals, possibly as a way to generate energy without robust access to oxygen.
“Taken together, our data suggest that similar processes were likely to have occurred in other ancient salt giant basins,” the researchers wrote.
But the researchers also performed geochemical analyses, finding that brine pool sediments were subsequently rich in metallic elements, including manganese, iron, molybdenum, and copper—sometimes at concentrations over 100 times higher than sediments outside these brine pools. The team now believes that these microbes may also be natural accumulators of certain precious metals.
“[Many] of these metals are essential for clean energy technologies, and understanding how they accumulate could help guide future resource exploration,” the researchers explained.
