Brine pools – deadly yet full of life

Brine pools are one of the greatest mysteries of oceanography. The dense, unmixed brine lies at the bottom of the ocean, forming what appears to be a separate body of water. However, it is so toxic that any creature entering it should die almost instantly. Yet, against all logic, some life forms have not only survived there but have actually thrived.

What are brine pools?

Although the term “underwater lake” sounds like an oxymoron, from a scientific perspective, it makes perfect sense. Brines with extraordinarily high salt concentrations do not mix with seawater and are so dense that they form distinct structures on the ocean floor, beyond which a completely different world begins.

The salinity in such lakes often reaches values five to eight times higher than in the surrounding waters. The boundary is so clear that footage from underwater cameras can give the impression of a completely separate water body lying at the bottom.

But that’s not the only peculiarity. Within these lakes, toxic gases such as hydrogen sulfide and methane are often present in lethal concentrations. The occasional presence of fish and crustacean graveyards serves as evidence of how deadly this environment truly is.

How do brine pools form?

Like many other geological phenomena, brine pools form through complex processes occurring over thousands or even millions of years. Several mechanisms contribute to the formation of these extraordinary reservoirs.

Ancient salt plains flooded by the sea

In some areas of the ocean floor, layers of salt were deposited when the region was still part of a landmass. Over time, as seawater flooded these areas, the salts partially dissolved, creating highly concentrated brines that settled in depressions.

Hydrothermal activity

Hot fluids released from hydrothermal vents contain numerous mineral compounds. If the pressure and geological conditions are favorable, the brine can concentrate on the ocean floor, forming a distinct lake.

Crustal fractures

In regions where tectonic plates diverge or overlap, fissures may form, allowing salt deposits and other minerals from within the Earth to escape. This is another mechanism leading to the accumulation of dense brine.

In each of these cases, the result is the same: an extremely saline area forms at the ocean’s bottom, behaving like an independent body of water, practically unmixed with the surrounding seawater.

A deadly trap for most creatures

The increased density and chemical composition of the water in these pools have extremely dangerous consequences. If a fish accidentally enters a brine pool, its fate is practically sealed. The difference in salt concentration causes its cells to lose water almost immediately, leading to rapid death. Over time, the bottom of brine pools becomes littered with the remains of creatures that wandered into these underwater death traps. Some research expeditions have documented entire graveyards of organisms that have succumbed to these extreme conditions.

An extreme environment teeming with life

Despite the seemingly hostile conditions in brine pools, some organisms have managed to thrive there. These are extremophiles – microorganisms, mainly bacteria and archaea, that have developed extraordinary adaptation mechanisms. Instead of using oxygen and sunlight for energy, they rely on chemical reactions such as sulfur or methane oxidation. Their unique cell wall and membrane structures allow them to function in extremely high salinity and toxic environments.

Although microscopic, these simple organisms form complex microecosystems in which each species plays a crucial role, supporting other inhabitants of this chemical abyss. Furthermore, scientists are increasingly finding evidence of small crustaceans and polychaete worms feeding near the edges of brine pools. These creatures survive primarily at the lake’s boundary, benefiting from the resources generated by extremophiles.

How are brine pools studied?

The largest known brine pools are found in the Gulf of Mexico, the Mediterranean Sea, and near underwater tectonic rift zones. Regardless of their location, studying them poses significant challenges for scientists and requires specialized equipment.

• Remotely operated underwater vehicles (ROVs) – These allow for long-term observations in extreme pressure conditions. ROVs are equipped with HD cameras, manipulators for collecting samples, and a variety of sensors that measure the chemical composition of the water.
• Manned submersibles and bathyscaphes – Although rarely used due to cost and risk, they provide humans with a direct view of these environments. They require advanced safety systems and meticulous mission planning.
• Deep-sea laboratories – Some research projects deploy special monitoring modules on the ocean floor to track long-term changes in brine composition and microbial activity.

A key to understanding life on Earth – and beyond

While brine pools might seem like mere geological curiosities, studying them provides science with invaluable insights.

• Models for astrobiology – The conditions in brine lakes resemble those that might exist on Jupiter’s moon Europa or Saturn’s moon Enceladus. Understanding how extremophiles survive in such a chemical soup aids in the search for extraterrestrial life.
• Understanding geochemical processes – Researching the composition of water and gases in brine pools sheds new light on the movement of elements in the oceans and the Earth’s deep crust.
• Answers to questions about the origins of life – Some hypotheses suggest that life on Earth may have evolved in similar environments. Analyzing extremophile biochemistry helps scientists understand what the early stages of evolution might have looked like.

On one hand, these areas have conditions so extreme that they almost instantly kill most species. On the other, they host micro-worlds full of life, dominated by remarkable organisms known as extremophiles.

For scientists, brine pools are not just a fascinating research subject but also a key to understanding how life began on Earth—and whether it could exist elsewhere. As climate change accelerates and the search for alternative energy sources and new biotechnological solutions continues, these deep-sea lakes may prove to be more important than anyone ever imagined.

main photo: NOAA Ocean Exploration / flicker