Found at depths of over 3000m, at pressure over 500 pounds per square inch. Found with salinity values of up to 30%, temperatures up to 63 degrees centigrade and toxically low levels of oxygen- every fish in the sea knows that brine pools are not for swimming. Nevertheless, whilst these factors combined may be enough to deter your common-or-garden extremophile from inhabiting this most extreme of niches, there are some organisms which thrive in these hypersaline, anoxic, torrid conditions.

Salt and Plate Tectonics: brine pool location and formation

Brine pools occur at most convergent or divergent plate margins, most notably in the Red Sea where the African and Arabian plates moving apart in opposite directions has created, not only the terrestrial East African Rift Valley, but also the unusual submarine topography seen there; including brine pools. The largest deep-sea brine pool in the world, named Atlantis Deep II is a direct result of these plate tectonics. 6 by 13 km wide with a 200m layer of brine at a depth of over 2000m, it is also the hottest brine pool in the world with water temperatures rising to 68.2 degrees C at its deepest point. Salinity also increases with depth here and reaches a maximum of 25.7% salt- hypersaline by any definition.

Image courtesy of Gulf of Mexico Expedition 2002
Location of the two salt layers buried under the Gulf of Mexico since the Jurassic Period which give rise to brine pools today. SOURCE: Image courtesy of Gulf of Mexico Expedition 2002

Yet brine pools are not just particular to continental plate margins. They can also be generated through a process known as salt tectonics which can be best described using the example of the Gulf of Mexico. During the late Jurassic period the shallow sea in the Gulf of Mexico became landlocked, the water evaporated leaving behind a layer of salt which was then buried in sediment before the Gulf opened up to the ocean again preventing it from dissolving away. The increasing weight of sediment, deforms the less dense salt layer which is forced to the surface in isolated hotspots forming brine pools.

There are two salt layers under the Gulf of Mexico called the Louann and Campeche Salt Layers with the Louann being twice the size of Arkansas.

 Waves at the Bottom of the Ocean

So apart from being one of the most inhospitable, extra-terrestrial environments on the planet, what makes brine pools interesting?

Saltwater is denser than freshwater and hypersaline water is denser still, therefore this creates a distinct and visible interface between the water in a brine pool and the surrounding ocean. This phenomenon- a halocline– is clearly noticeable in the video below taken as part of the NOAA Nautilus Live series. Waves created by the ROV’s thrusters are clearly visible propagating across the surface and breaking at the rim of the pool.

SOURCE: E/V Nautilus Live 

As aesthetically pleasing as this phenomenon is however, arguably the most captivating characteristic about brine pools is that organisms can survive here at all.

Deep-Dwelling Denizens

Brine pools are truly hell-holes of this watery underworld and few creatures are able to tolerate the extremity of the environment. Fish cannot survive immersion in such anoxic conditions, as the scientists in the video alluded to. Even fish swimming close to the surface of brine pools appear disorientated by the toxicity of the gas emitted by the pool.

Contrary to expectations however, there is often abundant life surrounding brine pools. Bathymodiolus childressi is a species of mussel which commonly aggregates around the brine-filled pockmarks forming the seafloor of the Gulf of Mexico. Bathynerita naticoidea, a species of gastropod and Methanoaricia dendrobranchiata a species of polychaete are both relatively common as are many organisms commonly associated with cold seeps in the ocean floor. However, although these latter two organisms are able to tolerate exceedingly high salinities, up to 85ppt, neither are able to withstand long submersion within the brine pools. Even the mussels would perish if the surface level of the brine pool were to rise by a few centimetres and they became submerged. Notice the key word in the first sentence of this paragraph: surrounding. Organisms may adapt and become highly derived to simply survive surrounding the rim of brine pools but there is only one domain of organisms which could have a prayer of surviving in a brine pool; frequently overlooked and underappreciated: Bacteria.

baymodiolus-childressi
Bathymodiolus childressi situated at the distinct interface of a brine pool SOURCE: NOAA (National Oceanographic and Atmospheric Administration) Ocean Explorer
na058_cold_seeps_breakdown_5_20150515-01_51_23_15-still007
Zonation and proximity of communities around a brine pool. Bivalve and polychaete species can be seen indicating some diversity. SOURCE: NOAA Ocean Explorer

Hardcore Halophiles

It is thanks to bacteria that other organisms can survive in this sunless underworld at all. When observing images of the deep sea it is easy to forget that actually, these seafloor organisms spend their lives in the midnight aphotic zone. Some deep-water species still gain their energy from photosynthesis by ingesting marine snow– organic debris which would have grown in a photosynthetically-driven ecosystem, that sinks from the surface. Other however synthesise their energy through another process- that is chemosynthesis- which is only possible due to bacteria.

The diagram below shows how organisms which aggregate around hydrothermal vents and brine pools are able to take in dissolved carbon dioxide, water and hydrogen sulphide (or some other hydrocarbon gas) emitted by the vent or pool and convert it into glucose– energy essential for survival. What it does not show, is that it is not actually the Bathymodiolus species or tube worm species which carries out this conversion but the chemotrophic bacteria they contain which are responsible for the conversion. A prime example of a symbiotic relationship.

chemosynthesis
Diagram showing the cycle of chemosynthesis and the chemical reaction involved. what it does not show is the role of endosymbiotic bacteria living inside the bivalve mollusc. SOURCE: The Encyclopedia of New England

But bacteria need no helpful host to infiltrate and inhabit brine pools. From within the brine pools of the Red Sea, bacteria belonging to seven different phyla have been isolated. From Flexistipes sinusarabici to Salinisphaera shabanensis, many of the species discovered are new to science and have never been described before. Diverse communities of bacteria and the related halobacteria are also commonly found in brine pools in the Gulf of Mexico and the Mediterranean brine pools however differences in the hydrothermal activity and salt evaporate composition means that each brine pool has an almost unique community of bacterial species.

Extraterrestrial to the Extreme

It has long been thought that hydrothermal vents may prove to shed light on the origins of life on the planet. On the other hand, it is the study of interactions between microbial life in hypersaline environments, such as brine pools, which is providing clues as to their distribution and metabolism on the early Earth. It is postulated that the probability of brine pools occurring on extraterrestrial bodies such as Europa is likely. Therefore if life can survive the most extreme places on Earth- why not extreme places off of Earth?

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