A brine pool, sometimes called an underwater lake, deepwater lake or brine lake, is a volume of brine collected in a seafloor depression. These pools are dense bodies of water that have a salinity that is typically three to eight times greater than the surrounding ocean. Brine pools are commonly found below polar sea ice and in the deep ocean. This below-sea ice forms through a process called brine rejection. For deep-sea brine pools, salt is necessary to increase the salinity gradient. The salt can come from one of two processes: the dissolution of large salt deposits through salt tectonics or geothermally-heated brine issued from tectonic spreading centers.
The brine often contains high concentrations of hydrogen sulfide and methane, which provide energy to chemosynthetic organisms that live near the pool. These creatures are often extremophiles and symbionts.Extremophile life near brine pools Deep-sea and polar brine pools are toxic to marine animals due to their high salinity and anoxic properties, which can ultimately lead to toxic shock and possibly death.
Characteristics Brine pools are sometimes called sea floor "lakes" because the dense brine does not easily mix with overlying seawater, creating a distinct interface between water masses. The pools range in area from less than to as large as the Orca Basin. Depending on concentration, some minerals such as baryte (barium sulfate) precipitate out of the brine and form crystalline crusts around the edge of the pool.
Because of the brine's high density and lack of mixing currents in the deep ocean, brine pools often become anoxic and deadly to respiring organisms. Brine pools supporting chemosynthetic activity, however, form life on the pool's shores where bacteria and their symbionts grow near the highest concentrations of nutrient release. Patchy, reddish layers can be observed floating above the dense brine interface due to high densities of halophilic archaea that are supported by these environments. These shores are complex environments with significant shifts in salinity, oxygen concentration, pH, and temperature over a relatively small vertical scale. These transitions provide a variety of environmental niches.
Formation Brine pools are created through three primary methods: brine rejection below sea ice, dissolution of salts into bottom water through salt tectonics, and geothermal heating of brine at tectonic boundaries and hot spots.
Brine rejection: When sea water freezes, salts do not fit into the crystalline structure of ice, so the salts are expelled. The expelled salts form a cold, dense brine that sinks below the sea ice to the sea floor. Brine rejection on an oceanic scale is associated with the formation of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW), which play a large role in global thermohaline circulation (THC). On a localized scale, that rejected brine collects in seafloor depressions, forming a brine pool. In the absence of mixing, the brine will become anoxic in a matter of weeks. Salt tectonics: During the Middle Jurassic period, the Gulf of Mexico was a shallow sea that dried out, producing a thick layer of salt and seawater-derived minerals up to 8 km thick. When the Gulf refilled with water, the salt layer was preserved from dissolution by sediments accumulating over the salt. Subsequent sedimentation layers became so heavy that they began to deform and move the more malleable salt layer below. In some places, the salt layer now protrudes at or near the seafloor, where it can interact with seawater. Where seawater comes in contact with the salt, the deposits dissolve and form brines. The location of these surfacing Jurassic-period salt deposits is also associated with methane releases, giving deep-ocean brine pools their chemical characteristics. Geothermal heating: At earth's oceanic tectonic spreading centers, plates are moving apart, allowing new magma to rise and cool. This process is involved in creating new sea floor. These mid-ocean ridges allow seawater to seep downward into fractures, where they come in contact with and dissolve minerals. In the Red Sea, for example, Red Sea Deep Water (RSDW) seeps into the fissures created at the tectonic boundary. The water dissolves salts from deposits created in the Miocene epoch much like the Jurassic-period deposits in the Gulf of Mexico. The resulting brine is then superheated in the hydrothermal active zone over the magma chamber. The heated brine rises to the seafloor, where it cools and settles in depressions as brine pools. The locations of these pools are also associated with methane, hydrogen sulfide, and other chemical releases, setting the stage for chemosynthetic activity.
Support of life Due to the methods of their formation and lack of mixing, brine pools are anoxic and deadly to aerobic organisms, including most eukaryotes and multicellular organisms. When an organism enters a brine pool, it attempts to "breathe" the environment and experiences cerebral hypoxia due to the lack of oxygen and toxic shock from the hypersalinity. Organisms that cannot surface long enough to retreat to the rim quickly die. When observed by submarines or remotely operated underwater vehicles (ROVs), brine pools are found to be littered with dead fish, crabs, amphipods, and various other organisms that ventured too far into the brine. Dead organisms are then preserved in the brine for years without decay due to the anoxic nature of the pool.
Despite the harsh conditions, life in the form of macrofauna such as bivalves can be found in a thin area along the rim of a brine pool. A novel genus and species of bivalves, known as Apachecorbula muriatica, has been found along the edge of the "Valdivia Deep" brine pool in the Red Sea. There have also been recorded instances of macrofauna brine pools at the seawater interface. Inactive sulfur chimneys have been found with affiliated epifauna such as polychaetes and hydroids. Fauna like gastropods, capitellid polychaetes, and top snails have also been found to be associated with brine pools in the Red Sea. Such species typically feed on microbial symbionts or bacterial and detritus films.
While organisms can typically flourish on the outskirts of a brine pool, they are not always safe from harm there. One possible reason for this is that underwater landslides can impact brine pools and cause waves of hypersaline brine to spill out into surrounding basins, thus negatively affecting the biological communities which live there.
Despite their inhospitable nature, brine pools can also provide a home, allowing organisms to flourish. Deep-sea brine pools often coincide with cold seep activity, allowing for chemosynthetic life to thrive. Methane and hydrogen sulfide released by the seep is processed by bacteria, which have a symbiotic relationship with organisms such as seep mussels. The seep mussels create two distinct zones: the inner zone, which is at the edge of the pool, provides the best physiological conditions and allows for maximum growth, while the outer zone is near the transition between the mussel bed and the surrounding seafloor, and this area provides the worst conditions, causing these mussels to have lower maximum sizes and densities. This ecosystem is dependent on chemical energy and, relative to almost all other life on Earth, has no dependence on energy from the Sun.World Wildlife Fund. "Deep sea ecology: hydrothermal vents and cold seeps". March 23, 2006. Accessed October 3, 2007.
An important part of the study of extreme environments such as brine pools is the function and survival of microbes. Microbes help support the larger biological community around environments like brine pools and are key to understanding the survival of other extremophiles. Biofilms contribute to the creation of microbes and are considered the foundation by which other micro-organisms can survive in extreme environments. The research into the growth and function of artificial extremophile biofilms has been slow due to the difficulty in recreating the extreme deep-sea environments they are found in.
Microbial diversity and community composition
Microbial composition Red Sea brine pool microbiology is among the most intensively studied using metagenomics and amplicon sequencing.
Metagenomic analysis Metagenomic analysis is a powerful approach for characterizing microbial communities in a variety of environments. Previously, genetic analysis required having the microorganisms in culture, which is problematic since most microorganisms in nature have not been cultivated. Metagenomics overcomes these problems by allowing researchers to directly sample and analyze and genetically characterize microbial communities sampled from the desired environment. Metagenomic analyses has revealed previously-uncharacterized microbial communities in multiple brine pools. Common procedures for characterizing marine microbial communities by metagenomic analysis includes sampling, filtration and extraction, DNA sequencing, and comparison to databases.
Main clades The taxonomic makeup of the main microbial communities found at Atlantis II and Discovery without including minor or unknown species to avoid ambiguity is summarized in the following list that is based on the data from primary articles.{6,7}
+DomainMicrobesBacteria[Order] Actinomycetales[Family] Microbacteriaceae[Genus] Bacillus[Phylum] Bacteroidetes[Class] Flavobacteria[Phylum] Candidatus division od1[Phylum] Chloroflexi[Class] Anaerolineae[Class] SAR202 clade[Phylum] Cyanobacteria[Phylum] Deinococcota[Genus] Meiothermus[Class] Deferribacteres[Phylum] Firmicutes[Order] Thermotoaea[Class] Candidatus Scalindua[Class] Candidatus Brocadiales l[Class] Alphaproteobacteria[Genus] Phyllobacterium[Genus] Afipia[Genus] Bradyrhizobium[Order] SAR11(Pelagibacterales)[Class] Betaproteobacteria[Genus] Rhodoferax[Genus] Malikia[Genus] Cupriavidus[Genus] Ralstonia[Class] Deltaproteobacteria[Class] Gammaproteobacteria[Order] Alteromonadales[Order] Oceanospirillales[Genus] Acinetobacter[Genus] Alkanindiges[Genus] StenotrophomonasArchaea[Class] Group c3[Class] Marine benthic group a (MBG-A)[Class] Marine benthic group b (MBG-B)[Class] Marine group I (MGI)[Class] Misc crenarchaeotic group[Class] Psl12[Order] Desulfurococcales[Order] Sulfolobales[Order] Thermoproteales[Phylum] Euryarchaeota[Class] Archaeoglobi[Order] Archaeoglobales[Order] Halobacteria[Order] Halobacteriales[Class] Methanomicrobia[Order] Methanobacteriales[Order] Methanocellales[Order] Methanococcales[Order] Methanomicrobiales[Order] Methanopyrales[Order] Methanosarcinales[Class] Thermoplasmata[Genus] Candidatus Korarchaeum cryptofilum[Genus] Candidatus Caldiarchaeum
Environmental challenges and adaptations The lack of mixing with the water column in combination with high salinity, anoxia, extremes in water temperature, and hydrostatic pressure results in microbial assemblages that are specific to these environments.
Challenges The high salinity levels present challenges for the retention of water by cells and consequent effects on cell turgor and functioning. Brine pools also exert ionic, kosmotropic, and chaotropic effects on the cells, which also causes additional challenges for the organisms to survive these extreme environments.
In addition, the lack of oxygen increases the difficulty of organisms to yield energy, as oxygen is the most energy-yielding electron acceptor.
Adaptations Organisms have developed different strategies to solve the challenges imposed by high levels of salinity. In order to decrease the risk of chaotropic effects on the cells, halophilic archaea have a "salt-in" approach and "compatible-solute" strategy, which increases intracellular ionic concentration (mostly K+) to decrease the osmotic pressure; thus, these organisms have adapted their entire metabolic machinery to maintain salt concentration inside of their cells.