Brine pool
A brine pool, sometimes called an underwater, deepwater or brine lake, is a volume of brine collected in a seafloor depression. The pools are dense bodies of water that have a salinity that is three to eight times greater than the surrounding ocean. Brine pools are commonly found below polar sea ice and in the deep ocean. Those below sea ice form 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. 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. The frequency of brine pool formation coupled with their uniquely high salinity has made them a candidate for research regarding ways to harness their properties to improve human science. 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. The high salinity raises the density of the brine, which creates a surface and shoreline for the pool. 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. Support of life. Due to the methods of their formation and lack of mixing, brine pools are anoxic and deadly to most organisms. When an organism enters a brine pool, they attempt to "breathe" the environment and experience cerebral hypoxia due to the lack of oxygen and toxic shock from the hyper-salinity. If organisms cannot surface long enough to retreat to the rim, they quickly die. When observed by submarines or Remotely Operated Vehicles (ROV), brine  pools are found to be eerily littered with dead fish, crabs, amphipods, and various 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 preventing decay and creating a fish "graveyard." 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", have been found along the edge of the "Valdiva 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. In 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 here. 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. 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. 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. Metagenomics Analysis. Metagenomics is a powerful genomic analysis to identify the microbiome communities in a variety of environments. Previous gene analysis requires culturing microorganisms that is problematic since most microorganisms present in nature are not cultivable. Metagenomics overcomes this problems by allowing  researchers to directly sample and analyze the microbe community from the desired environment. Despite the harsh environment for living organisms, metagenomics has revealed the presence of previously unknown microbiome communities in multiple brine pools. Common procedures of marine microbe metagenomics include sampling, filtration and extraction, DNA sequencing, and database analysis. Shotgun sequencing and 454 pyrosequencing are typically used to sequence genes, and 16S rRNA is used for the identification of species. Main clades. The taxonomy of the main microbe community found in Atlantis II and Discovery without minor or unknown species to avoid ambiguity. It is important to note that  the list is based on the data provided from the primary articles. Because identification of microbes and construction of phylogeny is controversial, phylogeny has been reconstructed indicating some taxonomic rank might not be up-to-date. Environmental challenges and adaptations. The lack of mixing of the water column in combination with the extreme conditions of salinity, anoxia, temperature and hydrostatic pressure, has resulted in an environment which has been separated from the upper water column. These unique environmental characteristics promote unique extreme challenges and selective pressures on the organisms that live in these ecosystems, and promoting unique microbial assemblages that  were able to adapt to the environment. Challenges. Due to high levels of salinity of the environment, cell membranes cannot avoid the rapid loss of intracellular water, and thus need to develop extreme strategies to decrease their metabolic activity as much as possible, impairing 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 multiple stressors in combination with the lack of oxygen in Brine Pools also increase the difficulty of organisms to yield energy and to  adapt to these extreme environments, as oxygen is usually extensively used as the most energy-yielding electron acceptor. Adaptations. Different organisms developed a number of strategies to solve the challenges imposed by these extreme environments, such as the high levels of salinity. In order to decrease the risk of chaotropic effects on the cells, organisms like halophilic archaea successfully developed the "salt-in" and the "compatible-solute" strategy, which increases intracellular ionic concentration (mostly K+) to decrease the osmotic pressure, and thus forcing these organisms to adapt their entire metabolic machinery to increase salt concentration inside of their cells to  ensure their survival. In addition, in order to survive the extreme water temperatures and hydrostatic pressures, piezophilic microorganisms that synthesized thermoprotective molecules (e.g. hydroxyketone) were selected for, and thus successfully avoiding denaturation of their proteins, decrease of fitness due to extreme hydrostatic pressures, decrease the risk of desiccation imposed by the environment, and developing novel characteristics in the microorganisms present in the brine pools. Lastly, another important adaptation is the usage of alternative electron acceptors molecules to yield energy, such as iron, manganese, sulfate, elemental sulfur, carbon dioxide, nitrite and nitrate, which are abundantly available in these deep water layers . Animals have also been found living in these anaerobic brine pools, such as the first ever described metazoan by Danovaro et al., in 2010., even though much research still has to be done about their entire life cycle. Many novel undefined taxa that live in these extreme environments are still unknown. Nutrient Cycling in Brine pools. Chemical composition and metabolic significance. As the name suggests, Brine pools, or Deep Hypersaline Anoxic Basins (DHABs), are characterized by a very high salt concentration and anoxic conditions. Sodium, chloride, magnesium, potassium, and calcium ion concentration are all extremely high in brine pools. And , due to low mixing rates between the above seawater and the brine water, brine pool water becomes anoxic within the first ten centimeters or so. While there are large variations in the geochemical composition of individual pools, as well as extreme chemical stratification within the same pool, conserved chemical trends are present. Deeper layers of DHABs will be saltier, hotter, more acidic and more anaerobic than the preceding layers. The concentration of heavy metals (Fe, Mn, Si, Cu) and certain nutrients (NO2-, NH4+, NO3-, and PO4-) will tend to increase with depth, while the concentration of SO4- and both  organic and inorganic carbon decrease with depth. While these trends are all observed to some capacity in DHABs, the intensity and distance over which these trends take effect can vary in depth from one meter, to tens of meters. The heavy stratification within DHABs has led to increased microbial metabolic diversity and varying cell concentrations between layers. The majority of cell biomass has been found at the interfaces between the distinct chemical layers (with the highest concentrations of cells located at the brine-surface interface). Microbes exploit the sharp chemical gradients between the layers to make their metabolisms more thermodynamically  favorable. Four heavily studied DHABs are Urania, Bannock, L’Atalante, and Discovery. All four of these Brine pools are located in the Mediterranean sea, yet they all exhibit distinct chemical properties: Urania has the highest concentration of Sulfuric Acid observed (at ~16mM)--compared to normal sea water (2.6 x 10^-6mM) or the next highest [HS-] in the Bannock basin (~3mM). Discovery has an extremely low concentration of Na+ (68mM) and an extremely high concentration of Mg2+ (4995mM)--compared to the surrounding seawater with concentrations of 528mM and 60mM respectively. The L’Atalante basin has a high [SO4 2-] compared to the  other pools. This extreme variability in environmental conditions leads to each brine pool having a unique metabolic composition. Main Metabolisms and nutrient cycling. Carbon Cycling. While it was initially thought that particulate organic matter (POM) was an important source of carbon for DHABs, due to their depth the concentration of POM reaching the pools it is not significant as originally thought. The majority of fixed carbon is now thought to come from autotrophy, specifically methanogenesis. Direct measurements of methane production in DHABs have provided extensive molecular evidence of methanogenesis in these environments. Proteomic analysis further support the presence of methanogenesis  by identifying the enzyme RuBisCo in various DHABs. Interestingly, it has been suggested that instead of CO2 or acetoclastic methanogenesis, prokaryotes in DHABs use methylotrophic methanogenesis as it allows for a higher energy yield and the intermediates can be used for osmoprotectants. Nitrogen Cycling. One of the key metabolic features of DHABs is the dissimilatory reduction of nitrogen. This is predominantly due to the thermodynamic favorablilty of nitrogen based metabolisms in anaerobic environments. In Bannock basin and L’Atalante basin anammox and denitrification pathways have been identified using a combination of transcriptomics and direct isotope tracking. Other DHABs have been analyzed  for anammox pathways using metatranscriptomic techniques with little positive results, which may be due to the limitations of transcriptomic sensitivity. In deeper DHAB layers, nitrogen fixation and ammonium assimilation has been observed. These reductive pathways require a lot of energy and are mainly performed by methanogens to synthesize osmoprotectants. Sulfur Cycling. Due to the high concentration of sulfate (especially in the Uranian Basin), sulfate reduction is extremely important in the biogeochemical cycling of DHABs. The highest rates of sulfate reduction tend to be found in the deepest DHAB layers, where redox potential is lowest. Sulfate reducing bacteria have been found  in the brines of Kebrit deep, Nereus Deep, Erba deep, Atlantis II deep, and Discovery Deep. Oxidative sulfur pathways help close the biogeochemical sulfur loops within the DHABs. There are three main sulfur oxidizing pathways which are likely found in DHABs: 1) sulfur-oxidizing multienzyme complex which can oxidize sulfide or thiosulfate to sulfate (w/ elemental sulfur or sulfite as an intermediate). 2) a sulfide/quinone complex which oxidizes hydrogen sulfide to elemental sulfur. 3) polysulfide reductase, which reduces precipitate sulfur to sulfide. A combination between the second and third pathway would allow for increased energetic yield. In addition, some  novel groups have been isolated from saline lakes which can anaerobically respire sulfur using acetate, pyruvate, formate, or hydrogen as a sole electron donors. Microbial Symbiosis. There is a high concentration of bacteria present in Brine Pools that serve essential roles for the ecosystem, such as being part of Symbiotic Relationships or acting as a food source for several organisms in this habitat. Examples include tubeworms and clams having a symbiotic relationship with many of these bacteria to convert chemical energy from hydrogen sulfide, and in exchange providing them food to allow reproduction and development; or mussels providing a safe  habitat for bacteria that feed on methane while thriving due to the chemosynthetic, carbon-fixing symbionts that are inhabiting their gill tissues. Thus, these symbiotic relationships with bacteria allow organisms to be abundant and have high biomass in these harsher environments. Bacteria can also act as epibiotic symbiont, which were found to play an important role in the adaptations of microorganisms to these environments, such as organisms from the flagellated group Euglenozoa that have been thriving in brine pools due to this relationship. Future uses. One major idea involves harnessing the salinity of brine pools to use as a power  source. This would be done using an osmotic engine which draws the high salinity top water through the engine and pushes it down due to osmotic pressure. This would cause the brackish stream (which is less dense and has a lighter salinity) to be propelled away from the engine via buoyancy. The energy created by this exchange can be harnessed using a turbine to create a power output. It is possible to study liquid brine in order to harness its electrical conductivity to study if liquid water is present on Mars. A HABIT (Habitability: Brines, Irradiation, and Temperature) instrument will  be part of a 2020 campaign to monitor changing conditions on Mars. This device will include a BOTTLE (Brine Observation Transition to Liquid Experiment) experiment to quantify the formation of transient liquid brine as well as observe its stability over time under non-equilibrium conditions. A third idea involves using microorganisms in deep-sea brine pools to form natural product drugs. These microorganisms are important sources of bioactive molecules against various diseases due to the extreme environment they inhabit, giving potential to an increasing number of drugs in clinical trials. In particular, a novel finding in a study used microorganisms  from the Red Sea brine pools as potential anticancer drugs. Deep sea brine pools have also been a large interest in bioprospecting in the hope that unlikely environments might serve as sources of biomedical breakthroughs due to unexplored biodiversity. Some areas have been found to host antibacterial and anticancer activities in biosynthetic clusters. Other novel antibiotic resistance enzymes have been found that are useful in various biomedical and industrial applications.