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A soda lake or alkaline lake is a highly alkaline lake due to its high concentrations of carbonate salts, typically sodium carbonate and related salt complexes, and with a pH value over 8.5 to 9. Many soda lakes also contain high concentrations of sodium chloride and other dissolved salts, making them saline or hypersaline lakes.

They are among the most extreme aquatic environments on Earth. However, despite their apparent inhospitability, they are often highly productive ecosystems, compared to their (pH-neutral) freshwater counterparts, and the most productive aquatic environments on Earth. An important cause for their high productivity is the abundance of dissolved carbon dioxide.

They are considered as environments that conserve and/or mimic ancient life conditions and as "a recreated model of late Precambrian ocean chemistry" — that is, the "soda lake" environment that prepared the great explosion of life during the Cambrian. This links to the Soda Ocean Hypothesis, characterizing the primitive ocean with a higher carbonate mineral supersaturation.

Soda lakes occur naturally throughout the world (see table below). Many are in arid and semi-arid areas and most are on the margins of tectonic plates, in connection to tectonic rifts like the East African Rift Valley and other such places. In Brazil, the Pantanal has around 600 shallow soda lakes.

Geology, geochemistry and genesis
Soda lakes form instead of saline chloride lakes when the concentration of bicarbonate in the recharging ground or surface water is more than two times that of Ca. As the water evaporates and calcite precipitates from the solution it crosses a ‘‘geochemical divide’’ (Hardie and Eugster 1970), with the resulting residual solution evolving toward a Na:HCO3–CO3–Cl type with variable SO4-2, but with high pH, and little Ca or Mg in solution.

A lake becomes alkalic when particular geographical, geological and climatic conditions are combined.

topography : endorheic lake

high alkalinity

relative deficit in soluble magnesium or calcium, so that they do not alter the saturation of carbonate ions.

Association with active tectonic and volcanic zones
Soda lakes seem to be associated with active tectonic and volcanic zones.

Endorheic lake
An endorheic lake is a lake with no outflow of water. Such lakes can be of two types:
 * soda lakes in endorheic depressions. Pit craters or depressions formed by tectonic rifting often provide such topography — they may be enclosed subsequently to tectonic movements or by volcanic dams. Most of the larger soda lakes seem to be in that category. The largest soda lake on Earth is Lake Van (Van Gölü) in eastern Anatolia, by volume the third largest closed basin lake on Earth (after Lake Aral dried up); its water exit was dammed by an eruption of the Nemrut volcano (Turkey).
 * soda lakes in volcanic craters or in calderas, that typically have no surface tributaries. Apart from Satonda lake, other volcanic soda lakes are Niuafoʻou lake (one of the Niua Islands in the southern Pacific Ocean between Fiji and Samoa), Kauhakō Crater lake (in the center of the Kalaupapa Peninsula on the island of Molokaʻi, Hawaii), Empakai Crater (Tanzania), a crater on the island of Pantelleria (Italy), and very possibly many others.

But there are exceptions to the "no outflow" rule: both Lake Kivu and Lake Tanganyika have outlets but also have the characteristics of soda lakes, and Lake Tanganyika even grows microbialites.

Climate
Climate-wise, soda lakes typically occur in hot, arid regions. But here again there are exceptions, most reknown of these being the soda lakes in Inner Mongolia, with a pH around 10 and each of their total inorganic carbon (TIC) and chlorine ion (Cl−) rising up to more than 1,000 mM. They are close to the southernmost extent of permafrost of the Russian taiga, north of the Gobi Desert, and they freeze solid in winter.

Alkalinity
Another condition for the formation of a soda lake is the relative scarcity of soluble magnesium or calcium. This is because dissolved magnesium (Mg2+) or calcium (Ca2+) remove the carbonate ions, through the precipitation of minerals such as calcite, magnesite or dolomite, effectively neutralizing the pH of the lake water. This results in a neutral (or slightly basic) salt lake instead. A good example is the Dead Sea, which is very rich in Mg2+.

The basic condition of a soda lake is that the total alkalinity (or TA) — that is, the amount of carbonates and bicarbonates, commonly measured in milliequivalents (of dissolved carbonates) per liter (meq/l) — is superior to the alkaline earth ions magnesium (Mg) and calcium (Ca): TA > (Mg + Ca) or, in more detailed form: $∑$($[HCO_{3}^{-}] carbonate$ + $2[CO_{3}^{2-}] bicarbonate$) > $∑$($2[Mg^{2+}] ion magnesium$ $2[Ca^{2+}] ion calcium$)

This ratio is most likely to occur in areas with fresh volcanic rocks, where carbon dioxide (CO2) can react with fresh silicates, mobilizing sodium (Na), potassium (K), magnesium (Mg) and calcium (Ca) in the appropriate proportions. But alkalinity can also rise by sulfate reduction (see "Sulfur cycle" below), through bacteria.

In some soda lakes, inflow of Ca2+ through subterranean seeps, can lead to localized precipitation. In Mono Lake, California and Lake Van, Turkey, such precipitation has formed columns of tufa rising above the lake surface.

Water evaporation is an important factor: it increases the concentration of bicarbonate ([CO32-]), which causes a rise in pH, which in turn induces sodium carbonates (soda ash, Na2CO3 and its hydrates) to precipitate.

Ongoing weathering within the crater lakes (as for lake Niuafo‘ou), or continuing evaporation in endorheic lakes lead to mature soda lake chemistry and to a CaCO3 super-saturation (saturation index or SI = 1) that can sustain microbialite growth.

Stratification
Many soda lakes are strongly stratified, with a well-oxygenated upper layer (epilimnion) and an anoxic lower layer (hypolimnion), without oxygen and often high concentrations of sulfide. Stratification can be permanent, or with seasonal mixing. The depth of the oxic/anoxic interface separating the two layers varies from a few centimeters to near the bottom sediments, depending on local conditions. In either case, it represents an important barrier, both physically and between strongly contrasting biochemical conditions.

Biodiversity
Soda lakes are unusually highly productive ecosystems, compared to their (pH-neutral) freshwater counterparts. Gross primary production (photosynthesis) rates above 10 g C m−2 day−1 (grams of carbon per square meter per day), over 16 times the global average for lakes and streams (0.6 g C m−2 day−1), have been measured. This makes them one of the most productive aquatic environments on Earth.


 * In the soda lake Nhecolândia (Pantanal, Brazil) and in a deep sea environment (Campos dos Goytacazes, Brazilian Atlantic Ocean)

Mimiviridae members that surprisingly harbored a long, thick tail as they grew on Acanthamoeba castellanii and Vermamoeba vermiformis. We named these strains Tupanvirus soda lake and Tupanvirus deep ocean This tail is the longest described in the virosphere these giant viruses present the largest translational apparatus within the known virosphere

Micro-organisms
Contrary to freshwater ecosystems, their living organisms are often completely dominated by prokaryotes, i.e. bacteria and archaea, particularly in lakes with more "extreme" conditions (higher alkalinity and salinity or lower oxygen content).

Their microbial richness and activity are also very different from that of other high-salt systems. This is essentially due to the main physico-chemical features of two dominant salts: sodium chloride (NaCl) in neutral saline systems and sodium carbonates in highly alkaline soda lakes, that influence the amount of energy required for osmotic migration of molecules.

Microbial diversity
Soda lakes are inhabited by a rich diversity of microbial life, often in dense concentrations. This leads to permanent or seasonal "algae blooms" with visible colouration in many lakes. The colour varies between particular lakes, depending on their predominant life forms and can range from green to orange or red.



In general, the microbial biodiversity of soda lakes is relatively poorly studied. Many studies have focused on the primary producers, namely the photosynthesizing cyanobacteria or eukaryotic algae (see Carbon cycle). As studies have traditionally relied on microscopy, identification has been hindered by the fact that many soda lakes harbour species that are unique to these relatively unusual habitats and in many cases thought to be endemic, i.e. existing only in one lake. The morphology (appearance) of algae and other organisms may also vary from lake to lake, depending on local conditions, making their identification more difficult, which has probably led to several instances of taxonomic confusions in the scientific literature.

Molecular methods such as DNA fingerprinting or sequencing have been used to study the diversity of organisms in soda lakes. For instance, 16S ribosomal RNA gene has revealed that the bacterial community of the lake with the highest salinity was characterized by a higher recent accelerated diversification than the community of a freshwater lake, whereas the phylogenetic diversity in the hypersaline lake was lower than that in a freshwater lake.

Biogeography and uniqueness (endemism)
In addition to their rich biodiversity, soda lakes often harbour many unique species, adapted to alkalic conditions and unable to live in environments with neutral pH. These are called alkaliphiles. Among alkaliphiles organisms, those also adapted to high salinity are called haloalkaliphiles. Culture-independent genetic surveys have shown that soda lakes contain an unusually high amount of alkaliphilic microorganisms with low genetic similarity to known species. This indicates a long evolutionary history of adaptation to these habitats with few new species from other environments becoming adapted over time.

In-depth genetic surveys also show an unusually low overlap between the microbial communities present in the various soda lakes with only slightly different conditions such as pH and salinity. This trend is especially strong in the bottom layer (hypolimnion) of stratified lakes, probably because of the isolated character of such environments. Diversity data from soda lakes suggest the existence of many endemic microbial species, unique to individual lakes. This is a controversial finding, since conventional wisdom in microbial ecology dictates that most microbial species are cosmopolitan and dispersed globally, thanks to their enormous population sizes, a famous hypothesis first formulated by Lourens Baas Becking in 1934 ("Everything is everywhere, but the environment selects").

Macro-organisms
A rich diversity of eukaryotic algae, protists and fungi have also been encountered in many soda lakes.

Multicellular animals such as crustaceans (notably the brine shrimp Artemia and the copepod Paradiaptomus africanus) and fish (e.g. Alcolapia), are also found in many of the less extreme soda lakes, adapted to the conditions of these alkalic and often saline environments. Particularly in the East African Rift Valley, microorganisms in soda lakes also provide the main food source for vast flocks of the lesser flamingo (Phoeniconaias minor). The cyanobacteria of the genus Arthrospira (formerly Spirulina) are a particularly preferred food source for these birds, owing to their large cell size and high nutritional value. Declines in East African soda lake productivity due to rising water levels threaten this food source. This may force lesser flamingos to move north and south, away from the equator.

Carbon cycle, photosynthesis and methanogenesis


a combination of high phytoplankton standing crop and above-average biomass-specific rates, partly due the large reserve of CO2 for localized photosynthetic activity

Photosynthesis produces chemical energy stored in intracellular organic compounds containing carbon. It dominates the activity at the surface of soda lakes and this process provides the primary energy source for life in the lake. The most important photosynthesizers are typically cyanobacteria, but in many less "extreme" soda lakes, eukaryotes such as green algae (Chlorophyta) can also dominate. The major genera of cyanobacteria typically found in soda lakes include Arthrospira (formerly Spirulina) (notably A. platensis), Anabaenopsis, Cyanospira, Synechococcus or Chroococcus. In more saline soda lakes, haloalkaliphilic archaea such as Halobacteria and bacteria such as Halorhodospira dominate photosynthesis. However, it is not clear whether this is an autotrophic process or if these require organic carbon from cyanobacterial blooms, occurring during periods of heavy rainfall that dilute the surface waters.

Below the surface, anoxygenic photosynthesizers using other substances than carbon dioxide for photosynthesis also contribute to primary production in many soda lakes. These include purple sulfur bacteria such as Ectothiorhodospiraceae and purple non-sulfur bacteria such as Rhodobacteraceae (for example the species Rhodobaca bogoriensis isolated from Lake Bogoria ).

The photosynthesizing bacteria provide a food source for a vast diversity of aerobic and anaerobic organotrophic microorganisms from phyla including Pseudomonadota, Bacteroidota, Spirochaetota, Bacillota, Thermotogota, Deinococcota, Planctomycetota, Actinomycetota, Gemmatimonadota, and more. The anaerobic fermentation of organic compounds originating from the primary producers, results in one-carbon (C1) compounds such as methanol and methylamine.

At the bottom of lakes (in the sediment or hypolimnion), methanogens use these compounds to derive energy, by producing methane, a procedure known as methanogenesis. A diversity of methanogens including the archaeal genera Methanocalculus, Methanolobus, Methanosaeta, Methanosalsus and Methanoculleus have been found in soda lake sediments. When the resulting methane reaches the aerobic water of a soda lake, it can be consumed by methane-oxidizing bacteria such as Methylobacter or Methylomicrobium.

Sulfur cycle
Sulfur-reducing bacteria are common in anoxic layers of soda lakes. These reduce sulfate and organic sulfur from dead cells into sulfide (S2−). Anoxic layers of soda lakes are therefore often rich in sulfide. As opposed to neutral lakes, the high pH prohibits the release of hydrogen sulfide (H2S) in gas form. Genera of alkaliphilic sulfur-reducers found in soda lakes include Desulfonatronovibrio and Desulfonatronum. These also play important an ecological role besides in the cycling of sulfur, as they also consume hydrogen, resulting from the fermentation of organic matter.

Sulfur-oxidating bacteria instead derive their energy from oxidation of the sulfide reaching the oxygenated layers of soda lakes. Some of these are photosynthetic sulfur phototrophs, which means that they also require light to derive energy. Examples of alkaliphilic sulfur-oxidizing bacteria are the genera Thioalkalivibrio, Thiorhodospira, Thioalkalimicrobium and Natronhydrogenobacter.

Nitrogen and other nutrients
Nitrogen is a limiting nutrient for growth in many soda lakes, making the internal nitrogen cycle very important for their ecological functioning. One possible source of bio-available nitrogen is diazotrophic cyanobacteria, which can fix nitrogen from the atmosphere during photosynthesis. However, many of the dominant cyanobacteria found in soda lakes such as Arthrospira are probably not able to fix nitrogen. Ammonia, a nitrogen-containing waste product from degradation of dead cells, can be lost from soda lakes through volatilization because of the high pH. This can hinder nitrification, in which ammonia is "recycled" to the bio-available form nitrate. Nevertheless, ammonia oxidation seems to be efficiently carried out in soda lakes in either case, probably by ammonia-oxidizing bacteria as well as Thaumarchaea.

Limiting nutrients for phytoplankton growth
A recurrent assertion still found in many publications up to the 2020s, is that nitrogen is the dominant limiting nutrient for phytoplankton growth in tropical lakes(soda lakes and non-soda lakes). This assertion is due to the difficulty in measuring phytoplankton nutrient limitation in situ and to the paucity of tropical inland water research. A 2023 review of studies regarding limitation factors in 114 tropical lakes has found that there is a wide range of nutrient limitation factors, and each lake often has several dominant limiting nutrients according to seasonal patterns of water column stratification and precipitation, land use and land cover, and the interaction of these characteristics with lake morphology.

Apart from straightforward nutrients limitations, some of the other limitation factors may be light limitation caused by mixing depth or increased suspended matter from various causes, or seasonal changes in stratification, or a number of other factors..

List of soda lakes


The following table lists some examples of soda lakes by region, listing country, pH and salinity. NA indicates 'data not available':

Industrial use
Many water-soluble chemicals are extracted from soda lake waters worldwide. Lithium carbonate (see Lake Zabuye), potash (see lake Lop Nur and Qinghai Salt Lake Potash), soda ash (see Lake Abijatta and Lake Natron), etc. are extracted in large quantities. Lithium carbonate is a raw material in production of lithium which has applications in lithium storage batteries widely used in modern electronic gadgets and electrically powered automobiles. Water of some soda lakes are rich in dissolved uranium carbonate. Algaculture is carried out on a commercial scale with soda lake water.