Halophilia

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Organisms that live in environments with high salt concentrations are referred to as halophiles or halotolerants (derived from the Greek hals , halos = salt) . Salt is not only considered to be table salt , but also any other mineral salt .

Halophilic (i.e. "salt-loving") organisms are so adapted to high salt concentrations that they stop growing or die when salinity drops below a certain threshold. Depending on the degree of adaptation, a distinction is made between weak, moderate or extreme halophiles.

Halotolerant (ie “salt-bearing”) organisms generally thrive in low-salt environments. However, many of them are weak and are being displaced into saline habitats. Due to their ability to adapt to higher salt concentrations, their broad ecological niche allows them to colonize saline biotopes .

Habitats

The Mono Lake , a salt lake in California

All habitats with increased salinity are characterized by reduced water activity . Water is bound here by soluble salts and can only be kept in the cell body through special adaptations. In addition, high ion concentrations have a detrimental effect on metabolic processes. Such environments are, for example, salt lakes , saline evaporation basins, coastal strips, but also small-scale locations such as the surface of desert plants. The salinity of these habitats varies and can reach that of a saturated (30 percent) saline solution . The salt composition of the individual locations can differ considerably, however: If the salinity of thalassohaline locations is largely determined by sodium chloride, many athalassohaline salt lakes are rich in calcium , magnesium or carbonate . In this case, the so-called soda lakes , the high pH value means that the organisms living here are also alkaliphilic or alkali-tolerant . In addition, the salinity can be subject to continuous or sudden changes, e.g. B. when a lake dries up, when water masses mix in estuaries , in tidal zones or during heavy rainfall.

Adaptation

If the salt concentration in the environment of an organism is higher or lower than in the cell body, this always forces an adjustment, because different salt concentrations always try to balance each other. However, only water can diffuse between the cell interior and the environment; Salt ions can only cross cell membranes with difficulty. This leads to the following situation: If the concentration in the environment is lower, water diffuses into the cell. This is the case, for example, in the “freshwater” habitat, the environment is hypoosmotic . If the concentration is higher, the organism loses water. This is the case, for example, in salt lakes, where the environment is hyperosmotic . In both cases there is a change in the salt concentration inside the organism. However, life processes are always linked to a certain availability of water - and thus to certain salt concentrations - in the cell. Almost all organisms therefore actively regulate their internal salt concentration.

Multicellular organisms that live in a hyperosmotic environment have developed special organs for this purpose, for example salt glands or kidneys .

Halotolerant and halophilic unicellular organisms achieve adaptation through two different strategies:

  • The first possibility is the absorption of the inorganic salts into the cytoplasm ("salt-in" strategy).

This variant is mainly found in halophilic protozoa. Their life processes (i.e. especially their enzymes ) are so adapted to high salt concentrations that their functionality is lost when the salt concentration decreases.

  • The second possibility is the accumulation of organic compounds inside the cell, which are referred to as compatible solutes or osmoprotectants (“organic osmolyte” strategy).

This variant is preferred by halotolerant single-cell organisms. If the salt concentration in the environment increases, the cell produces osmotically active organic substances ( e.g. certain carbohydrates , amino acids , polyols , betaines and ectoins ). Like salts, these small molecules are readily soluble in water and develop the same osmotic effect. However, they do not negatively affect cell metabolism.

Extremely halophiles

Phototrophic halophilic single-celled cells color the water of this salt production plant. Depending on the salt content, certain species dominate in the individual basins.
Brine shrimp or brine shrimp Artemia salina
Haloquadratum walsbyi

Halophilic and halotolerant organisms are found in all domains of life. The successful colonization of the particularly salty habitats is reserved for single cells such as bacteria , archaea and some algae . In some of these unicellular organisms, unusual and previously unique cell shapes were found, namely triangular and square cells.

Extremely halophilic protozoa live in salt lakes, salt pans or cured foods. Certain archaea can even live in saturated saline solution (5 mol / l NaCl), but they grow only slowly. These microorganisms usually live aerobic , chemoorganotrophic or phototrophic - that is, they carry out photosynthesis . Some of these phototrophic unicellular organisms can use light through bacteriorhodopsin for the outward transport of protons. The resulting proton gradient across the cell membrane can be used for ATP synthesis. This process is a simple and probably original type of photosynthesis.

Phototrophic halophilic single cells are the cause of the intense color that salt and soda lakes or sea salt extraction systems have. The pigments of these organisms are so highly concentrated that they virtually migrate through the entire food chain: they color halophilic crabs that feed on the microorganisms and finally the flamingos , which in turn eat the crabs.

Extreme biotopes are often poor in species. This also applies to locations with high salt concentrations. Even alkaline soda lakes are poor in species, but extremely rich in individuals. In addition to estuaries and reefs, they have the highest rate of biomass production and are among the most productive ecosystems in the world.

Some genera of halophilic protozoa:

Halobacterium noricense and various Halococcus salifodinae were u. a. Found in the Bad Ischler Salzberg and in the Altaussee salt mine .

Haloquadratum walsbyi ("Salt Square"), a species of the genus Haloquadratum in the Halobacteriaceae family, is adapted to the highest salt concentrations. Green and flat, it optimizes photosynthesis by floating and dominates the world in pools in which seawater thickens for salt production before all life dies with an increase in the magnesium chloride concentration in the liquid phase.

See also

literature

  • Klaus Hausmann, et al .: Extremophiles - Microorganisms in unusual habitats. VCH-Verl.-Ges., Weinheim 1995, ISBN 3-527-30068-6 , pp. 87 ff.
  • Francisco Rodriguez-Valera: Halophilic bacteria. CRC Press, Boca Raton 1988, ISBN 0-8493-4366-6 .

Individual evidence

  1. Jump up ↑ Sergiu Fendrihan, Andrea Legat, Marion Pfaffenhuemer, Claudia Gruber, Gerhard Weidler, Friedrich Gerbl, Helga Stan-Lotter: Extremely halophilic archaea and the issue of long-term microbial survival . In: Reviews in Environmental Science and Bio / Technology . tape 5 , no. 2–3 , July 2006, pp. 203-218 , doi : 10.1007 / s11157-006-0007-y .
  2. ↑ Helga Stan-Lotter: Extreme Biotopes - Microorganisms in Permian Salt Sediments. P. 10–13 in: Scientific Spectrum - Dossier Life in Space . 3/2002, Spektrum-d.-Wiss.-Verl., Heidelberg 2002, ISBN 3-936278-14-8 .
  3. ↑ M. Pfaffenhuemer, MN Spilde, PJ Boston, H. Stan-Lotter: Analysis of Ancient Austrian RockSalt by using Electronmicroscopic techniques . ( Online [PDF; 10 kB ]).
  4. ↑ Helga Stan-Lotter, et al .: Microorganisms in the ancient terrestrial Subsurface. in: Joseph Seckbach, et al .: From fossils to astrobiology - records of life on Earth and the search for extraterrestrial biosignatures. Springer, Dordrecht 2009, ISBN 978-1-4020-8836-0 , p. 240.
  5. ↑ Survivors - Special Features of Halobacteria from Permian Salt. In: fwf.ac.at. Fund for the Promotion of Scientific Research , May 17, 2002, accessed on July 13, 2016 .