HALOPHILES: CLASSIFICATION, OSMOSIS, APPLICATIONS, EXAMPLES - BIOLOGY - 2024

The halophilic organisms are a category of microorganisms, both prokaryotes and eukaryotes, able to reproduce and live in environments with high concentrations of salt such as seawater and hypersaline arid areas. The term halophile comes from the Greek words halos and filo, which means "lover of salt."

Organisms classified within this category also belong to the large group of Extremophilic organisms since they proliferate in habitats of extreme salinity, where the majority of living cells would be unable to survive.

Salinas, environments of extreme salinity where extreme halophilic cells proliferate. By H. Zell, from Wikimedia Commons.

In fact, the vast majority of existing cells rapidly lose water when exposed to media rich in salt and it is this dehydration that in many cases quickly leads to death.

The ability of halophilic organisms to be able to live in these environments is due to the fact that they can balance their osmotic pressure in relation to the environment and maintain their isosmotic cytoplasm with the extracellular environment.

They have been classified based on the concentration of salt, in which they can live in extreme, moderate, weak and halotolerant halophiles.

Some halophilic representatives are the green alga Dunaliella salina, the crustacean of the genus Artemia or water flea, and the fungi Aspergillus penicillioides and Aspergillus terreu.

Classification

Not all halophilic organisms are capable of proliferating in a wide range of salt concentrations. On the contrary, they differ in the degree of salinity that they are able to tolerate.

This tolerance level, which varies between very specific concentrations of NaCl, has served to classify them as extreme, moderate, weak, and halotolerant halophiles.

The group of extreme halophiles includes all those organisms capable of populating environments where NaCl concentrations exceed 20%.

These are followed by moderate halophiles that proliferate at NaCl concentrations between 10 and 20%; and weak halophiles, which do so at lower concentrations that vary between 0.5 and 10%.

Finally the halotolerantes, are organisms that are only capable of supporting low concentrations of salt.

Osmosis and salinity

There is a wide variety of prokaryotic halophiles capable of resisting high concentrations of NaCl.

This ability to resist salinity conditions that vary from low, but higher than those that most living cells are capable of tolerating, to very extreme ones, have been acquired thanks to the development of multiple strategies.

The main or central strategy is to avoid the consequences of a physical process known as osmosis.

This phenomenon refers to the movement of water through a semi-permeable membrane, from a place with a low concentration of solutes to one with a higher concentration.

Therefore, if in the extracellular environment (environment where an organism develops) there are concentrations of salt higher than those in its cytosol, it will lose water to the outside and it will dehydrate to death.

Meanwhile, to avoid this loss of water, they store high concentrations of solutes (salts) in their cytoplasm in order to compensate for the effects of osmotic pressure.

Adaptive strategies to cope with salinity

Halophilic bacteria. By Maulucioni based on images from Commons, from Wikimedia Commons.

Some of the strategies used by these organisms are: the synthesis of enzymes capable of maintaining their activity at high concentrations of salt, purple membranes that allow growth through phototrophy, sensors that regulate the phototactic response such as rhodopsin, and gas vesicles that promote their growth. floatation.

In addition, it should be noted that the environments where these organisms grow are quite changeable, which creates a risk for their survival. Therefore, they develop other strategies adapted to these conditions.

One of the changing factors is the concentration of solutes, which is not only important in hypersaline environments, but in any environment where rains or high temperatures can cause desiccation and consequently variations in osmolarity.

To cope with these changes, halophilic microorganisms have developed two mechanisms that allow them to maintain a hyperosmotic cytoplasm. One of them called "salt-in" and the other "salt-out"

Salt-in mechanism

This mechanism is carried out by Archeas and Haloanaerobiales (strict anaerobic moderate halophilic bacteria) and consists in raising the internal concentrations of KCl in their cytoplasm.

However, the high concentration of salt in the cytoplasm has led them to make molecular adaptations for the normal functioning of intracellular enzymes.

These adaptations basically consist of the synthesis of proteins and enzymes rich in acidic amino acids and poor in hydrophobic amino acids.

A limitation for this type of strategy is that those organisms that carry it out have a poor capacity to adapt to sudden changes in osmolarity, restricting their growth to environments with very high saline concentrations.

Salt-out mechanism

This mechanism is used by both halophilic and non-halophilic bacteria, in addition to moderate halophilic methanogenic archaea.

In this, the halophilic microorganism performs the osmotic balance using small organic molecules that can be synthesized by it or taken from the medium.

These molecules can be polyols (such as glycerol and arabinitol), sugars such as sucrose, trehalose or glucosyl-glycerol or amino acids and derivatives of quaternary amines such as glycine-betaine.

All of them have a high solubility in water, have no charge at physiological pH and can reach concentration values ​​that allow these microorganisms to maintain the osmotic balance with the external environment without affecting the functioning of their own enzymes.

Additionally, these molecules have the ability to stabilize proteins against heat, desiccation or freezing.

Applications

Halophilic microorganisms are very useful for obtaining molecules for biotechnological purposes.

These bacteria do not present major difficulties to be cultivated due to the low nutritional requirements in their media. Their tolerance to high saline concentrations minimizes the risks of contamination, which makes them more advantageous alternative organisms than E. coli.

Additionally, by combining its production capacity with its resistance to extreme salinity conditions, microorganisms are of great interest as a source of industrial products, both in the pharmaceutical, cosmetic and biotechnological fields.

Some examples:

Enzymes

Many industrial processes are developed under extreme conditions, which offers a field of application for enzymes produced by Extremophilic microorganisms, capable of acting at extreme values ​​of temperature, pH or salinity. Thus, amylases and proteases, used in molecular biology, have been described.

Polymers

Similarly, halophilic bacteria are producers of polymers with surfactant and emulsifying properties of great importance in the oil industry because they contribute to the extraction of crude oil from the subsoil.

Compatible solutes

The solutes that these bacteria accumulate in their cytoplasm have a high stabilizing and protective power of enzymes, nucleic acids, membranes and even whole cells, against freezing, desiccation, heat denaturation and high salinity.

All of this has been used in enzyme technology as well as in the food and cosmetic industry to extend the life of products.

Waste biodegradation

Halophilic bacteria are capable of degrading toxic wastes such as pesticides, pharmaceuticals, herbicides, heavy metals, and oil and gas extraction processes.

Foods

In the field of food they participate in the production of soy sauce.

References

1. Dennis PP, Shimmin LC. Evolutionary divergence and salinity-mediated selection in halophilic Archaea. Microbiol Mol Biol Rev. 1997; 61: 90-104.
2. González-Hernández JC, Peña A. Adaptation strategies of halophilic microorganisms and Debaryomyces hansenii (halophilic yeast). Latin American Journal of Microbiology. 2002; 44 (3): 137-156.
3. Oren A. Bionergetic aspects of halophilism. Microbiol Mol Biol Rev. 1999; 63: 334-48.
4. Ramírez N, Sandoval AH, Serrano JA. Halophilic bacteria and their biotechnological applications. Rev Soc Ven Microbiol. 2004; 24: 1-2.
5. Wood JM, Bremer E, Csonka LN, Krämer R, Poolman B, Van der Heide T, Smith LT. Osmosensing and osmoregulatory compatible solutes accumulation by bacteria. Comp Biochem Physiol. 2001; 130: 437-460.