Psychrophiles: Definition, Adaptation, and Examples

The present world is full of diversity. Biodiversity or biological diversity refers to the variety and variability among living organisms and the ecological complexes in a given area. On Earth, there are numerous varieties of organisms and a high magnitude of biodiversity. This diversity goes to the next level when we talk of microorganisms. Worth to mention, microorganisms are present everywhere ranging from air, water, and soil to hot geysers, thermal vents, arctic habitats, etc. However, microorganisms found in extreme conditions like thermal vents, and cold arctic regions are called extremophiles. Moreover, these extremophiles can be classified into various types. Out of these, the psychrophiles are the ones that survive at freezing temperatures where other organisms lose their functionality or even die. The speciality of psychrophiles is they survive and are metabolically active at lower temperatures.

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Extremophiles

Extremophiles are organisms that thrive under extreme conditions, from a human point of view, are clearly hostile. Moreover, these refer to organisms that are unicellular and prokaryotic. These are the members of Domain Archaea. These extremophiles can be any: acidophile, alkalophile, halophile, thermophile, hyperthermophile, psychrophile and so on. Further, to elaborate a little:

Acidiphiles: These are extremophiles that can grow at extremely low pH that is in an acidic environment. Optimum pH between 0-5.5.
Alkalophiles: These are extremophiles that love an alkaline environment and prefer to live at high pH. Optimum pH between 8- 11.5.
Halophiles: These inhabit saline conditions and can tolerate high salt conditions.
Thermophiles: These are capable of tolerating high temperatures of 55 degrees to 85 degrees Celsius. For example, Thermus aquaticus
Hyperthermophiles: These don’t grow below 55 degrees Celsius and show growth between 85 degrees to 113 degrees Celsius.

However, in this article, we are focussing on Psychrophiles that grow in regions that are periodically or permanently cold. They are inhabitable in continuously frozen regions that are inhospitable to life.

Psychrophiles: Definition

Psychrophiles are a group of organisms that are cold-loving and grow best at low temperatures of less than 5 degrees Celsius without losing their ability to reproduce. Thus, these are the extremophiles that love extreme cold temperatures and are metabolically active. These are contradictory in nature to mesophiles which prefer optimal growth conditions and other extremophiles like thermophiles grow at extremely high temperatures.

Psychrophilic bacteria. The small fluorescing dots and rods in the photomicrograph grow to high numbers in sea ice. Image source: [Photograph by S. Grossmann] – Thomas, D. N., & Dieckmann, G. S. (2002). Antarctic sea ice–a habitat for extremophiles. Science, 295(5555), 641-644.

Specific environmental conditions for growth

Environmental conditions play a very important role in the survival of nearly all organisms. All microorganisms require certain conditions of temperatures, pH, salinity, humidity, etc. So do the psychrophiles. These are mainly dependent on the temperature conditions of the surrounding environment. Other organism membranes solidify at low temperatures and disintegrate at high temperatures. But psychrophiles require a temperature range between -20 degrees to 10 degrees Celsius. These grow well at 0 degree Celsius and need optimum temperatures of 10 degree Celsius or lower. On Earth, these are found in arctic or antarctic habitats.

Adaptations of Psychrophiles

Since at low temperatures cells become metabolically inactive, growth gets ceased, cell organelles stop dividing, and membrane composition changes. Therefore, microorganisms require certain modifications in normal morphology to adapt successfully to the environment. Catering to the need, psychrophiles have evolved mechanisms to successfully combat additional stress factors associated with cold environments, such as desiccation, high or low pH, radiation, excessive UV, high osmotic pressure, and low nutrient availability. Several adaptations or modifications can be:

Modifications in the cell membrane
Genome structure
Cryoprotectants
Antifreeze proteins

Image source- Tendulkar, S., Hattiholi, A., Chavadar, M., & Dodamani, S. (2021). Psychrophiles: A journey of hope. Journal of Biosciences, 46(3), 64.

Modifications in the cell membranes of psychrophiles

The abnormal thermosensitivity of psychrophiles indicates the adaptation of cold-loving bacteria to their cold environment. Since microbes do not have thermoregulatory mechanisms they solely depend on physiological adaptations.

To maintain normal functions of the cell, psychrophilic bacteria adapt by changing the composition of cell membrane fatty acids. Generally, the majority of bacterial cell membrane fatty acids are straight-chain monounsaturated fatty acids (n-MUFAs) and branched fatty acids (br-FAs). This account for more than 70% of the fatty acids. Moreover, the production of polyunsaturated fatty acids (PUFAs) is only seen at colder temperatures. Thus, polyunsaturated fatty acids maintain the cell membrane of bacteria inhabiting polar habitats. This adaptation is based on the fact that unsaturated fatty acids are fluid at cold temperatures.

Carotenoid pigments also aid membrane fluidity modulators. Antarctic bacteria produce both polar and non-polar carotenoid pigments that aid to buffer membrane fluidity and assist in maintaining viscosity during temperature fluctuations.

Genome structure

Extreme cold temperatures induce an enhanced regulation of genes. This involves membrane biogenesis, such as fatty acid, peptidoglycan biosynthesis, glycosyltransferases, and outer membrane proteins.

A high capacity for translational and post-translational processing is vital for growth at low temperatures. Psychrophiles are cold-sensitive which results in transposons inactivation. Moreover, clustered regularly interspaced short palindromic repeats (CRISPR) that are associated with reverse transcriptase are more abundant in the Alteromonas sp genome of cold habitats.

Cryoprotectants

Hostile temperatures freeze cells to form cytoplasmic ice crystals. This damages the cell and causes osmotic imbalance. Thus, the psychrophiles accumulate some compatible solutes such as glycine, betaine, sucrose, mannitol, etc. The main function these perform is to lower the cytoplasmic freezing point Thereby providing protection against freezing, desiccation, and hyper-osmolality. In Escherichia coli, trehalose formation takes place at cold temperatures. Disaccharide trehalose plays its role in preventing denaturation, aggregation of proteins, removing free radicals, stabilizing cellular membranes under cold temperatures, etc.

Antifreeze proteins

Several psychrophiles produce antifreeze or ice-binding proteins. They bind to control ice crystal growth and recrystallization by lowering the freezing point. This lowering of the freezing point is called thermal hysteresis. Ice-nucleating (IN) proteins prevent supercooling of water and facilitate the formation of ice crystals at temperatures close to the melting point. Many psychrophiles produce exopolysaccharides under cold conditions as a potential cryoprotection mechanism.

Examples of psychrophiles

Various examples of psychrophilic bacteria are Psychrobacter cryohalolentis, Anthrobacter tumbae, Psychrobacter arcticus, Psychrobacter immobilis, Psychrobacter pacificensis, P. piscatorii, P. marincola, P. phenylpyruvicus, Chlamydomonas nivalis, Psychrophiles phenylpyruvicus, Pseudoglutamici cumminisii, Anthrobacter psychrolactophilus, etc.

Applications of psychrophiles

Psychrophilic bacteria have applications like the production of enzymes. These enzymes are catalytically very active, cost-effective, eco-friendly, and adapted to cold temperatures. These enzymes can be used in food processing, bioremediation, waste treatments, molecular biology, etc.

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