Survival in the Extremes: Introduction, Types, Adaptations, and Examples of Extremophiles

 What are Extremophiles?

Introduction:

    Extremophile means "extreme-loving". Organisms that can survive in harsh and extreme conditions are called extremophiles. They can tolerate high and low temperatures, extreme pH, and intense pressure. Common examples of extremophiles include tardigrades, Archae, and certain fungi and protozoa.  

Tardigrade:

 An animal, the tardigrade, is known as the toughest animal.  It is a polyextremophile and can tolerate all types of radiation, stress, and temperature extremes. 

• Tardigrades are famous for surviving extreme stress, but their ordinary life span is short — and they are not truly immortal. They have a lifespan of only a few months to 2 years.
• It is very interesting that tardigrade can survive exposure to -272°C to 150°C(in a laboratory) for a short time. However, they can die if exposed to 87°C hot water for one hour. 
• They can also survive without food, oxygen, and other factors(also in vaccume).
• Tardigrades are microscopic, eight-legged animals also known as "water bears" or "moss piglets".
• They are usually 0.5 mm long when they are mature. 
• They curl up and dehydrate themselves in order to survive in a harsh environment. This dry ball is called a tun, and this state is called cryptobiosis. 
• In this state, it reduces its metabolism to approximately 0.01% from its normal level and survives in harsh environments and radiation(even gamma rays).
• These are oviparous, which means they lay eggs. Their eggs hatch in 30 to 40 days.
• Due to their unique qualities, their ancestor may survive five major mass extinction events. In addition, they existed from approximately 600 million years ago(before dinosaurs).
• They are omnivores and feed on the fluid of algae, bacteria, and plants.

Tardigrade

Types of extremophiles:

    They are divided into several types, based on their adaptations. 

• Halophiles
• Alkaliphiles
• Thermophiles
• Barophiles
• Psychrophiles

Halophiles:

    Halophiles means salt-loving. They are adapted to survive in high concentrations of salt. They live in salty lakes, salt mines, and soil with high concentrations of  salt. Some common examples are Halobacterium, Halococcus, and Dunaliella salina.

Halophiles are generally divided into three groups.

• Slight halophiles: Survive in 1% to 3% salt concentration(example: Marine phytoplankton and algae).
• Moderate halophiles: Thrive in 3% to 15% salt concentration(example: Chromohalobacter salexigens).
• Extreme halophiles: Live in 15% to 30% salt concentration(example: Haloarchaea).

Adaptation:

    Halophiles thrive in salt by maintaining osmotic balance. They used two strategies.

• Salt-In strategy: In this strategy, they accumulate K⁺ and Cl⁻ in their cytoplasm. By doing this, their internal salt concentrations become equal to the environment.
• Salt-Out strategy: In this strategy, they produce compatible solutes instead of accumulating salt.

Both these strategies maintain osmotic balance.

Alkaliphiles:

    Alkaliphiles means alkali-loving. They live in high pH(almost 9-11) or alkaline environment. Mostly found in deep-sea sediments and carbonated soil. Some common examples are Oceanobacillus iheyensis, vibrio cholerae, and serpentinomonas raichei. 

There are two primary types of alkaliphiles.

• Facultative alkaliphiles: These organisms can survive under 9 pH or below. They can tolerate in low pH near to neutral(example: Bacillus pseudofirmus).
• Obligate alkaliphiles: These organisms need a higher pH(more than 9) in order to survive and multiply(example:Sporosarcina pasteurii).

Adaptation:

    Alkaliphiles have to maintain their pH at almost 7.5. For doing this, they have the following mechanisms. 

• They have modified the cell membrane with a high level of saturated fatty acids. It is rigid and less permeable, which is a strict barrier to stop hydroxide ions.
• Their enzymes are also modified to prevent denaturation. They have strong hydrogen bonding due to more negative amino acids that enhance hydrophobic interaction.
• Negatively charged functional groups on the cell membrane and cell wall attract hydrogen ions and repel hydroxide ions.

Thermophiles:

    The term "therm" is used for "heat". So, thermophiles means heat-loving. Those organisms that live in high temperatures between 41°C and 122 °C are called thermophiles. They are found in hydrothermal vents, hot springs, decomposition and volcanic sites. 

They are divided into different types based on survival requirements and growth temperature.

Types based on survival requirements:

• Obligate thermophiles: They require high temperatures for survival.
• Facultative thermophiles: They can survive in both higher and low temperature.

Types based on temperature:

• Moderate thermophiles: Live in temperatures between 50-64 °C(example: Thermus aquaticus).
• Extreme thermophiles: Thrive in temperatures between 65-79 °C(example: Pyrobaculum islandicum).
• Hyperthermophiles: live in temperatures above 80 °C(example: Pyrolobus fumarii).

Adaptation:

    Thermophiles possess special adaptations to prevent the melting of fats and lipids and also the denaturation of enzymes.

• In the cell membrane, lipids are linked by ether bonds, which are much stronger than an ester bond.
• Their enzymes contain extra ionic bonds and dense hydrophobic cores, which protect them from unfolding or denaturation.  
• Their DNA is coated with heat-stable proteins that protect from breaking the hydrogen bond in the double helix.

Barophiles:

    Those organisms that can survive in high hydrostatic pressure are called barophiles. They are also known as piezophiles. They are found in deep sea, hydrothermal vents, and ocean trenches. They found in the Mariana Trench, which is the deepest part of the ocean.

They are classified into three types.

• Barotolerant: They prefer normal conditions but can survive in high hydrostatic pressure(example: Alcanivorax borkumensis).
• Barophilic: They live at pressure of 400-500 atm(example: Shewanella benthica).
• Obligate barophiles: They require high pressure, more than 500 atm(example: Pyrococcus yayanosii) .

Adaptation:

    At high pressure cell membrane can solidify. Body fluid is balanced at optimum pressure. But barophiles have specialized techniques or structures that prevent fluid from solidifying.

• Their enzymes are rigid and contains a high proportion of specific amino acids (arginine). Prevents them from unfolding under high pressure.
• For keeping fluid, their cell membrane contains unsaturated fatty acids.

Psychrophiles:

    Those organisms that live in a cold environment permanently are called psychrophiles. They are found in polar areas like the Arctic and Antartic ocean, ice and glaciers, alpine regions, and the deep sea floor.

There are two main groups of psychrophiles.

• True psychrophiles: They live in cold environments and cannot tolerate above 20 °C(example: Psycromonas ingrahamii).
• Psychrotrophs: They can grow at freezing point but prefer moderate temperature(example: Bacillus cereus).

Adaptation:

    Psycrophiles have special adaptations to survive at freezing point. 

• They produce cryoprotectants such as trehalose and synthesize antifreeze proteins that protect from the formation of ice crystals.
• For protecting solidifying they contain unsaturated fattyacid in their cell membrane.
• Their enzymes contain a higher proportion of alpha-helices, which makes them flexible to work in cold temperatures.
• Chaperones(a specific molecule) fold and preserve proteins at sub-zero temperatures.

Conclusion:

    Extremophiles have the ability to live in harsh environments. They have a unique adaptation and specialized structure to survive. This fact leads scientists to think that life can exist in extreme conditions on other planets. These organisms live all around the world. Now scientists are more curious about knowing these animals for finding aliens.