Extremophiles are organisms that have evolved to survive in environments once thought to be entirely uninhabitable. These environments are inhospitable, reaching extreme conditions of heat, acidity, pressure, and cold that would be fatal to most other life forms. Because extremophiles live on extreme ends of the spectrum, they can indicate the range of conditions under which life is possible.
One important thing to note, however, is that extremophiles are “extreme” only from an anthropocentric perspective. For example, while oxygen is indispensible to ourselves and much of life on Earth, many organisms flourish in environments without oxygen at all.
Extremophiles can be divided into two broad categories: extremophilic organisms, and extremotolerant organisms. As the suffix “philic,” translated to “loving,” suggests, extremophilic organisms require one or more extreme conditions in order to thrive, while extremotolerant organisms grow optimally at more ‘normal’ conditions but are still able to survive one or more extreme physiochemical values.
Most extremophiles are microscopic organisms belonging to a domain of life known as archaea. However, to say extremophiles are restricted to this domain would be incorrect. Some extremophiles belong in the bacteria domain, and some are even multicellular eukaryotes!
Importance in Research
The enzymes secreted by extremophiles, termed “extremozymes,” that allow them to function in such forbidding environments are of great interest to medical and biotechnical researchers. Perhaps they will be the key to creating genetically based medications, or creating technologies that can function under extreme conditions.
Astrobiologists have also taken interest in extremophiles for their remarkable resilience in freezing environments. Extremophiles, or “psychrophiles,” that are active in such environments raise the possibility of life on other planets, as the majority of bodies in the solar system are frozen. Additionally, the biochemical properties of such psychrophiles, such as the ability to use arsenic rather than phosphorus to create energy, furthers the possibility of extraterrestrial life. And, because extremophiles can indicate the range of conditions under which life is possible, they can also provide clues about how and where to look for life on other solar bodies.
Types of Extremophiles
Of course, different environmental conditions require different adaptations by the organisms that live those conditions. Extremophiles are classified according to the conditions under which they grow. Usually, however, environments are a mix of different physiochemical conditions, requiring extremophiles to adapt to multiple physiochemical parameters. Extremophiles found in such conditions are termed “polyextremophiles.”
Acidophiles are adapted to conditions with acidic pH values that range from 1 to 5. This group includes some eukaryotes, bacteria, and archaea that are found in places like sulfuric pools, areas polluted by acidic mine drainage, and even our own stomachs!
Acidophiles regulate their pH levels through a variety of specialized mechanisms— some of which are passive (not exerting energy), and some of which are active (exerting energy). Passive mechanisms usually involve reinforcing the cell membrane against the external environment, and may involve secreting a biofilm to hinder the diffusion of molecules into the cell, or changing their cell membrane entirely to incorporate protective substances and fatty acids. Some acidophiles can secrete buffer molecules to help raise their internal pH levels. Active pH regulation mechanisms involve a hydrogen ion pump that expels hydrogen ions from the cell at a constantly high rate.
Alkaliphiles are adapted to conditions with basic pH values of 9 or higher. They maintain homeostasis by both passive and active mechanisms. Passive mechanisms include pooling cytoplasmic polyamines inside the cell. The polyamines are rich with positively charged amino groups that buffer the cytoplasm in alkaline environments. Another passive mechanism is having a low membrane permeability, which hinders the movement of protons in and out of the cell. The active method of regulation involves a sodium ion channel that carries protons into the cell.
Thermophiles thrive in extremely high temperatures between 113 and 251 degrees Fahrenheit. They can be found in places like hydrothermal vents, volcanic sediments, and hot springs. Their survival in such places can be accredited to their extremozymes. The amino acids of these types of enzymes do not lose their shape and misfold in extreme heat, allowing for continued proper function.
Psychrophiles (also known as Cryophiles) thrive in extremely low temperatures of 5 degrees Fahrenheit or lower. This group belongs to all three domains of life (bacteria, archaea, and eukarya), and they can be found in places like cold soils, permafrost, polar ice, cold ocean water, and alpine snow packs.
One way they survive in extreme cold can be attributed to their extremozymes, which continue to function at low temperatures, and a little more slowly at even lower temperatures. Psychrophiles are also able to produce proteins that are functional in cold temperatures, and contain large amounts of unsaturated fatty acids in their plasma membranes that help buffer the cells from the cold. Most notably, however, some psychrophiles are able to replace the water in their bodies with the sugar trehalose, preventing the formation of harmful ice-crystals.
Xerophiles grow in extremely dry conditions which can be very hot or very cold. They have been found in places like the Atacama Desert, the Great Basin, and the Antarctic. Like their psychrophilic friends, some xerophiles have the ability to replace water with trehalose, which can also protect membranes and other structures from periods with low water availability.
Barophiles are organisms that grow best under high pressures of 400 atm or more. They can survive by regulating the fluidity of the phospholipids in the membrane. This fluidity compensates for the pressure gradient between the inside and outside of the cell, and the external environment. Extreme barophiles grow optimally at 700 atm or higher, and will not grow at lower pressures.
Halophiles are organisms that require high salt concentrations for growth. At salinities exceeding 1.5M, prokaryotic bacteria are predominant. Still, this group belongs to all three domains of life, but in smaller numbers.
Overcoming the challenges of hypersaline environments starts with minimizing cellular water loss. Halophiles do this by accumulating solutes in the cytoplasm via varying mechanisms. Halophilic archaea use a sodium-potassium ion pump to expel sodium and intake potassium. Halotolerant bacteria balance the osmotic pressure by using glycerol as compatible solutes.
Examples of Extremophiles
Also known as a “snoticle,” snottites are made up of colonies of cave-dwelling, extremophilic, single-cell bacteria. These colonies look similar to stalactites, but have the consistency of, well, snot. These bacteria colonies survive extreme toxicity and acidity, among other extreme physiochemical conditions. They survive by using chemosynthesis to turn volcanic sulfur compounds into energy and sulfuric acid waste.
Giant Tube Worms
The giant tube worm is a deep-sea extremophile found near hydrothermal vents living in conditions of high pressure, high heat, and no sunlight. Waters near hydrothermal vents can reach temperatures of 600 degrees Fahrenheit, and the pressure can reach up to nearly 9,000 psi! With no digestive tract of their own, they survive in such conditions with the help of their symbiotic partners: extremophilic bacteria that live in the midgut of the giant tube worm. The bacteria, which may account for up to half of the worm’s weight, use chemosynthesis to turn oxygen, hydrogen sulfide, and carbon dioxide into organic molecules that the worm can use as food.
Technically more extremotolerant than extremophilic, these eight-legged microscopic creatures are one of the most resilient organisms known to man. They have two survival strategies: one in case of flooding, and one in case of freezing or drought. In the occurrence of a flood, tardigrades inflate themselves like balloons, allowing them to float up to the surface where they have access to oxygen. In the case of drought or freezing conditions, tardigrades have the remarkable ability to replace more than 97% of the water in their bodies with a type of sugar called trehalose. This reduces the need for water and prevents ice crystals from forming that would otherwise form with water and harm these organisms. Using these survival techniques, these creatures have survived temperatures from -458 degrees Fahrenheit to 300 degrees Fahrenheit, pressures six times greater than that found in the deepest parts of the ocean, lethal doses of radiation, and even the vacuum of space! Still, the longer tardigrades stay in non-optimal conditions, the lower their chances survival.
These microscopic organisms were first collected from the depths of a Mediterranean Sea basin where the salt-saturated brine they inhabit does not mix with or get watered down by the waters above it. They inhabited the marine sediment, thriving in this salty, sulphidic, freezing, highly pressurized environment without oxygen or light. This is possible because, unlike us, Loricifera have hydrogenosomes that require no oxygen, instead of mitochondria, to produce energy!
Grylloblattidae is a family of psychrophilic insects that are found in cold environments like mountain tops, glaciers, and ice sheets. They prefer temperatures between 33.8 and 39.2 degrees Fahrenheit—just above freezing. When temperatures drop below freezing, these insects burrow down through the snow and stay near the soil—otherwise, they risk death by ice-crystals forming in their bodies.