Một sinh vật ưa cực (tiếng Anh: "extremophile", từ extremus tiếng Latinh nghĩa là "cực hạn" và philiā (φιλία) tiếng Hy Lạp nghĩa là "yêu") là một sinh vật sinh trưởng trong điều kiện vật lý hay địa hóa học mà khắc nghiệt đến nỗi có thể dễ dàng gây hại cho đa phần sự sống trên Trái Đất.^[1][2]

• Sinh vật ưa axit: sinh vật có khả năng phát triển tối ưu ở mức pH 3.0 hoặc thấp hơn.

• Sinh vật ưa kiềm: sinh vật có khả năng phát triển tối ưu ở mức pH 9.0 hoặc hơn.

• Capnophile: sinh vật phát triển mạnh trong môi trường có nồng độ Carbon dioxide cao. Một ví dụ là Mannheimia succiniciproducens, một loại vi khuẩn sống trong hệ tiêu hóa của động vật nhai lại.^[3]

• Halophile: an organism with optimal growth at a concentration of dissolved salts of 50 g/L (= 5% m/v) or above (for comparison, the ocean salinity is about 35 g/L (= 3.5% m/v)).

• Hyperpiezophile: an organism with optimal growth at hydrostatic pressures above 50 MPa (= 493 atm = 7,252 psi).

• Hyperthermophile: an organism with optimal growth at temperatures above 80 °C (176 °F).

• Metallotolerant: an organism capable of tolerating high levels of dissolved heavy metals in solution, such as copper, cadmium, arsenic, and zinc. Examples include Ferroplasma sp., Cupriavidus metallidurans and GFAJ-1.^[4][5][6]

• Oligotroph: an organism with optimal growth in nutritionally limited environments.

• Osmophile: an organism with optimal growth in environments with a high sugar concentration.

• Piezophile or barophile: an organism with optimal growth in hydrostatic pressures above 10 MPa (= 99 atm = 1,450 psi).

• Polyextremophile (mixed Latin/Greek compound for affection for many extremes): not a well-defined category itself – an organism that qualifies as an extremophile under more than one category.

• Psychrophile or cryophile: an organism with optimal growth at temperatures of 15 °C (59 °F) or lower.

• Radioresistant: organisms resistant to high levels of ionizing radiation, most commonly ultraviolet radiation. This category also includes organisms capable of resisting nuclear radiation, notably gamma rays.

• Sulphophile: an organism with optimal growth conditions in high concentrations of sulfur. An example would be Sulfurovum epsilonproteobacteria, a sulfur-oxidizing bacteria that inhabits deep-water sulfur vents.^[7]

• Thermophile: an organism with optimal growth at temperatures above 45 °C (113 °F).

• Xerophile: an organism with optimal growth at water activity below 0.8.

There are many classes of extremophiles that range all around the globe; each corresponding to the way its environmental niche differs from mesophilic conditions. These classifications are not exclusive. Many extremophiles fall under multiple categories and are classified as polyextremophiles. For example, organisms living inside hot rocks deep under Earth's surface are thermophilic and piezophilic such as Thermococcus barophilus.^[11] A polyextremophile living at the summit of a mountain in the Atacama Desert might be a radioresistant xerophile, a psychrophile, and an oligotroph. Polyextremophiles are well known for their ability to tolerate both high and low pH levels.^[12] Note that "tolerant" or "resistant" organisms are not necessarily extremophiles: tolerant or resistant organisms may survive despite harsh conditions instead of thriving in harsh conditions. For example, the tardigrade (Tardigrada spp.), despite being highly resistant to many stresses, is not an extremophile properly speaking.^[13]

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