UPSC Prelims 2023 Q18: Extremophile Microorganisms — Explained
Consider the following statements:
1. Some microorganisms can grow in environments with temperature above the boiling point of water.
2. Some microorganisms can grow in environments with temperature below the freezing point of water.
3. Some microorganisms can grow in highly acidic environment with a pH below 3.
How many of the above statements are correct?
- Only one
- Only two
- All three
- None
Answer: (c) All three
Statement-wise explanation
- 1: correct — Water boils at 100 °C only at sea-level pressure. At deep-sea hydrothermal vents the high pressure keeps water liquid well above 100 °C. Microbes that grow there are called hyperthermophiles. The archaeon Methanopyrus kandleri strain 116, from the Kairei hydrothermal field on the Central Indian Ridge, multiplied at 122 °C under 40 MPa pressure (Takai et al., PNAS, 2008), the highest growth temperature on record.
- 2: correct — Psychrophiles ('cold-loving' microbes) grow below 0 °C. Inside permafrost and sea ice, salty brine stays liquid below that temperature. Planococcus halocryophilus Or1, a bacterium from Canadian high-Arctic permafrost, was shown to grow and divide at −15 °C and to stay metabolically active at −25 °C (ISME Journal, 2013).
- 3: correct — Acidophiles grow best at low pH. Many of them, such as the iron- and sulphur-oxidising bacteria in acid mine drainage, live comfortably below pH 3. The archaeon Picrophilus torridus grows best at about pH 0.7, grows at pH values near 0, and its cells burst above pH 4. That makes it the most acid-loving organism described so far.
How to eliminate
Every statement says 'some microorganisms'. To make it true you need just one example, so 'some' works the opposite way to absolute words like 'all' or 'only'. In science questions about what life can tolerate, a statement saying 'some organism can do X' is usually true, because microbes have been found in almost every extreme habitat on Earth. Basic knowledge settles two statements. Deep-sea vents and hot springs are well-known homes of heat-loving microbes, and the Hindu Kush–Himalaya and polar regions host cold-loving ones. Statement 3 is safe once you remember that acid mine drainage and volcanic springs often have pH below 2–3, and microbes such as Acidithiobacillus are used to leach copper from ore in such conditions. Once all three look true, 'None' and 'Only one' are out. The facts to remember: life has been recorded growing at 122 °C (Methanopyrus kandleri), at −15 °C (Planococcus halocryophilus) and at about pH 0 (Picrophilus).
Know more
Extremophiles are organisms that need, or tolerate, conditions that would kill most life: extreme heat, cold, acidity, alkalinity, salt, pressure or radiation. Most are archaea or bacteria. Many are polyextremophiles, meaning they handle more than one extreme at once. Picrophilus, for example, lives at about 60 °C and about pH 0.7.
| Type | Extreme | Example | Record / range |
|---|---|---|---|
| Hyperthermophile | Heat (best growth above 80 °C) | Methanopyrus kandleri 116 | Grows at 122 °C under high pressure |
| Psychrophile | Cold | Planococcus halocryophilus | Divides at −15 °C |
| Acidophile | Low pH | Picrophilus torridus | Best growth at about pH 0.7 |
| Alkaliphile | High pH | Soda-lake bacteria | pH 9–11 |
| Halophile | Salt | Halobacterium | Saturated brine |
| Piezophile | Pressure | Deep-trench microbes | Ocean floor |
| Radioresistant | Radiation | Deinococcus radiodurans | Survives very high doses |
How they survive
- Heat: proteins with tighter folding, and archaeal membranes built from ether-linked lipids that do not melt.
- Cold: more flexible enzymes, more unsaturated fats in the membrane, and antifreeze (cryoprotectant) compounds.
- Acid: the cell pumps protons out so its inside stays close to neutral.
Complex cells have a lower ceiling. Eukaryotes such as amoebae were long thought to stop growing at about 60 °C. In 2026, Incendiamoeba cascadensis from a Californian hot spring was shown to reproduce at 63 °C. That is still far below the 122 °C prokaryote record.
Why UPSC cares: applications
- Taq polymerase, a heat-stable enzyme from Thermus aquaticus, made PCR testing possible.
- Cold-active enzymes are used in detergents that wash at low temperatures.
- Acidophiles such as Acidithiobacillus are used in bioleaching of copper and gold.
- Astrobiology: extremophiles show what kind of life might exist on Mars or on icy moons such as Enceladus.