Textbook in PDF format Recent studies on extremophiles have focused on thermophiles, microbes that thrive in high temperatures, while psychrophiles, which prefer cold environments, have received less attention. However, interest in cold-adapted microbes, especially those in polar regions, has increased significantly in recent decades. Given that much of Earth’s biosphere remains below 5°C, psychrophiles likely outnumber thermophiles. Oceans, covering over 75% of the planet, maintain deep water temperatures between 2–4°C. Antarctica offers both terrestrial and aquatic cold habitats, including ice that melts seasonally. Other cold ecosystems include permafrost, alpine soils, cold deserts, marine sediments, glaciers, and sea ice