Submarine hydrothermal vent environments span the temperature range from 2°C (36°F) for ambient deep seawater to over 400°C (750°F) for some hydrothermal fluids. These high-temperature fluids remain liquid because of the hydrostatic pressure at deep-sea vent depths (2–4 km or 1–2.5 mi). The microbial world at hydrothermal vents includes organisms capable of growing at 2°C (36°F) or lower to above the boiling point of water (100°C or 212°F at atmospheric pressure). The ability to grow above 60°C (140°F) is confined to prokaryotes (cells that lack a nucleus) divided into two groups: moderate thermophiles that have maximal growth temperatures of 50–60°C (122–140°F), and hyperthermophiles that grow optimally at 80°C (176°F) or higher with maximal growth temperatures of 90°C (194°F) or higher. The highest temperature for growth by a pure culture (Pyrolobus fumaris) is 113°C (235°F), although there is some evidence that deep-sea vent hyperthermophiles may grow and survive at higher temperatures under elevated pressures.