6.2.2: Pressure

Microbial life can exist over multiple orders of magnitude in pressure. Most species that have been tested do not grow at pressures below \(0.0025 \ \text{MPa}\), although growth has been observed for a few species at \(0.0007 \ \text{MPa}\), a pressure within the range measured on the surface of Mars (Verseux, 2020). At the high end, microorganisms capable of growing at \(100 \ \text{MPa}\) \((= 1000 \ \text{atm})\) have been isolated from water samples collected at a 10.5 km depth in the ocean (Yayanos et al., 1979) and mud samples from the Mariana Trench (Takami et al., 1997). Laboratory studies have found that cell viability decreases sharply at pressures ranging from about \(100\) to \(150 \ \text{MPa}\) (Picard and Daniel, 2013), suggesting that the upper limit for growth may be within or near that range.

Life at each extreme provides unique challenges. Low pressure can inhibit growth by causing cells to dry out (i.e., desiccate) or by limiting the supply of a necessary gas such as carbon dioxide or oxygen (Verseux, 2020). At high pressure, lipid volumes decrease, which impacts membrane fluidity and function (Pikuta et al., 2007).

Based on their pressure requirements, individual species can be described as barotolerant microorganisms, barophiles, and extreme barophiles. Barotolerant microorganisms can grow at \(0.1 \ \text{MPa}\) \((\text{~} 1000 \ \text{atm})\) but cannot grow at pressures above \(50 \ \text{MPa}\) (Madigan et al., 2003). Barophilic microorganism can also grow at \(0.1 \ \text{MPa}\) but have optimal growth at pressures greater than \(40 \ \text{MPa}\) (Madigan et al., 2003; Pikuta et al., 2007; Zobell and Johnson, 1949). Extreme or obligate barophiles require high pressure for growth. They do not grow at pressures below about \(40 \ \text{MPa}\) (Madigan et al., 2003).