ABSTRACT
There is hardly any place on our planet, however extreme, that is not inhabited by at least some types of living organisms adapted to the environment. Thus, the physico-chemical boundaries for life on Earth are very broad. One can almost state that any place where a minimal amount of liquid water is available can support life (Mazur 1980; Madigan 2000; Xu and Glansdorff 2007; Stan-Lotter 2012). We find microbial life in very cold and very hot environments, even at temperatures exceeding 100°C in undersea springs, where the boiling point of water is elevated because of the hydrostatic pressure. We know microorganisms adapted to life in concentrated acids and in highly alkaline lakes. Life is possible in salt-saturated brines, under high levels of ionizing radiation and under pressure in the deepest parts of the ocean. Microorganisms even live in deserts, hot as well as cold, thriving on very small amounts of water that become available occasionally. The lowest water activity that still allows active growth of microorganisms appears to be around 0.63–0.64 (Stevenson et al. 2015), but many types can survive prolonged periods of desiccation until conditions again become suitable for growth.
