adapted to life under the “unusual” harsh conditions in which they live. More than that,
their adaptation is often so extensive that they are unable to live in the more “moderate”
environments that we normally associate with life on Earth. An organism such as
Natrialba magadii that inhabits the salt-saturated and extremely alkaline Lake Magadi
(Kenya) will not grow when the pH is lowered to values below 8.5, and will lyse when
suspended in solutions that contain less than 90 g/l salt. For such an organism an
encounter with a high water activity, neutral pH environment will signify immediate
death. Similarly, hyperthermophilic Archaea that grow optimally at temperatures above
100o C will experience any temperature below 70-80o C as “freezing” conditions, under which they may be able to survive for some time, but not to actively metabolize and multiply. Many extremophiles experience “relatively normal” environments as uninhabitable and are not able to exist in milder habitats.