UC Riverside

There are few places on Earth that are truly aseptic. Even environments that we may consider 'extreme', such as glaciers, deserts, or hypersaline bodies of water (Figure 1), can harbour life. The organisms that thrive in such environments - mostly microbes - are often referred to as 'extremophiles'. However, what constitutes extreme is in the eye of the beholder. Extremophilic organisms are so adapted to their environment that they perceive extreme conditions as optimal for their growth and can sometimes even be stressed by what we perceive as moderate. Stress is therefore not an optimal criterion for defining what is extreme. Instead, extreme conditions can be seen as those in which the majority of species cannot grow or even survive.