The study of interrelationships between microorganisms and their living and nonliving environments. Microbial populations are able to tolerate and to grow under varying environmental conditions, including habitats with extreme environmental conditions such as hot springs, salt lakes, and deep-sea thermal vents. Some microorganisms, referred to as extremophiles, grow only under conditions usually considered hostile to life. The archaebacteria (Archaea) have many extremophiles that grow at high temperatures, high salinities, and low pH. Understanding the environmental factors controlling microbial growth and survival offers insight into the distribution of microorganisms in nature, and many studies in microbial ecology are concerned with examining the adaptive features that permit particular microbial species to function in particular habitats.