consist of microorganisms that are capable of surviving and thriving in harsh
environments and conditions that are detrimental to the majority of life on earth.
Extremophiles have been isolated in diverse zones that possess extreme conditions.
Life in extreme environments have been studied intensively focussing on molecular
mechanisms involved as well as the diversity of organisms. Moderate environments
are important to sustain life which means environments with temperature between
20 and 40
C, air pressure about one atmosphere, pH near neutral and adequate
levels of available nutrients water and salts. The presently known upper limit is
50
C for multicellular eukaryotes, 62
C for single-celled eukaryotes, 95
C for
bacteria and 121
C for archaea. Many extreme environments such as saline and/or
alkaline lake/ponds, acidic or hot springs, deserts and the ocean beds are found in
nature on the earth which are too harsh for normal life to exist. These extremophilic
organisms not only tolerate specific extreme conditions but also require these for
growth and survival. Many species can survive but are unable to reproduce or grow
indefinitely under such conditions. Extreme environments include high pH, temperature, pressure, salt concentration, low temperature, pH, nutrients concentration,
water availability, harmful heavy metals, toxic compounds and high levels of
radiation.
Extremophiles are categorized according to conditions in which they grow. There
are many terms used to describe extremophiles as shown in Table 12.1. Thermophiles/hyperthermophiles grow in habitats with high or very temperatures such as
volcanic sites, hydrothermal vents, hot springs; psychrophiles thrive in cold habitats
such as on the mountains at high altitude, polar region; barophiles which love high
pressure conditions which are mainly found deep inside the oceans and sea; halophiles love very high salt concentrations such as in saline alkaline lakes, sea;
alkalophiles thrive at highly alkaline pH such as sodic lakes; acidophiles grow at
habitats with pH less than 5, such conditions are found in acid mine drainage sites
and acidic lakes; metallophiles can tolerate and grow in the presence of high
concentration of heavy metals; xerophiles can grow in conditions with very low
water availability which include deserts; anoxiphiles are the organisms having
colonized ecosystems deprived of oxygen. Multiple stresses are present in the
niche simultaneously and extremophiles which are able to thrive in such habitats
are known as polyextremophiles (e.g., thermoacidophiles, haloalkaliphiles).
Polyextremophiles organisms are adapted to live in habitats where various physicochemical parameters reach extreme values. For example, many hot springs are acid
or alkaline with rich metal content at the same time. Similarly, the deep ocean is
generally cold, very low nutrient content (oligotrophic) and exposed to high pressure. Haloalkalophilic Halomonas campisalis which can grow at pH up to 12 has
been reported from Soap Lake, USA. Recently, the most acidophilic microorganisms
such as Picrophilus oshimae and Picrophilus torridus, which can grow at pH as low
as 0.06, have been discovered from hot spring in Noboribetsu, Japan. The most
halophilic microbe Halarsenatibacter silvermanii has been discovered from a salt
lake in USA, which can survive in salt concentration of about 35%. Microorganisms
such as Methanothermococcus thermolithotrophicus and Methanocaldococcus
jannaschii are examples of barophilic and thermophilic methanogens, which have
been isolated from high pressure niches of deep sea beds.
298
S. Kaushik et al.
environments and conditions that are detrimental to the majority of life on earth.
Extremophiles have been isolated in diverse zones that possess extreme conditions.
Life in extreme environments have been studied intensively focussing on molecular
mechanisms involved as well as the diversity of organisms. Moderate environments
are important to sustain life which means environments with temperature between
20 and 40
C, air pressure about one atmosphere, pH near neutral and adequate
levels of available nutrients water and salts. The presently known upper limit is
50
C for multicellular eukaryotes, 62
C for single-celled eukaryotes, 95
C for
bacteria and 121
C for archaea. Many extreme environments such as saline and/or
alkaline lake/ponds, acidic or hot springs, deserts and the ocean beds are found in
nature on the earth which are too harsh for normal life to exist. These extremophilic
organisms not only tolerate specific extreme conditions but also require these for
growth and survival. Many species can survive but are unable to reproduce or grow
indefinitely under such conditions. Extreme environments include high pH, temperature, pressure, salt concentration, low temperature, pH, nutrients concentration,
water availability, harmful heavy metals, toxic compounds and high levels of
radiation.
Extremophiles are categorized according to conditions in which they grow. There
are many terms used to describe extremophiles as shown in Table 12.1. Thermophiles/hyperthermophiles grow in habitats with high or very temperatures such as
volcanic sites, hydrothermal vents, hot springs; psychrophiles thrive in cold habitats
such as on the mountains at high altitude, polar region; barophiles which love high
pressure conditions which are mainly found deep inside the oceans and sea; halophiles love very high salt concentrations such as in saline alkaline lakes, sea;
alkalophiles thrive at highly alkaline pH such as sodic lakes; acidophiles grow at
habitats with pH less than 5, such conditions are found in acid mine drainage sites
and acidic lakes; metallophiles can tolerate and grow in the presence of high
concentration of heavy metals; xerophiles can grow in conditions with very low
water availability which include deserts; anoxiphiles are the organisms having
colonized ecosystems deprived of oxygen. Multiple stresses are present in the
niche simultaneously and extremophiles which are able to thrive in such habitats
are known as polyextremophiles (e.g., thermoacidophiles, haloalkaliphiles).
Polyextremophiles organisms are adapted to live in habitats where various physicochemical parameters reach extreme values. For example, many hot springs are acid
or alkaline with rich metal content at the same time. Similarly, the deep ocean is
generally cold, very low nutrient content (oligotrophic) and exposed to high pressure. Haloalkalophilic Halomonas campisalis which can grow at pH up to 12 has
been reported from Soap Lake, USA. Recently, the most acidophilic microorganisms
such as Picrophilus oshimae and Picrophilus torridus, which can grow at pH as low
as 0.06, have been discovered from hot spring in Noboribetsu, Japan. The most
halophilic microbe Halarsenatibacter silvermanii has been discovered from a salt
lake in USA, which can survive in salt concentration of about 35%. Microorganisms
such as Methanothermococcus thermolithotrophicus and Methanocaldococcus
jannaschii are examples of barophilic and thermophilic methanogens, which have
been isolated from high pressure niches of deep sea beds.
298
S. Kaushik et al.