6.2
Definition and Ecological Diversity of Cold Tolerant
Microorganisms
Cold tolerant microorganisms exhibit distinctly different properties than
representatives of mesophiles and thermophiles. According to Morita (1975)
microorganisms with low temperature optima are generally referred to as
psychrophiles (cold loving). He defined cold-adapted bacteria based on their cardinal
growth temperature, viz. lower limit, optimum and upper limit (Russell 2006). A true
psychrophile can be defined as an organism whose optimal growth temperature is
below 20
C and which is capable to grow at 0
C or below. On the other hand,
microorganisms which can grow at 0
C or at subzero temperature but have optimum
growth at 20–30
C are referred to as psychrotolerant (also named as cold tolerant or
psychrotrophic). However, any such classification is artificial and individual coldadapted microorganisms may not fit into these definitions. Psychrotolerants may
grow as fast as psychrophiles at low temperatures. The main difference between the
two groups is that psychrotolerants have a much broader growth temperature range
(30–40
C) comparable to psychrophiles (~20
C) (Russell 2006).
The new terms “eurypsychrophile” and “stenopsychrophile” have been proposed
to substitute psychrophile and psychrotolerant, respectively (Cavicchioli 2006).
“Steno-” and “eury-” are ecological terms derived from Shelford’s law of tolerance
that respectively describes a narrow and wide tolerance to an environmental determinant. The term stenopsychrophile (“true psychrophile”) describes microorganisms
with a restricted growth temperature range that cannot tolerate higher temperature
for growth. Eurypsychrophile (“psychrotolerant”/cold tolerant or “psychrotrophic”)
describes microorganisms that like permanently cold environment, but that can
tolerate a wide range of temperature extending into the mesophilic range and are,
therefore, literally “mesotolerant,” not “psychrotolerant.” In general, the term
psychrophile describes microorganisms that grow in cold environment. It is noteworthy that the term “trophic” pertains to a nutritional state and is not a useful term
for clarifying the temperature that can tolerate by any organisms (Cavicchioli 2006).
Psychrotolerant microorganisms are much more widely distributed than true
psychrophiles. They persist in permanently cold habitats such as polar regions,
high altitude areas or regions characterized with low winter temperatures. Diverse
microorganisms have remained viable within glacial ice cores for over 120,000 years
(Miteva et al. 2004).
The lowest temperature limit for life seems to be around À20
C, which is the
value reported for bacteria living in permafrost soil and in sea ice. During the past
two decades, microbial life has been extensively investigated in frozen natural
habitats (snow, glacial and sea ice, permafrost area, ice clouds, etc.) due to the
increasing interest in life on distance frozen planets (astrobiology) (Price 2004).
Bacteria, archaea, and eukaryotes like yeast occur in cold environments. While
bacteria dominate and are present in greater diversity than archaea in polar
environments, archaea are widespread in cold and deep ocean waters (Karner et al.
2001; Deming 2002). Diverse morphological types of microbes encountered in cold
environments include spore-formers, non-spore formers, and filamentous bacteria.
188
P. K. Mishra et al.
Definition and Ecological Diversity of Cold Tolerant
Microorganisms
Cold tolerant microorganisms exhibit distinctly different properties than
representatives of mesophiles and thermophiles. According to Morita (1975)
microorganisms with low temperature optima are generally referred to as
psychrophiles (cold loving). He defined cold-adapted bacteria based on their cardinal
growth temperature, viz. lower limit, optimum and upper limit (Russell 2006). A true
psychrophile can be defined as an organism whose optimal growth temperature is
below 20
C and which is capable to grow at 0
C or below. On the other hand,
microorganisms which can grow at 0
C or at subzero temperature but have optimum
growth at 20–30
C are referred to as psychrotolerant (also named as cold tolerant or
psychrotrophic). However, any such classification is artificial and individual coldadapted microorganisms may not fit into these definitions. Psychrotolerants may
grow as fast as psychrophiles at low temperatures. The main difference between the
two groups is that psychrotolerants have a much broader growth temperature range
(30–40
C) comparable to psychrophiles (~20
C) (Russell 2006).
The new terms “eurypsychrophile” and “stenopsychrophile” have been proposed
to substitute psychrophile and psychrotolerant, respectively (Cavicchioli 2006).
“Steno-” and “eury-” are ecological terms derived from Shelford’s law of tolerance
that respectively describes a narrow and wide tolerance to an environmental determinant. The term stenopsychrophile (“true psychrophile”) describes microorganisms
with a restricted growth temperature range that cannot tolerate higher temperature
for growth. Eurypsychrophile (“psychrotolerant”/cold tolerant or “psychrotrophic”)
describes microorganisms that like permanently cold environment, but that can
tolerate a wide range of temperature extending into the mesophilic range and are,
therefore, literally “mesotolerant,” not “psychrotolerant.” In general, the term
psychrophile describes microorganisms that grow in cold environment. It is noteworthy that the term “trophic” pertains to a nutritional state and is not a useful term
for clarifying the temperature that can tolerate by any organisms (Cavicchioli 2006).
Psychrotolerant microorganisms are much more widely distributed than true
psychrophiles. They persist in permanently cold habitats such as polar regions,
high altitude areas or regions characterized with low winter temperatures. Diverse
microorganisms have remained viable within glacial ice cores for over 120,000 years
(Miteva et al. 2004).
The lowest temperature limit for life seems to be around À20
C, which is the
value reported for bacteria living in permafrost soil and in sea ice. During the past
two decades, microbial life has been extensively investigated in frozen natural
habitats (snow, glacial and sea ice, permafrost area, ice clouds, etc.) due to the
increasing interest in life on distance frozen planets (astrobiology) (Price 2004).
Bacteria, archaea, and eukaryotes like yeast occur in cold environments. While
bacteria dominate and are present in greater diversity than archaea in polar
environments, archaea are widespread in cold and deep ocean waters (Karner et al.
2001; Deming 2002). Diverse morphological types of microbes encountered in cold
environments include spore-formers, non-spore formers, and filamentous bacteria.
188
P. K. Mishra et al.