Together they cover a wide range of metabolic types ranging from aerobes to
anaerobes and include both heterotrophs and autotrophs. The most commonly
reported microorganisms are Gram-negative α, β, and γ-proteobacteria (Pseudomonas spp. and Vibrio spp.), and the cytophaga-Flavobacterium-Bacteroides phylum.
Coryneforms, Arthrobacter sp. and Micrococcus sp. are the most frequently found
Gram-positive bacteria (D’Amico et al. 2006). Also, diverse cold tolerant
microorganisms are widely encountered in refrigerated environments and have
become a major cause of concern in the food processing and storage industry.
Cold-adapted microorganisms are known to contribute to the processes of nutrient turnover, biomass production, and litter decomposition in cold ecosystems.
There are evidences of a wide range of metabolic activities in cold habitats,
e.g. nitrogen fixation, photosynthesis, methanogenesis, and degradation of natural
or xenobiotic organic compounds such as proteins, carbohydrates, lignin, and
hydrocarbons (Cummings and Black 1999; Margesin et al. 2002; Trotsenko and
Khmelenina 2005). Metabolism remains active at subzero temperature, as microbial
DNA and protein precursor synthesis are noticed in glacial ice at À15
C (Christner
2002) and in snow at À12 to À17
C (Carpenter et al. 2000). Metabolic activity is
found in permafrost bacteria at temperatures up to À20
C (Rivkina et al. 2000).
Bacteria are capable of performing basic life functions at temperatures far below
0
C. For example, the Arctic bacterium Colwellia psychrerythraea is motile at
temperatures of À10
C and its swimming speeds are comparable at À5 and
À10
C (Junge et al. 2003). Despite all challenges, life thrives in these environments
with a remarkable microbial biodiversity.
6.3
Effect of Temperature on Growth and Metabolic Activity
Decrease in temperature causes an exponential reduction of the reaction rate and the
magnitude of which depends on the value of the activation energy. Consequently,
most biological systems display a reaction rate 2–3 times lower when the temperature is decreased by 10
C (Q 10 value). Temperatures outside the linear range of the
Arrhenius plot (log of growth rate vs the reciprocal of the absolute temperature) are
stress inducing temperatures. For psychrophiles, Arrhenius plots remain linear down
to 0
C, for psychrotolerants and mesophiles they deviate from linearity at 5–10 and
at 20
C, respectively (Gounot and Russell 1999). Optimal growth temperature is
often correlated to the maximum growth rate. The temperature, at which the growth
rate is maximum, reflects only kinetic effects and occurs above the linear part of the
Arrhenius curve, which means that the physiological conditions are not ideal
(Gounot and Russell 1999; Glansdorff and Xu 2002) and growth rate may not be
as relevant as growth yield.
Low temperature can influence the response of microorganisms either directly or
indirectly. Direct effects include decreased growth rate and enzyme activities,
alteration of cell composition, differential nutritional requirements, etc. Indirect
effects are usually observed on the solubility of solute molecules, diffusion of
nutrients, membrane osmosis, and cell density (Herbert 1986). As temperature
6 Plant Growth Promoting Rhizobacteria: Mechanisms and Alleviation of Cold Stress. . . 189
anaerobes and include both heterotrophs and autotrophs. The most commonly
reported microorganisms are Gram-negative α, β, and γ-proteobacteria (Pseudomonas spp. and Vibrio spp.), and the cytophaga-Flavobacterium-Bacteroides phylum.
Coryneforms, Arthrobacter sp. and Micrococcus sp. are the most frequently found
Gram-positive bacteria (D’Amico et al. 2006). Also, diverse cold tolerant
microorganisms are widely encountered in refrigerated environments and have
become a major cause of concern in the food processing and storage industry.
Cold-adapted microorganisms are known to contribute to the processes of nutrient turnover, biomass production, and litter decomposition in cold ecosystems.
There are evidences of a wide range of metabolic activities in cold habitats,
e.g. nitrogen fixation, photosynthesis, methanogenesis, and degradation of natural
or xenobiotic organic compounds such as proteins, carbohydrates, lignin, and
hydrocarbons (Cummings and Black 1999; Margesin et al. 2002; Trotsenko and
Khmelenina 2005). Metabolism remains active at subzero temperature, as microbial
DNA and protein precursor synthesis are noticed in glacial ice at À15
C (Christner
2002) and in snow at À12 to À17
C (Carpenter et al. 2000). Metabolic activity is
found in permafrost bacteria at temperatures up to À20
C (Rivkina et al. 2000).
Bacteria are capable of performing basic life functions at temperatures far below
0
C. For example, the Arctic bacterium Colwellia psychrerythraea is motile at
temperatures of À10
C and its swimming speeds are comparable at À5 and
À10
C (Junge et al. 2003). Despite all challenges, life thrives in these environments
with a remarkable microbial biodiversity.
6.3
Effect of Temperature on Growth and Metabolic Activity
Decrease in temperature causes an exponential reduction of the reaction rate and the
magnitude of which depends on the value of the activation energy. Consequently,
most biological systems display a reaction rate 2–3 times lower when the temperature is decreased by 10
C (Q 10 value). Temperatures outside the linear range of the
Arrhenius plot (log of growth rate vs the reciprocal of the absolute temperature) are
stress inducing temperatures. For psychrophiles, Arrhenius plots remain linear down
to 0
C, for psychrotolerants and mesophiles they deviate from linearity at 5–10 and
at 20
C, respectively (Gounot and Russell 1999). Optimal growth temperature is
often correlated to the maximum growth rate. The temperature, at which the growth
rate is maximum, reflects only kinetic effects and occurs above the linear part of the
Arrhenius curve, which means that the physiological conditions are not ideal
(Gounot and Russell 1999; Glansdorff and Xu 2002) and growth rate may not be
as relevant as growth yield.
Low temperature can influence the response of microorganisms either directly or
indirectly. Direct effects include decreased growth rate and enzyme activities,
alteration of cell composition, differential nutritional requirements, etc. Indirect
effects are usually observed on the solubility of solute molecules, diffusion of
nutrients, membrane osmosis, and cell density (Herbert 1986). As temperature
6 Plant Growth Promoting Rhizobacteria: Mechanisms and Alleviation of Cold Stress. . . 189