The Effect of Temperature per se

The temperature per se is related to kinetic energy of reactants. The cooling should progressively slow-down metabolic reactions but cannot stop them completely, due to the exponential nature of energy distribution (i.e., it approaches zero when temperature is approaching 0 K). In the real processes, we can't vary incubation temperature without affecting soil water content, but we can do it mathematically by using multiple regression. Figure 9.8 shows temperature-dependent changes in metabolic activity (respiration and DF) and available water (UW and air humidity). These data were fitted to multiple non-linear regression of metabolic activity (v) on two factors, temperature, Celsius (T) and available water content (W). The best results were obtained with double exponential regression:

where A, l and k are kinetic constants.

The empirical exponential term exp(lT) could be replaced with the more meaningful Arrhenius term containing the temperature in Kelvin (K) and the parameter Ea (energy of activation):

Si 30

I 10

-100

Si 30

-100

-100

Temperature ("C)

Fig. 9.7 Dependence of unfrozen water content on ambient temperature in permafrost. Top: The calculated unfrozen water content [continuous line, (9.1)] plotted vs experimental data points (Romanovsky and Osterkamp 2000) for Sagwon site, AL (□). Bottom: The relationship between unfrozen water content (o) and relative humidity of air over frozen soil (dotted line)

-100

Temperature ("C)

Fig. 9.7 Dependence of unfrozen water content on ambient temperature in permafrost. Top: The calculated unfrozen water content [continuous line, (9.1)] plotted vs experimental data points (Romanovsky and Osterkamp 2000) for Sagwon site, AL (□). Bottom: The relationship between unfrozen water content (o) and relative humidity of air over frozen soil (dotted line)

v = A x exp(kW) x exp

(-E >

= A x exp

/

E 1

a

kW -

a

1RK)

V

The agreement between (9.2) or (9.3) and experimental points was good enough to carry out a separate account of factors T and W. We prefer to use the Celsius temperature (equation 9.2) rather than K, since this is more common in biological literature. The influence of T alone was expressed by the parameter X which is related to the traditional parameter Q10, which explains how many times the reaction rate is accelerated per every l0 degrees of temperature shift-up: Q10= exp(10X).

0.0001

0.0001

10000

7

1000 -

(g soil

100 -

T3

10

C

C

ta

0.1

C

0.0001

Temperature (°C)

Fig. 9.8 The effect of below-freezing temperature on available water and microbial activity. Top (Panikov et al. 2006): the rate of 14CO2 production from the 14C-glucose (•) added to Barrow (AL) soil, the total rate of CO2 evolution (o) and unfrozen water content (dotted line) determined in the field by Romanovsky and Osterkamp (2000). Bottom: the rate of 14CO2 uptake (•) and water vapor concentration (dotted line) during Sagwon soil laboratory incubation (Panikov and Sizova 2007). Continuous solid lines were calculated from equation 9.2 with the following parameters: X= 0.078, k = 1.65 (CO2 evolution); X = 0.135, k = 1.77 (14C-glucose oxidation); X = 0.063, k = 1.36 (DF)

A cooling by 10 degrees caused a 1.9-fold decrease in DF and a 2.1-3.8-fold decrease in respiration, which is very close to the "typical" Q10 value of 2-3 observed in a majority of above-zero biological processes. Therefore, the effect of temperature alone remains the same above and below the freezing point of water, and the experimentally observed steep decline in metabolic activity below the freezing point should be caused by the abrupt decrease in availability of water, not by temperature.

Surprisingly two metabolic processes, 14C-glucose oxidation and respiration in Barrow (Fig. 9.8, top) and DF in Sagwon soil (Fig. 9.8, bottom), correlated with different forms of available water: the first was more closely related to UW, while the second one correlated better with air humidity. Further studies are needed to clarify this discrepancy and decide whether the source of the samples or the type of metabolic processes is more important.

What is the lower temperature limit for microbial growth and activity? We still do not have a clear answer, and there is a wide range of opinions from extreme skepticism denying metabolic activity at -10°C (Warren and Hudson 2003) to the overoptimistic statement that "there is no evidence of a minimum temperature for metabolism" (Price and Sowers 2004). We have found that even at the lowest tested temperature of -40°C the rate of 14CO2 incorporation exceeded the background level of the killed control. Moreover, the entire "activity-temperature" plot was smooth and continuous, indicating a progressive decline with cooling below the freezing point rather than some threshold. Therefore, we are inclined to support the opinion expressed by Price and Sowers (2004) with the truistic reminder that any processes should stop completely before approaching 0 K, metabolic reactions being no exception. In future, it would be important to find out whether at temperatures approaching 0K (-273°C) its effect on microbial activity would deviate from equations 9.2 or 9.3. Such deviation would indicate an existence of the minimal temperature or implication of factors other than temperature and available water content. Note that testing of temperature effects below -40°C would require a rather expensive experimental setup, longer incubation times and highly sensitive analytical instruments.

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