Effect of Environment on Growth Kinetics- Temperature

Growth kinetics can be affected by three main environmental factors:

1. Temperature
2. pH
3. Dissolved oxygen concentration

Temperature

    According to temperature optima (T_opt), organisms are classified into three types:

•     Psychrophiles (T_opt < 20°C)
•     Mesophiles (T_opt  50°C-80°C)
•     Thermophiles (T_opt > 50°C)

 Figure 1: Schematic representation of effect of Temperature in growth

      Now, As the temperature increase towards this optimal growth temperature, the growth rate actually doubles for every 10 degree Celsius. 

Please look into the figure above. Consider the Car is the cell and as it moves towards the optimal growth temperature, its growth rate i.e µg doubles for every 10 degree Celsius increase in temperature. 

but, beyond this optimal temperature, µg drops slowly and finally thermal death occurs. So What could be the net replication rate for temperature above optimal level?

First the net replication rate in the absence of cell death i.e kd is said to be 

N (µ’R) = dN/dt

Now thermal death occurs above the optimal growth temperature and that could be explained by another equation. i.e

N (-k'd) =dN/dt

Combining both these equation, we get 

dN/dt = (µ’R - k'd) N

So, at higher temperature the thermal death rate exceeds the growth rate, which causes a net decrease in the concentration of viable cells. 
    
 Both the net replication rate constant and death rate constant vary with temperature which is according to Arrhenius Equation. 

µ’R = Ae^(-Ea/RT)
k’d = A'e^(-Ed/RT)

Ea and Ed are the activation energies for growth and thermal death.
For growth Ea is 10-20 kcal/mol and thermal death Ed is 60-80 kcal/mol. So, Thermal death is more sensitive to temperature changes than microbial growth.

     Temperature affects four things:

   Why maintenance coefficient increase with increase in temperature?

   

   As the temperature increases, the proteins and cell organelles starts to ware out and in order to replace the losses, the cell utilizes more of the substrate. 

      

    Maintenance Coefficient is the specific rate of substrate uptake for cellular maintenance. And increase in 'm' happens with activation energy range between 15-20 kcal/mol, which results in decrease in the yield coefficient. 

 
 
 
 

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