9: Kinetic Controls
- Page ID
- 131876
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“Chemotrophic metabolism involves catalysis of energetically possible, but kinetically hindered redox reactions.”
—Banfield and Welch (2000)
Thermodynamics determine which reactions have the potential to be useful as a source of energy for microorganisms. If those reactions proceed rapidly on their own, without any microbial catalysis, then microbial populations may not be able to capture any energy from them. However, if those reactions occur slowly without catalysis, then the opportunity exists for microorganisms to catalyze the reaction and capture energy for growth.
Microorganisms can make a living from such a reaction, avoiding a population decline, if the reaction allows them to grow new cells faster than the cells are removed from the environment. Viable cells are effectively removed when they die or when they are transported away by, for example, flowing water. The rate of biomass production is tied to the kinetics of the group’s catabolic reaction and the efficiency with which they use nutrients and energy resources. Species that can grow faster generate more biomass and are thus more likely to avoid decline. The kinetic properties of different species, even some using the same catabolic reaction, can vary widely. These properties, together with the physical and chemical conditions of the environment, help determine the proportions of species within microbial communities.
This chapter examines how microorganisms speed up redox reactions using enzymes. We will briefly consider factors that influence rates of enzymatic reactions and then examine a kinetic rate law for microbial reactions, which can quantify links between the kinetic impacts of substrate concentrations, enzyme properties, and reaction free energy yields.
- 9.1: Role of enzymes
- Effect of enzymes on reaction activation energy. Overview of how enzymes catalyze reactions.
- 9.2: Influence of pH and temperature on enzyme activity
- Effects of changes in pH and temperature on enzyme activity.
- 9.3: Influence of substrate concentrations
- Effects of variation in substrate concentration on the rates of enzymatic reactions.
- 9.4: Kinetic rate laws
- Discussion of the kinetic rate laws for microbial growth, including the Monod, dual-Monod, and Jin & Bethke rate law equations.
- 9.5: Thermodynamic constraints on rates
- Applying the thermodynamic potential factor to extend the analysis of thermodynamic controls on microbial reactions.
- 9.6: Kinetic constraints on competition between sulfate reducers and methanogens
- Competition between sulfate reducers and methanogens as an example of how differences in growth yield and half-saturation electron donation constant can influence the competition outcome. Definition and calculation of substrate thresholds.
- 9.7: External controls on microbial reaction rates
- Overview of factors external to microorganisms that can determine microbial reaction rates within an environment.