Respiration in Microorganisms: Chemolithotrophy & Anaerobic Respiration

Respiration in Microorganisms: Chemolithotrophy & Anaerobic Respiration

Introduction

This study guide covers the diversity of catabolic strategies in microorganisms, focusing on chemolithotrophy and anaerobic respiration. It explains key concepts such as redox chemistry, electron donors and acceptors, and the use of the redox tower to predict metabolic feasibility and energy yield.

Environments and Microenvironments

Chemical Gradients

• Chemical gradients arise in environments due to the interplay of biogeochemical processes and diffusion rates.

• The greater the difference between the process rate and the diffusion rate, the steeper the gradient.

• Key zones include the sediment/water interface and the oxic/anoxic transition zone (OATZ), where chemical and microbial processes change rapidly.

• These gradients are crucial for understanding microbial metabolism in natural environments.

Learning Objectives

• Understand the diversity of catabolic strategies and terminology.

• Review redox chemistry and its role in respiration.

• Define electron donor and terminal electron acceptor.

• Use the redox tower to determine feasible and most exergonic respirations.

• Comprehend the basics of anaerobic respiration and chemolithotrophy.

Physiological Groups of Life

Energy Sources and Classification

• Organisms are classified by their energy source:

  • Chemotrophs: Use chemicals as energy sources.

  • Phototrophs: Use light as an energy source.

  • Chemoorganotrophs: Use organic chemicals (e.g., Escherichia coli).

  • Chemolithotrophs: Use inorganic chemicals (e.g., Thiobacillus species).

• ATP is produced via different mechanisms depending on the group.

Chemotrophy

Definitions

• Respiration: The process in which a compound is oxidized with O2 (or a substitute) as the terminal electron acceptor, usually with ATP production by oxidative phosphorylation.

• Fermentation: Anaerobic catabolism where an organic compound serves as both electron donor and acceptor, with ATP produced by substrate-level phosphorylation.

Respiration

Definition and Process

• A catabolic process where a redox reaction catalyzed by organisms is coupled with the generation of an ion-motive force (often via an electron transport chain) that drives ATP synthesis.

Catabolism

What is Catabolism?

• Catabolism refers to energy-yielding reactions that generate biological energy currency (e.g., ATP), enabling anabolism and reproduction.

• These reactions are exergonic and provide free energy for cellular processes.

Respiration Basics

Redox Reactions

• Respirations are redox reactions requiring a reducing agent and an oxidizing agent.

• Reducing agent (electron donor): Also called reductant, fuel, or food. Examples: glucose, hydrogen.

• Oxidizing agent (terminal electron acceptor): Also called oxidant. Examples: oxygen (O2), sulfate (SO42−), perchlorate (ClO4−).

Key Terms and Concepts

Electron Donor and Acceptor

• Electron donor: The substance that is oxidized during respiration, donating electrons to the electron transport chain.

• Terminal electron acceptor (TEA): The substance that is reduced during respiration, accepting electrons from the electron transport chain.

• The oxidized product of the electron donor and the reduced product of the electron acceptor are typically waste products.

Gibbs Free Energy ($\Delta G$)

• Gibbs free energy ($\Delta G$) is the amount of energy available to do work in a reaction.

• If $\Delta G$ is negative, the reaction is exergonic (spontaneous); if positive, it is endergonic (requires energy input).

• Standard biological conditions are denoted as $\Delta G^\circ'$ (pH 7, 25°C, 1 atm, 1 M concentrations).

• The relationship between standard and actual free energy change is:

$\Delta G = \Delta G^\circ' + RT \ln(Q)$

• Where $Q$ is the activity quotient, $R$ is the gas constant, and $T$ is temperature in Kelvin.

Examples of Respiration

Aerobic Respiration of Glucose

• Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$

• Electron donor: Glucose ($C_6H_{12}O_6$)

• Oxidized waste product: CO2

• Terminal electron acceptor: O2

• Reduced waste product: H2O

• Electrons from glucose are transferred via NADH and FADH2 to the electron transport chain, generating a proton motive force for ATP synthesis.

The Redox Tower

Understanding the Redox Tower

• The redox tower lists redox couples by their standard reduction potential ($E_0'$) in volts.

• Redox couples at the top are better electron donors; those at the bottom are better electron acceptors.

• The difference in $E_0'$ between donor and acceptor determines the energy yield.

• The relationship between $\Delta G^\circ'$ and $E_0'$ is:

$\Delta G^\circ' = -nF\Delta E_0'$

• Where $n$ is the number of electrons transferred and $F$ is the Faraday constant.

Chemolithotrophy

Definition and Importance

• Chemolithotrophy is the process by which organisms use inorganic compounds (e.g., H2, Fe2+, H2S, NH3) as electron donors.

• Only known in bacteria and archaea; plays a key role in nutrient cycling.

• Examples: Hydrogen oxidation, iron oxidation, sulfur oxidation, and ammonia oxidation.

Hydrogen Oxidation

• Hydrogen gas (H2) can be derived from fermentation or geologic processes.

• Oxidation is catalyzed by hydrogenase enzymes, splitting H2 into protons and electrons.

• Electrons flow through the electron transport chain to a terminal electron acceptor (O2 or others).

• Hydrogen oxidation can be aerobic or anaerobic.

Anaerobic Respiration

Definition and Significance

• Anaerobic respiration uses a terminal electron acceptor other than oxygen (e.g., nitrate, sulfate, iron(III), carbon dioxide).

• Common in bacteria and archaea, especially in anoxic environments (e.g., sediments, deep subsurface).

• Important for nutrient cycling and mineral formation.

Examples of Anaerobic Respiration

• Nitrate reduction: $NO_3^- \rightarrow NO_2^- \rightarrow N_2$ (denitrification)

• Sulfate reduction: $SO_4^{2-} \rightarrow H_2S$

• Iron reduction: $Fe^{3+} \rightarrow Fe^{2+}$

• Carbon dioxide reduction: $CO_2 \rightarrow CH_4$ (methanogenesis)

Summary Table: Electron Donors and Acceptors in Microbial Respiration

Type of Respiration	Electron Donor	Terminal Electron Acceptor	Example Organism
Aerobic Respiration	Glucose, H2	O2	Escherichia coli
Anaerobic Respiration	Glucose, H2	NO3-, SO42-, Fe3+, CO2	Paracoccus denitrificans, Desulfovibrio
Chemolithotrophy	H2, Fe2+, NH3	O2, NO3-, SO42-	Thiobacillus, Nitrosomonas

Key Terms Recap

• Electron donor: Reductant, reducing agent, fuel, food; becomes oxidized.

• Electron acceptor: Oxidant, terminal electron acceptor, "breathes"; becomes reduced.

• Catabolism: Energy-yielding, exergonic reactions fueling cellular processes.

• Redox tower: Tool for predicting energy yield and feasibility of microbial metabolic reactions.

Discussion Questions

• Can a chemolithotroph also be capable of anaerobic respiration?

• Can an aerobe also be capable of anaerobic respiration?

• Can a chemoheterotroph also be capable of chemolithotrophy?

• What can an organism use nitrate (NO3-) for?

• Are all anaerobes capable of anaerobic respiration?

Additional info: Some context and examples were expanded for clarity and completeness, including the summary table and more detailed explanations of redox chemistry and metabolic diversity.