Chapter 11 aerobes and anaerobes

Major Nutritional Types of Microorganisms

Overview of Nutritional Types

The major nutritional types of microorganisms are categorized based on their sources of energy, electrons, and carbon.

Nutritional Types and Their Sources

  • Photolithoautotroph

    • Energy Source: Light

    • Electron Source: Inorganic chemicals

    • Carbon Source: CO2

    • Representative Microorganisms: Purple and green sulfur bacteria, cyanobacteria, diatoms

  • Photoorganoheterotroph

    • Energy Source: Light

    • Electron Source: Organic compounds (often the same as carbon source)

    • Carbon Source: Organic carbon

  • Chemolithoautotroph

    • Energy Source: Inorganic chemicals

    • Electron Source: Inorganic chemicals

    • Carbon Source: CO2

    • Representative Microorganisms: Sulfur-oxidizing bacteria, hydrogen-oxidizing bacteria, methanogens, nitrifying bacteria, iron-oxidizing bacteria

  • Chemolithoheterotroph

    • Energy Source: Inorganic chemicals

    • Electron Source: Organic compounds (often the same as carbon source)

    • Carbon Source: Organic carbon

    • Representative Microorganisms: Some sulfur-oxidizing bacteria (e.g., Beggiatoa spp.)

  • Chemoorganoheterotroph

    • Energy Source: Organic chemicals (often the same as carbon source)

    • Electron Source: Organic compounds (often the same as carbon source)

    • Carbon Source: Organic carbon

    • Representative Microorganisms: Most non-photosynthetic microbes, including most pathogens, fungi, and many protists

Types of Chemoorganotrophic Metabolism

Chemoorganotrophs are characterized by their various metabolic pathways that utilize organic compounds.

Key Learning Objectives

This section outlines the objectives for understanding chemoorganotrophs:

  • Identify the three major types of metabolisms carried out by chemoorganotrophs.

  • Name the carbon, energy, and electron sources for chemoorganotrophs.

  • List types of organic molecules that chemoorganotrophs can utilize.

  • Explain examples of carbon compounds that microorganisms can use which human cells cannot.

  • Discuss how chemoorganotrophs utilize a diverse range of substrates.

Types of Catabolism in Chemoorganotrophs

Chemoorganotrophs can metabolize a wide variety of organic substrates:

  1. Polysaccharides (e.g., starch, cellulose)

  2. Lipids (e.g., triglycerides, phospholipids)

  3. Peptides (e.g., proteins, enzymes)

  4. Aromatic compounds (e.g., lignin, polychlorinated aromatics)

Breakdown of Specific Organic Compounds
A. Polysaccharides

  • Starch (amylose): A glucose polymer connected by α-1,4-acetal linkages.

  • Cellulose: Another glucose polymer, commonly found in plant cell walls.

  • Pectin: A polymer of galacturonic acid; provides structural support in plant cells.

B. Lipids

  • Triglycerides: Composed of glycerol and fatty acids. Structural details include:

    • Glycerol backbone connected to three fatty acids.

    • Various arrangements of fatty acids can affect their properties.

  • Phospholipids: Cellular membrane components, composed of hydrophilic heads and hydrophobic tails.

C. Peptides and Proteins

  • Involves the breakdown of complex proteins into simpler amino acids.

D. Aromatic Molecules

  • Lignin: A complex organic polymer that provides rigidity to plant cell walls. Microorganisms such as certain types can degrade lignin into simpler compounds like catechol.

Enzymatic Breakdown of Complex Fibers

Complex fibers such as xyloglucans can be broken down by microorganisms through specific enzymes known as CAZymes (carbohydrate-active enzymes).

  • The human genome has around 17 genes coding for CAZymes.

  • Gut microbes possess hundreds of such genes, allowing them to break down complex dietary fibers that humans cannot digest on their own.

Importance in Bioremediation

The ability of chemoorganotrophs to degrade aromatic compounds is significant in bioremediation efforts, facilitating the breakdown of environmental pollutants.

Central Metabolic Pathways

Chemoorganotrophic metabolism involves major pathways such as glycolysis, the Entner-Doudoroff pathway, and the pentose phosphate pathway. These pathways involve:

  • The oxidation of carbon compounds.

  • Production of ATP, NADH, and FADH2 during the oxidation process.

Glycolysis Overview

  • Glycolysis breaks down glucose into pyruvate, yielding 2 ATP and 2 NADH.

  • Various pathways, including fermentation, allow for the recycling of NADH back into NAD+.

Other Pathways to Oxidize Glucose

  1. Entner-Doudoroff Pathway: Utilized predominantly by gram-negative bacteria, yielding 1 ATP, 1 NADH, and 1 NADPH.

  2. Pentose Phosphate Pathway: Provides precursors for biosynthesis, yielding 1 ATP and 2 NADPH.

Energy Conservation and Carbon Fate in Metabolic Pathways

  • Glucose oxidation does not conserve sufficient energy without further metabolic processes.

  • Pyruvate can be directed towards fermentation or respiration pathways based on aerobic or anaerobic conditions.

Products of Fermentation

During fermentation, pyruvate is reduced to regenerate NADH, resulting in various products such as lactate, ethanol, and other organic acids.

  • Fermentation pathways adjust based on environmental conditions, optimizing growth and energy production.

Applications of Fermentation

Fermentation is harnessed in various applications, such as the production of alcoholic beverages, yogurt, and cheeses. Byproducts of fermentation can also provide nutritional benefits, such as short-chain fatty acids that enhance gut health.

Anaerobic Respiration

Bacteria can utilize a variety of electron acceptors other than oxygen, leading to anaerobic respiration. Important aspects include:

  • Denitrification: Nitrate as an electron acceptor, producing nitrogen gas, relevant for agricultural nitrogen management and water quality.

  • Sulfate Reduction: Utilizing sulfate as an electron acceptor, prevalent in anaerobic environments such as wetlands.

  • Metal Reduction: Involves the reduction of metals like iron and manganese, important for biomining and bioremediation applications.

Summary of Electron Transport Chains (ETC)

  • The electron transport chain requires both an electron donor (e.g., NADH) and an electron acceptor (e.g., O2, different metals).

  • The efficiency of energy conservation varies significantly between aerobic and anaerobic respiration based on the redox potential of the electron acceptors used.

The Electron Tower

This provides an overview of the standard reduction potentials (E°) for various electron acceptors, illustrating the energetics involved in both aerobic and anaerobic processes, essential for understanding metabolism in various microorganisms.

Concluding Remarks

Understanding the metabolic capacities of chemoorganotrophs and their diverse pathways illuminates their ecological roles, industrial applications, and their potential for biotechnological innovations in sustainable practices.

Future Directions

Further research into the optimizations and adaptations of these metabolisms can lead to advancements in biofuel production and bioremediation technologies, capitalizing on microbial diversity and metabolic flexibility.