chapter 14

Metabolic Diversity in Microorganisms

Introduction

Microorganisms exhibit remarkable metabolic diversity, allowing them to thrive in a wide range of environments. Their metabolic pathways enable the cycling of key elements and the transformation of various compounds, which is essential for ecosystem function and human applications.

Major Differences Between Prokaryotes and Eukaryotes

Cellular Organization and Metabolism

• Prokaryotes (Bacteria and Archaea) lack a nucleus and membrane-bound organelles. Their metabolic processes often occur in the cytoplasm or across the cell membrane.

• Eukaryotes (Fungi, Protists, Plants, Animals) possess a nucleus and organelles such as mitochondria and chloroplasts, which compartmentalize metabolic functions.

• Prokaryotes display greater metabolic diversity, including unique pathways such as nitrogen fixation, methanogenesis, and anaerobic respiration.

Example: Only prokaryotes can perform nitrification and denitrification, crucial steps in the nitrogen cycle.

Gene Regulation: The Lac Operon

Regulation in Response to Glucose and Lactose

• The lac operon in Escherichia coli is a classic example of gene regulation in response to environmental nutrients.

• In the absence of glucose and presence of lactose, the operon is activated, allowing the cell to metabolize lactose.

• When glucose is present, catabolite repression inhibits the lac operon, prioritizing glucose metabolism.

Equation:

$\text{Lactose} \xrightarrow{\text{Lac Operon Enzymes}} \text{Glucose} + \text{Galactose}$

Additional info: The lac operon is regulated by both the lac repressor and catabolite activator protein (CAP).

Horizontal Gene Transfer

Mechanisms and Importance

• Horizontal gene transfer (HGT) refers to the movement of genetic material between organisms other than by descent.

• Main mechanisms: Transformation (uptake of free DNA), Transduction (phage-mediated), and Conjugation (plasmid transfer).

• HGT accelerates microbial evolution, spreading traits such as antibiotic resistance.

Example: The spread of antibiotic resistance genes among pathogenic bacteria.

Nitrogen Cycle: Ammonia Oxidation and Reduction

Ammonia Oxidizers and Anammox Bacteria

• Ammonia oxidizers convert ammonia (NH3) to nitrite (NO2-) and then to nitrate (NO3-).

• Anammox bacteria perform anaerobic ammonia oxidation, converting NH4+ and NO2- directly to N2 gas.

Equation:

$\mathrm{NH}_4^+ + \mathrm{NO}_2^- \rightarrow \mathrm{N}_2 + 2\mathrm{H}_2O$

Additional info: Anammox is important in oxygen-free environments and for nitrogen removal in wastewater treatment.

Denitrification and Nitrogen Reduction

• Denitrification is the reduction of nitrate (NO3-) to nitrogen gas (N2), passing through intermediates such as nitric oxide (NO) and nitrous oxide (N2O).

• This process removes fixed nitrogen from the environment and is essential for soil fertility and wastewater treatment.

• Nitrous oxide (N2O) is a potent greenhouse gas, making denitrification environmentally significant.

Oxidation States of Nitrogen:

Compound	Oxidation State
NH3 (Ammonia)	-3
N2 (Dinitrogen)	0
NO (Nitric oxide)	+2
NO2- (Nitrite)	+3
NO3- (Nitrate)	+5

Additional info: Denitrification is harmful to agriculture when it removes nitrogen needed for plant growth, but beneficial for sewage treatment.

Anaerobic Respiration and Electron Acceptors

Types of Anaerobic Respiration

• Microorganisms can use alternative electron acceptors in the absence of oxygen, such as nitrate, sulfate, sulfur, metals, and halogenated compounds.

• This allows survival in diverse environments and contributes to biogeochemical cycles.

Example: Sulfate-reducing bacteria use SO42- as an electron acceptor, producing H2S.

Fermentation

Overview and Types

• Fermentation is a metabolic process that does not require external electron acceptors.

• Microbes achieve redox balance by producing organic waste products such as acids or alcohols.

• Fermentation is classified as obligate (must ferment) or facultative (can ferment or respire).

Equation:

$\text{Glucose} \rightarrow \text{2 Lactate} + 2 ATP$

Lactic Acid Fermentation

• Homolactic fermentation yields a single product, lactate.

• Heterolactic fermentation yields multiple products, including lactate, ethanol, and CO2.

• Lactic acid fermentation is used in food production (yogurt, sauerkraut, kimchi) and preservation.

Mixed Acid Fermentation

• Characteristic of enteric bacteria (e.g., Escherichia coli), which inhabit animal and human intestines.

• Produces a mixture of acids: lactic, acetic, succinic, and formic acids.

• Adaptation to low-oxygen, variable environments like the gut.

Primary and Secondary Fermentations

Classification and Substrates

• Primary fermenters break down polymers (carbohydrates, proteins, fats) into monomers.

• Secondary fermenters utilize products of primary fermentation as substrates.

Additional info: Secondary fermentation is important in complex microbial communities, such as soil and animal digestive tracts.

Syntrophy

Definition and Importance

• Syntrophy is a cooperative interaction between two or more microbial species, where each depends on the other to carry out a metabolic reaction that neither can perform alone.

• Often involves interspecies electron transfer, where electrons are exchanged between cells via direct contact or diffusion.

• Syntrophy is critical for the degradation of complex organic compounds in anaerobic environments.

Example: Methanogenic archaea and fermentative bacteria cooperate to degrade fatty acids, producing methane.

In your own words: Syntrophy is when different microbes work together to complete a metabolic process that neither could do by itself, often by sharing electrons or metabolic products.