6.1 Taxonomy of Prokaryotes

Introduction

Prokaryotes are the simplest and most ancient forms of life on Earth. They include bacteria and archaea. The taxonomy of prokaryotes has evolved significantly over time. Early classification systems placed all prokaryotes in a single kingdom, but modern molecular techniques have revealed that prokaryotes consist of two distinct domains: Bacteria and Archaea. This topic covers the taxonomic and phylogenetic position of prokaryotes.

Definition

Taxonomy of Prokaryotes: The taxonomy of prokaryotes is the science of classifying and naming prokaryotic organisms based on their evolutionary relationships and characteristics.

In simple words: Taxonomy of prokaryotes is how we classify and name bacteria and archaea.

Core Concept

• In 1969, American biologist Robert H. Whittaker introduced a five-kingdom system that included the fundamental distinction between prokaryotes and eukaryotes.
• This system was later revised by Lynn Margulis and Karlene V. Schwarts in 1988, where they designated a distinct kingdom called Monera for all prokaryotic organisms.
• Phylogenetics is the study of evolutionary relatedness among various groups of organisms.
• The term "bacteria" traditionally referred to all tiny, single-celled prokaryotes.
• Advancements in molecular systematics revealed that prokaryotic life consists of two distinct domains:
• These domains evolved independently from a common ancient ancestor.
• Together with Eukarya, they form the foundation of the widely accepted three-domain system in bacteriology.
• A significant milestone in bacterial research occurred in 1977 when Carl Woese identified that archaea have a separate evolutionary lineage from bacteria.
• This new discovery led to the division of prokaryotes into the three-domain system:
• The ancestors of contemporary bacteria were ancient, single-celled microorganisms that emerged on Earth approximately 4 billion years ago.
• For roughly 3 billion years, all life forms remained microscopic, with bacteria and archaea dominating the biosphere.
• The most recent common ancestor of bacteria and archaea likely existed around 2.5 to 3.2 billion years ago and was probably a hyperthermophile.

Detailed Explanation

Historical Background of Prokaryotic Taxonomy

Five-Kingdom System (1969)

• Monera (prokaryotes)
• Protista
• Fungi
• Plantae
• Animalia

Kingdom Monera (1988)

• Proposed by Lynn Margulis and Karlene V. Schwarts.
• Designated a distinct kingdom called Monera for all prokaryotic organisms.
• Included all bacteria and archaea.

Three-Domain System

• Proposed by Carl Woese in 1977 and formalized in 1990.
• Based on molecular systematics (rRNA sequences).
• Divides life into three domains:

• Bacteria and Archaea are prokaryotes.
• Eukarya includes protists, fungi, plants, and animals.
• Bacteria and Archaea evolved independently from a common ancient ancestor.
• The three-domain system is the widely accepted system in bacteriology.

Phylogenetic Position of Prokaryotes

Definition: Phylogenetics is the study of evolutionary relatedness among various groups of organisms.

• Phylogeny is the origin and evolution of a set of organisms.
• The term "bacteria" traditionally referred to all tiny, single-celled prokaryotes.
• Advancements in molecular systematics revealed that prokaryotic life consists of two distinct domains:
• These domains evolved independently from a common ancient ancestor.
• Together with Eukarya, they form the foundation of the widely accepted three-domain system.

Timeline of Prokaryotic Evolution

• The ancestors of contemporary bacteria were ancient, single-celled microorganisms that emerged on Earth approximately 4 billion years ago.
• For roughly 3 billion years, all life forms remained microscopic, with bacteria and archaea dominating the biosphere.
• The most recent common ancestor of bacteria and archaea likely existed around 2.5 to 3.2 billion years ago and was probably a hyperthermophile.

Carl Woese's Contribution (1977)

• Carl Woese identified that archaea have a separate evolutionary lineage from bacteria.
• He used ribosomal RNA (rRNA) sequences to compare organisms.
• He found that archaea are as different from bacteria as they are from eukaryotes.
• This led to the division of prokaryotes into the three-domain system:

• Carl Woese (1977) identified archaea as a separate lineage.
• Used rRNA sequences for comparison.
• Led to the three-domain system.

Comparison of Bacteria and Archaea

Summary Table of Prokaryotic Taxonomy

Intermediate Level

Five-Kingdom System in Detail

Kingdom Monera in Detail

Three-Domain System in Detail

Carl Woese's Contribution in Detail

Timeline of Prokaryotic Evolution in Detail

Important Points

• 1. In 1969, American biologist Robert H. Whittaker introduced a five-kingdom system that included the fundamental distinction between prokaryotes and eukaryotes.
• 2. This system was later revised by Lynn Margulis and Karlene V. Schwarts in 1988, where they designated a distinct kingdom called Monera for all prokaryotic organisms.
• 3. Phylogenetics (Greek: phyle, tribe, race and genetikos, relative to birth, from genesis, birth) is the study of evolutionary relatedness among various groups of organisms (e.g., species, populations).
• 4. Phylogeny (or phylogenesis) is the origin and evolution of a set of organisms, usually a set of species.
• 5. The term "bacteria" traditionally referred to all tiny, single-celled prokaryotes.
• 6. However, advancements in molecular systematics revealed that prokaryotic life consists of two distinct domains, initially named Eubacteria and Archaeobacteria but now recognized as Bacteria and Archaea.
• 7. These domains evolved independently from a common ancient ancestor and, together with Eukarya, form the foundation of the widely accepted three-domain system in bacteriology.
• 8. A significant milestone in bacterial research occurred in 1977 when Carl Woese identified that archaea have a separate evolutionary lineage from bacteria.
• 9. This new discovery led to the division of prokaryotes into the three-domain system: eubacteria, archaea, and eukaryotes.
• 10. The ancestors of contemporary bacteria were ancient, single-celled microorganisms that emerged on Earth approximately 4 billion years ago.
• 11. For roughly 3 billion years, all life forms remained microscopic, with bacteria and archaea dominating the biosphere.
• 12. The most recent common ancestor of bacteria and archaea likely existed around 2.5 to 3.2 billion years ago and was probably a hyperthermophile.

Remember

• - Prokaryotes = bacteria and archaea
• - Five-kingdom system = Whittaker (1969)
• - Kingdom Monera = Margulis and Schwarts (1988)
• - **Three-domain system** = Woese (19771990)
• - Domains = Bacteria, Archaea, Eukarya
• - Bacteria and Archaea = prokaryotes
• - **Eukarya** = eukaryotes
• - Carl Woese = discovered archaea as a separate lineage
• - Common ancestor = hyperthermophile
• - Origin of life = ~4 billion years ago

Common Mistakes

• 1. Confusing the five-kingdom and three-domain systems – Five-kingdom = 1969; Three-domain = 19771990.
• 2. Thinking all prokaryotes are bacteria – Prokaryotes include both bacteria and archaea.
• 3. Thinking archaea are a type of bacteria – Archaea are a separate domain.
• 4. Forgetting the contribution of Carl Woese – He identified archaea as a separate lineage in 1977.
• 5. Confusing the terms "Eubacteria" and "Bacteria" – Eubacteria is the former name for Bacteria.
• 6. Confusing the terms "Archaeobacteria" and "Archaea" – Archaeobacteria is the former name for Archaea.
• 7. Thinking the three-domain system was proposed in 1969 – The three-domain system was proposed in 19771990.
• 8. Forgetting the timeline – Origin of life ~4 billion years ago; common ancestor ~2.5–3.2 billion years ago.
• 9. Thinking prokaryotes are not important – Prokaryotes are essential for life on Earth.
• 10. Confusing phylogenetics and taxonomy – Phylogenetics = evolutionary relatedness; Taxonomy = classification and naming.
• 11. Thinking all archaea live in extreme environments – Some archaea live in moderate environments.
• 12. Forgetting the importance of rRNA sequencing – rRNA sequencing was used to identify archaea as a separate lineage.

Quick Revision

• In 1969, American biologist Robert H. Whittaker introduced a five-kingdom system that included the fundamental distinction between prokaryotes and eukaryotes.
• This system was later revised by Lynn Margulis and Karlene V. Schwarts in 1988, where they designated a distinct kingdom called Monera for all prokaryotic organisms.
• Phylogenetics (Greek: phyle, tribe, race and genetikos, relative to birth, from genesis, birth) is the study of evolutionary relatedness among various groups of organisms (e.g., species, populations).
• Phylogeny (or phylogenesis) is the origin and evolution of a set of organisms, usually a set of species.
• The term "bacteria" traditionally referred to all tiny, single-celled prokaryotes.
• However, advancements in molecular systematics revealed that prokaryotic life consists of two distinct domains, initially named Eubacteria and Archaeobacteria but now recognized as Bacteria and Archaea.
• These domains evolved independently from a common ancient ancestor and, together with Eukarya, form the foundation of the widely accepted three-domain system in bacteriology.
• A significant milestone in bacterial research occurred in 1977 when Carl Woese identified that archaea have a separate evolutionary lineage from bacteria.
• This new discovery led to the division of prokaryotes into the three-domain system: eubacteria, archaea, and eukaryotes.
• The ancestors of contemporary bacteria were ancient, single-celled microorganisms that emerged on Earth approximately 4 billion years ago.
• For roughly 3 billion years, all life forms remained microscopic, with bacteria and archaea dominating the biosphere.
• The most recent common ancestor of bacteria and archaea likely existed around 2.5 to 3.2 billion years ago and was probably a hyperthermophile.

60-Second Revision

• Prokaryotes = bacteria and archaea
• Five-kingdom system = Whittaker (1969)
• Kingdom Monera = Margulis and Schwarts