Bacteria and Archaea: Diversity, Evolution, and Study Methods

Bacteria and Archaea

Introduction to Prokaryotes

Bacteria and Archaea are two of the three domains of life, representing the prokaryotic organisms. Prokaryotes are unicellular organisms that lack a membrane-bound nucleus and other organelles. They are among the most ancient and diverse forms of life on Earth.

• Prokaryotes include both Bacteria and Archaea.

• They are found in nearly every environment, from soil and water to extreme environments like hot springs and salt lakes.

• Prokaryotes play essential roles in ecological processes such as nutrient cycling and decomposition.

The Three Domains of Life

Classification of Life

All living organisms are classified into three domains: Bacteria, Archaea, and Eukarya. This classification is based on differences in cellular structure, genetics, and biochemistry.

• Bacteria: Prokaryotic cells with peptidoglycan in their cell walls.

• Archaea: Prokaryotic cells without peptidoglycan, often found in extreme environments.

• Eukarya: Organisms with eukaryotic cells, including animals, plants, fungi, and protists.

Example: The domain Bacteria includes cyanobacteria and proteobacteria, while Archaea includes methanogens and halophiles. Eukarya includes multicellular organisms such as animals and plants.

Estimated Diversity of Species

Prokaryotic Diversity

Prokaryotes represent a vast majority of the Earth's biodiversity. Estimates suggest that the number of prokaryotic species far exceeds that of eukaryotes.

• Pie charts show that prokaryotes make up the largest portion of species diversity on Earth.

• Other groups, such as insects and plants, represent much smaller fractions of total species diversity.

Example: According to Chapman (2009) and Locey & Lennon (2016), prokaryotes dominate the estimated diversity of life forms.

Origin and Evolution of Prokaryotes

The First Cells

The earliest life forms on Earth were prokaryotic. These cells likely evolved in environments lacking oxygen and used alternative metabolic pathways.

• The first cells lived in the absence of oxygen (anaerobic conditions).

• Early photosynthesis probably used H2S (hydrogen sulfide) as an electron donor, releasing sulfur as a byproduct.

• Later, photosynthetic organisms evolved to use H2O (water) as an electron donor, releasing O2 as a byproduct, which led to the oxygenation of Earth's atmosphere.

Example: Stromatolites are layered structures formed by the activity of ancient prokaryotes, providing evidence of early life.

Structure of Bacteria and Archaea

Cellular Features

Bacteria and Archaea share some structural similarities but also have key differences.

• Both lack a nucleus and membrane-bound organelles.

• Bacteria have cell walls made of peptidoglycan.

• Archaea have cell walls composed of unique molecules (not peptidoglycan), such as pseudopeptidoglycan or proteins.

• Both may have structures such as flagella (for movement), pili (for attachment), and a plasma membrane.

Example: Diagrams often show bacterial and archaeal cells with labeled features such as nucleoid, ribosomes, and cell wall composition.

Studying Prokaryotes

Methods and Techniques

Scientists use a variety of methods to study prokaryotes, including microscopy, culturing, and molecular techniques.

• Microscopy: Allows visualization of cell shape, arrangement, and staining properties.

• Culturing: Growing prokaryotes in laboratory media to study their physiology and genetics.

• Molecular techniques: DNA sequencing and genetic analysis provide insights into evolutionary relationships and diversity.

Example: Gram staining is a common technique to differentiate bacterial species based on cell wall structure.

Table: Comparison of Bacteria and Archaea

Feature	Bacteria	Archaea
Cell Wall Composition	Peptidoglycan	No peptidoglycan; may have pseudopeptidoglycan or protein
Membrane Lipids	Ester-linked fatty acids	Ether-linked isoprenoids
Habitat	Wide range, including soil, water, and living hosts	Often extreme environments (e.g., hot springs, salt lakes)
Examples	Cyanobacteria, Proteobacteria	Methanogens, Halophiles

Additional info:

• Prokaryotes are essential for processes such as nitrogen fixation, decomposition, and as part of the human microbiome.

• Modern techniques such as metagenomics have greatly expanded our understanding of prokaryotic diversity.