Learning Outcomes
i. Identify the three domains of life: Archaea, Bacteria, and Eukarya.
ii. Define prokaryotes and distinguish them from eukaryotes.
iii. Explain the taxonomic hierarchy of prokaryotes, including domains, kingdoms, phyla, classes, orders, families, genera, and species.
iv. Understand the phylogenetic position of prokaryotes in the tree of life.
Introduction
Prokaryotes are single-celled organisms that lack a true nucleus and membrane-bound organelles. They are the most abundant and diverse group of organisms on Earth, inhabiting a wide range of environments, from extreme conditions like hot springs and hydrothermal vents to the human body. For many years, prokaryotes were classified under the kingdom Monera, but recent advancements in molecular biology have revealed their evolutionary diversity, leading to a revised classification system that recognizes three domains of life: Archaea, Bacteria, and Eukarya.
i. Domains of Life
Archaea: Archaea are a distinct group of prokaryotes that share closer evolutionary relationships with eukaryotes than with bacteria. They are found in extreme environments, such as hydrothermal vents, hot springs, and hypersaline lakes. Archaea play important roles in global biogeochemical cycles, such as methane production and nitrogen fixation.
Bacteria: Bacteria are the most abundant and diverse group of prokaryotes, found in almost every environment on Earth. They play essential roles in ecosystems, including decomposition of organic matter, nutrient cycling, and symbiosis with other organisms. Some bacteria can cause diseases, while others are beneficial to humans, such as those used in food production and industrial processes.
Eukarya: Eukarya are a diverse group of organisms that include all multicellular organisms, as well as some single-celled organisms like protists and fungi. Eukaryotic cells have a true nucleus and membrane-bound organelles, such as mitochondria, Golgi apparatus, and endoplasmic reticulum.
ii. Taxonomy of Prokaryotes
Prokaryotes are classified into a hierarchical system based on their evolutionary relationships. The major ranks in the prokaryotic taxonomic hierarchy are:
Domain: Archaea and Bacteria
Kingdom: Monera
Phylum: Proteobacteria, Cyanobacteria, Firmicutes, Actinobacteria, and others
Class: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Cyanophyceae, Bacilli, Clostridia, Mollicutes, and others
Order: Enterobacteriales, Pseudomonadales, Rhizobiales, Cyanobacteriales, Bacillales, Clostridiales, Mycoplasmatales, and others
Family: Enterobacteriaceae, Pseudomonadaceae, Rhizobiaceae, Nostocales, Bacillaceae, Clostridiaceae, Mycoplasmataceae, and others
Genus: Escherichia, Pseudomonas, Rhizobium, Nostoc, Bacillus, Clostridium, Mycoplasma, and others
Species: Escherichia coli, Pseudomonas aeruginosa, Rhizobium leguminosarum, Nostoc punctiforme, Bacillus subtilis, Clostridium tetani, Mycoplasma pneumoniae, and others
iii. Phylogenetic Position of Prokaryotes
The tree of life depicts the evolutionary relationships between all living organisms. Prokaryotes are located at the base of the tree, indicating their early divergence from other life forms. Archaea and Bacteria form distinct branches on the tree, while eukaryotes represent a separate and more recent lineage.
Prokaryotes are a diverse and fascinating group of organisms that play essential roles in the biosphere. Understanding their taxonomy and phylogenetic relationships is crucial for appreciating their evolutionary history, ecological significance, and potential applications in various fields, including medicine, biotechnology, and environmental science.
