Three-domain System - Classification

Classification

The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the five-or-six-kingdom systems. This classification system recognizes the fundamental divide between the two prokaryotic groups, insofar as archaea appear to be more closely related to eukaryotes than they are to their fellow prokaryotic bacteria. The current system has the following kingdoms in the three domains:

Archaea Domain – prokaryotic, no nuclear membrane, distinct biochemistry and RNA markers from eubacteria, possess unique ancient evolutionary history for which they are considered some of the oldest species of organisms on Earth; traditionally classified as archaebacteria; often characterized by living in extreme environments

• Kingdom Archaebacteria
  Examples:

• Methanogens – metabolize hydrogen and carbon dioxide into methane
• Halophiles – thrive in salt
• Thermoacidophiles – thrive in acid and high temperatures (up to 110 degrees Celsius)

Bacteria Domain – prokaryotic, consists of prokaryotic cells possessing primarily diacyl glycerol diaster lipids in their membranes and bacterial rRNA, no nuclear membrane, traditionally classified as bacteria, contain most known pathogenic prokaryotic organisms (see for exceptions), studied far more extensively than Archaea

• Kingdom Eubacteria
  Examples:

• Cyanobacteria – photosynthesizing bacteria
• Spirochaete – Gram-negative bacteria that include those causing syphilis and Lyme disease
• Firmicutes – Gram-positive bacteria including Bifidobacterium animalis which is present in the human large intestine

Eukarya Domain – eukaryotes, nuclear membrane

• Kingdom Fungi or fungi
  Examples:

• Saccharomycotina – includes true yeasts
• Basidiomycota – includes shiitake mushrooms

• Kingdom Plantae or plants
  Examples:

• Bryophyta – mosses
• Magnoliophyta – flowering plants

• Kingdom Animalia or animals
  Examples:

• Arthropoda – includes insects, arachnids, and crustaceans
• Chordata – includes vertebrates and, as such, human beings

• Kingdom Protista or protists (recognized to be paraphyletic, and thus subject to dissolution and/or redefinition)
  Examples:

• Rhodophyta – red algae
• Chromalveolata – includes dinoflagellates