Concept 19.1 Life Consists of Three Domains That Share a Common Ancestor

• Two of life's three domains, Bacteria and Archaea, are prokaryotic. They are distinguished from Eukarya in several ways, including their lack of a nucleus and of membrane-enclosed organelles. Review Table 19.1
   
• Eukaryotes are related to both Archaea and Bacteria and appear to have formed through endosymbiosis between members of these two lineages. The last common ancestor of all three domains probably lived about 3 billion years ago. Review Figure 19.1 and ANIMATED TUTORIAL 19.1
   
• Early attempts to classify prokaryotes were hampered by the organisms' small size and the difficulties of growing them in pure culture.
   
• The cell walls of almost all bacteria contain peptidoglycan, whereas the cell walls of archaea and eukaryotes lack peptidoglycan.
   
• Bacteria can be differentiated into two groups by the Gram stain. Gram-negative bacteria have a periplasmic space between the plasma membrane and a distinct outer membrane. Gram-positive bacteria have a thick cell wall containing about five times as much peptidoglycan as a Gram-negative wall. Review Figure 19.2 and WEB ACTIVITY 19.1
   
• The three most common bacterial morphologies are cocci (spheres), bacilli (rods), and helices (spirals). Review Figure 19.3
   
• The cells of some bacteria aggregate, forming multicellular colonies.
   
• Phylogenetic classification of prokaryotes is now based principally on the nucleotide sequences of rRNA and other genes involved in fundamental cellular processes.
   
• Prokaryotes reproduce asexually by binary fission, but may exchange genetic material. Reproduction and genetic exchange are not directly linked in prokaryotes.
   
• Although lateral gene transfer has occurred throughout prokaryotic evolutionary history, elucidation of prokaryote phylogeny is still possible. Review Figure 19.4
   

Concept 19.2 Prokaryote Diversity Reflects the Ancient Origins of Life

• Prokaryotes are the most numerous organisms on Earth, only a small fraction of prokaryote diversity has been characterized to date.
   
• Some high-GC Gram-positives produce important antibiotics.
   
• The low-GC Gram-positives include the mycoplasmas, which are among the smallest cellular organisms ever discovered.
   
• The photosynthetic cyanobacteria release oxygen into the atmosphere. Cyanobacteria may live free as single cells or associate in multicellular colonies. Review Figure 19.9
   
• Spirochetes have unique structures called axial filaments that allow them to move in a corkscrew-like manner. Review Figure 19.10
   
• The proteobacteria embrace the largest number of known species of bacteria. Smaller groups include the hyperthermophilic bacteria, halobacteria, and chlamydias.
   
• The best-studied groups of Archaea are Crenarchaeota and Euryarchaeota.
   
• Many archaea are extremophiles. Most known crenarchaeotes are thermophilic, acidophilic, or both. Some euryarchaeotes are methanogens; extreme halophiles are also found among the euryarchaeotes. Review Figure 19.14
   
• Ether linkages in the branched long-chain hydrocarbons of the lipids that make up their cell membranes are a synapomorphy of Archaea. Review Figure 19.15
   

Concept 19.3 Ecological Communities Depend on Prokaryotes

• Prokaryotes form complex communities, of which biofilms are one example. Review Figure 19.19
   
• Prokaryote metabolism is very diverse. Some prokaryotes are anaerobic, others are aerobic, and still others can shift between these modes.
   
• Prokaryotes fall into four broad nutritional categories: photoautotrophs, photoheterotrophs, chemolithotrophs, and chemoheterotrophs. Review Table 19.2
   
• Prokaryotes play key roles in the cycling of elements such as nitrogen, oxygen, sulfur, and carbon. One such role is as decomposers of dead organisms.
   
• Some prokaryotes metabolize sulfur or nitrogen. Nitrogen fixers convert nitrogen gas into a form that organisms can metabolize. Nitrifiers convert that nitrogen into forms that can be used by plants, and denitrifiers return nitrogen to the atmosphere.
   
• Prokaryotes inhabit the guts of many animals (including humans) and help them digest food.
   
• Koch's postulates establish the criteria by which an organism may be classified as a pathogen. Relatively few bacteria—and no archaea—are known to be pathogens. Review Figure 19.21
   

Concept 19.4 Viruses Have Evolved Many Times

• Viruses have evolved many times from many different groups of cellular organisms. They do not represent a single taxonomic group.
   
• Some viruses are probably derived from escaped genetic elements of cellular species; others are thought to have evolved as highly reduced parasites.
   
• Viruses are categorized by the nature of their genomes.