Chapter Summary

• The earliest split in the tree of life is between Bacteria and Archaea. One lineage within the Archaea branch, the Eukarya, evolved specializations (such as a nucleus and membrane-enclosed organelles) that allowed them flourish. All organisms that are not eukaryotes are called prokaryotes. Review Figure 25.1, Table 25.1, Animation 25.1

• The cell walls of almost all bacteria contain peptidoglycan, whereas the cell walls of prokaryotic archaea lack peptidoglycan.

• Some groups of bacteria can be differentiated using the Gram stain. Gram-negative bacteria have a periplasmic space between the cell 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 cell wall. Review Figure 25.2, Activity 25.1

• The three most common bacterial shapes are cocci (spheres), bacilli (rods), and spirilla (helices). Review Figure 25.3

• Phylogenetic classification of prokaryotes is now based principally on nucleotide sequences of their genomes, especially of the rRNA genes.

• 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 many aspects of prokaryote phylogeny is still possible. Review Focus: Key Figure 25.4

• Prokaryotes are the most numerous organisms on Earth, but only a small fraction of prokaryote diversity has been characterized to date.

• Several early-diverging groups of bacteria, such as hadobacteria and hyperthermophilic bacteria, live mostly in hot environments.

• The firmicutes include the mycoplasmas, which are among the smallest cellular organisms known.

• Some actinobacteria produce important antibiotics.

• The photosynthetic cyanobacteria release oxygen into the atmosphere. Cyanobacteria may live free as single cells or associate in multicellular colonies. Review Figure 25.9

• Spirochetes have unique structures called axial filaments that allow them to move in a corkscrew-like manner. Review Figure 25.10

• Chlamydias are tiny parasitic bacteria (0.2–1.5 μm in diameter) that live within the cells of other organisms. Review Figure 25.11

• The proteobacteria embrace the largest number of known species of bacteria.

• Prokaryotic archaea live in highly diverse environments and include many extremophiles. Most known crenarchaeotes are thermophilic, acidophilic, or both. Some euryarchaeotes are methanogens; extreme halophiles are also found among the euryarchaeotes. Review Figure 25.14

• Ether linkages in the branched long hydrocarbon chains of the lipids that make up the cell membranes are characteristic of prokaryotic archaea. Review Figure 25.15

• Prokaryotes form complex communities, of which biofilms are one example. Review Figure 25.18

• Microbiomes are the communities of prokaryotes that live in and on the bodies of multicellular organisms. These communities are often important to the health of the hosts, and changes to the microbiome may lead to serious health consequences. Review Figure 25.19

• Communities of bacteria can communicate information about their density using chemical signals in a process known as quorum sensing. Review Investigating Life: How Do Bacteria Communicate with One Another?

• 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 prokaryotic archaea—are known to be pathogens. Review Figure 25.20

• Prokaryote metabolism is 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, chemoautotrophs, and chemoheterotrophs. Review Table 25.2

• Prokaryotes play key roles in the cycling of elements such as nitrogen, oxygen, sulfur, and carbon.

• 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.

• Viruses have evolved many times from many different groups of cellular organisms. They are placed in groups according to the structure of their genomes, but these groups are not thought to represent monophyletic taxa. Review Figure 25.22

• Some viruses are probably derived from escaped components of cellular organisms; others are thought to have evolved as highly reduced parasites. Review Figure 25.24

• Endogenous retroviruses can represent a substantial component of vertebrate genomes (8% in the case of the human genome).

• Viruses can be used to fight bacterial infections in a process known as phage therapy.