Published on October 9, 2007
Chapter 27: Chapter 27 Prokaryotes 張學偉 助理教授 生物醫學暨環境生物學系 http://genomed.dlearn.kmu.edu.tw [email protected] Slide2: Prokaryotes were the earliest organisms on Earth. Today, they still dominate the biosphere. Their collective biomass outweighs all eukaryotes combined at least tenfold. Prokaryotes are wherever there is life. They thrive in habitats that are too cold, too hot, too salty, too acidic, or too alkaline for any eukaryote. Lecture Outline-1 Slide3: Prokaryotes have even been discovered in rocks two miles below the surface of the Earth. Why have these organisms dominated the biosphere since the origin of life on Earth? Prokaryotes display diverse adaptations that allow them to inhabit many environments. They have great genetic diversity. Prokaryotes are classified into two domains, Bacteria and Archaea, which differ in structure, physiology and biochemistry. Lecture Outline-2 Slide4: Overview: They’re (Almost) Everywhere! Most prokaryotes are microscopic But what they lack in size they more than make up for in numbers. Slide5: Prokaryotes thrive almost everywhere Including places too acidic, too salty, too cold, or too hot for most other organisms Hot water of a Nevada geyser Slide6: Biologists are discovering That these organisms have an astonishing genetic diversity Slide7: Concept 27.1: Structural, functional, and genetic adaptations contribute to prokaryotic success Most prokaryotes are unicellular Although some species form colonies Slide8: Prokaryotic cells have a variety of shapes most common Figure 27.2a–c Cell-Surface Structures: Cell-Surface Structures One of the most important features of nearly all prokaryotic cells- cell wall. maintains cell shape, provides physical protection prevents the cell from bursting in a hypotonic environment Slide10: Using a technique called the Gram stain Scientists can classify many bacterial species into two groups based on cell wall composition, Gram-positive and Gram-negative Slide11: The cell wall of many prokaryotes Is covered by a capsule, a sticky layer of polysaccharide or protein Cell wall Slide12: Capsules allow cells to adhere to their substratum. They may increase resistance to host defenses. They glue together the cells of those prokaryotes that live as colonies. Slide13: Some prokaryotes have fimbriae and pili Which allow them to stick to their substrate or other individuals in a colony Slide14: Fimbriae are usually more numerous and shorter than pili. These structures can fasten pathogenic bacteria to the mucous membranes of the host. Sex pili are specialized for holding two prokaryote cells together long enough to transfer DNA during conjugation. Slide15: Conjugation: plasmid-directed transfer of DNA from one cell to another. 補充 Motility: Motility Most motile bacteria propel themselves by flagella 27.6 Salmonella Flagella: 27.6 Salmonella Flagella movie Some species can move at speeds exceeding 50 m/sec, about 100 times their body length per second. Slide18: Fig. 7-? Eukaryotic flagellum or cillum structurally and functionally different bwteen prokayotic & eukaryotic flagella Slide19: In a heterogeneous environment, many bacteria exhibit taxis The ability to move toward or away from certain stimuli Internal and Genomic Organization: Internal and Genomic Organization Prokaryotic cells Usually lack complex compartmentalization Slide21: Some prokaryotes Do have specialized membranes that perform metabolic functions Slide22: The typical prokaryotic genome Is a ring of DNA that is not surrounded by a membrane and that is located in a nucleoid region Slide23: Some species of bacteria Also have smaller rings of DNA called plasmids Reproduction and Adaptation: Reproduction and Adaptation Prokaryotes reproduce quickly by binary fission And can divide every 1–3 hours Slide25: Many prokaryotes form endospores Which can remain viable in harsh conditions for centuries When the environment becomes more hospitable, the endospore absorbs water and resumes growth. Sterilization in an autoclave kills endospores by heating them to 120°C under high pressure. Slide26: Rapid reproduction and horizontal gene transfer Facilitate the evolution of prokaryotes to changing environments Lacking meiotic sex, mutation is the major source of genetic variation in prokaryotes. Horizontal gene transfer: Horizontal gene transfer Conjugation can permit exchange of a plasmid containing a few genes or large groups of genes. Once the transferred genes are incorporated into the prokaryote’s genome, they are subject to natural selection. Horizontal gene transfer is a major force in the long-term evolution of pathogenic bacteria. Slide28: Concept 27.2: A great diversity of nutritional and metabolic adaptations have evolved in prokaryotes Species that use light energy are phototrophs. Species that obtain energy from chemicals in their environment are chemotrophs. Organisms that need only CO2 as a carbon source are autotrophs. [自營] Organisms that require at least one organic nutrient as a carbon source are heterotrophs. [異營] Slide29: Photoautotrophs are photosynthetic organisms that harness light energy to drive the synthesis of organic compounds from carbon dioxide. Among the photoautotrophic prokaryotes are the cyanobacteria. Among the photosynthetic eukaryotes are plants and algae. Slide30: Chemoautotrophs need only CO2 as a carbon source, but they obtain energy by oxidizing inorganic substances, rather than light. These substances include hydrogen sulfide (H2S), ammonia (NH3), and ferrous ions (Fe2+) among others. This nutritional mode is unique to prokaryotes. Slide31: Photoheterotrophs use light to generate ATP but obtain their carbon in organic form. This mode is restricted to prokaryotes. Chemoheterotrophs must consume organic molecules for both energy and carbon. This nutritional mode is found widely in prokaryotes, protists, fungi, animals, and even some parasitic plants. Slide32: Table 27.1 in prokaryotes Metabolic Relationships to Oxygen: Metabolic Relationships to Oxygen Prokaryotic metabolism Also varies with respect to oxygen Obligate aerobes Require oxygen Facultative anaerobes Can survive with or without oxygen Obligate anaerobes Are poisoned by oxygen Nitrogen Metabolism: Nitrogen Metabolism Prokaryotes can metabolize nitrogen nitrogen fixation Some prokaryotes convert atmospheric nitrogen to ammonia Metabolic Cooperation: Metabolic Cooperation Cooperation between prokaryotes Allows them to use environmental resources they could not use as individual cells Slide36: In the cyanobacterium Anabaena Photosynthetic cells and nitrogen-fixing cells exchange metabolic products Photosynthetic cells Heterocyst 20 m Figure 27.10 27.10 Oscillatoria 顫藻屬 : 27.10 Oscillatoria 顫藻屬 Slide38: In some prokaryotic species Metabolic cooperation occurs in surface-coating colonies called biofilms colored SEM for dental plaque Slide39: Concept 27.3: Molecular systematics is illuminating prokaryotic phylogeny Until the late 20th century Systematists based prokaryotic taxonomy on phenotypic criteria Applying molecular systematics to the investigation of prokaryotic phylogeny Has produced dramatic results Lessons from Molecular Systematics: Lessons from Molecular Systematics Molecular systematics Is leading to a phylogenetic classification of prokaryotes Is allowing systematists to identify major new clades Carl Woese and his colleagues used small-subunit ribosomal RNA (SSU-rRNA) as a marker for evolutionary relationships. Microbiologists have since analyzed larger amounts of genetic data, including whole genomes of some species. Slide41: A tentative phylogeny of some of the major taxa of prokaryotes based on molecular systematics Figure 27.12 Two important lessons from studies of prokaryotic phylogeny: Two important lessons from studies of prokaryotic phylogeny One is that the genetic diversity of prokaryotes is immense. Every year, new prokaryotes are identified that add major new branches to the tree of life. Another important lesson is the significance of horizontal gene transfer in the evolution of prokaryotes. As a result, significant portions of the genomes of many prokaryotes are actually mosaics of genes imported from other species. Slide43: Figure 27.13 Chlamydia (arrows) inside an animal cell (colorized TEM) Leptospira, a spirochete (colorized TEM) Streptomyces, the source of many antibiotics (colorized SEM) Two species of Oscillatoria, filamentous cyanobacteria (LM) Hundreds of mycoplasmas covering a human fibroblast cell (colorized SEM) 2.5 m 5 m 5 m 50 m 1 m Figure 27.13 Chlamydias, spirochetes, Gram +bact. cyanobacteria Proteobacteria 5 major groups of bacteria Bacteria: Bacteria The two largest groups are The proteobacteria and the Gram-positive bacteria Archaea: Archaea Archaea share certaintraits with bacteria And other traits with eukaryotes Slide46: Some archaea Live in extreme environments Extreme thermophiles Thrive in very hot environments Pyrococcus furiosus is as the source of DNA polymerase for the polymerase chain reaction (PCR). Slide47: Extreme halophiles Live in high saline environments Slide48: Methanogens Live in swamps and marshes Produce methane as a waste product Slide49: All known extreme halophiles and methanogens, plus a few extreme thermophiles, are members of a clade called Euryarchaeota. Most thermophilic species belong to a second clade, Crenarchaeota. Slide50: Concept 27.4: Prokaryotes play crucial roles in the biosphere Chemical Recycling Prokaryotes play a major role In the continual recycling of chemical elements between the living and nonliving components of the environment in ecosystems Chemoheterotrophic prokaryotes function as decomposers Nitrogen-fixing prokaryotes add usable nitrogen to the environment Slide51: In mutualism, both symbionts benefit. In commensalism, one symbiont receives benefits while the other is not harmed or helped by the relationship. In parasitism, one symbiont, the parasite, benefits at the expense of the host. Prokaryotes are involved in all three categories of symbiosis with eukaryotes. Symbiotic Relationships Organ haboring bioluminescent bact. Slide52: Pathogenic Prokaryotes Prokaryotes cause about half of all human diseases, e.g., Lyme disease. Concept 27.5: Prokaryotes have both harmful and beneficial impacts on humans Slide53: Exotoxins are proteins secreted by prokaryotes. Exotoxins can produce disease symptoms even if the prokaryote is not present. Clostridium botulinum, which grows anaerobically in improperly canned foods, produces an exotoxin that causes botulism. An exotoxin produced by Vibrio cholerae causes cholera. Pathogenic prokaryotes typically cause disease By releasing exotoxins or endotoxins Slide54: Endotoxins are components of the outer membranes of some gram-negative bacteria. Salmonella typhi, which are not normally present in healthy animals, causes typhoid fever. Other Salmonella species, including some that are common in poultry, cause food poisoning. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Prokaryotes in Research and Technology: Prokaryotes in Research and Technology Experiments using prokaryotes Have led to important advances in DNA technology Prokaryotes are the principal agents in bioremediation. The use of organisms to remove pollutants from the environment Slide56: Prokaryotes are also major tools in Mining (recovery metal from ores) The synthesis of vitamins Production of antibiotics, hormones, and other products Summary-1: Summary-1 A. Structural, Functional, and Genetic Adaptations Contribute to Prokaryotic Success 1. Prokaryotes are small. 2. Nearly all prokaryotes have a cell wall external to the plasma membrane. 3. Many prokaryotes are motile. 4. The cellular and genomic organization of prokaryotes is fundamentally different from that of eukaryotes. 5. Populations of prokaryotes grow and adapt rapidly. B. Nutritional and Metabolic Diversity 1. A great diversity of nutritional and metabolic adaptations have evolved in prokaryotes. Summary-2: Summary-2 C. A Survey of Prokaryotic Diversity 1. Molecular systematics is leading to a phylogenetic classification of prokaryotes. 2. Researchers are identifying a great diversity of archaea in extreme environments and in the oceans. D. The Ecological Impact of Prokaryotes 1. Prokaryotes are indispensable links in the recycling of chemical elements in ecosystems. 2. Many prokaryotes are symbiotic. 3. Some prokaryotes are human pathogens. 4. Humans use prokaryotes in research and technology.