CHAPTER 16 Microbial Life: Prokaryotes and Protists Chapter Objectives Prokaryotes 16.1 Describe the diversity, abundance, and importance of prokaryotic life. 16.2 Compare the different shapes, cell walls, and projections of prokaryotes. 16.3 Explain how bacteria can evolve quickly and how bacteria can survive stressful environments. 16.4 Describe the types of nutritional diversity found in prokaryotes. 16.5 Explain why biofilms are unique and potentially dangerous to human health. 16.6 Explain how prokaryotes are employed to address the needs of human society. 16.7 Compare the three domains of life based on differences in cellular and biochemical traits. 16.8 Describe the diverse types of Archaea living in extreme and more moderate environments. 16.9 Distinguish between the five subgroups of the domain Bacteria, noting the particular structure, special features, and habitats of each group. 16.10 Describe some of the diseases associated with bacteria. Distinguish between exotoxins and en- dotoxins, noting examples of each. 16.11 Describe the four parts of Koch’s postulates. Explain how Marshall was able to demonstrate that the bacterium Helicobacter pylori can cause gastritis. Protists 16.12 Describe the basic types of protists. Explain why biologists currently think that they represent many clades. 16.13 Explain how primary endosymbiosis and secondary endosymbiosis led to further cellular diversity. 16.14 Describe and compare the groups that together form the supergroup SAR. 16.15 Explain how cultured algae may be used as renewable fuels and the current challenges that re- main. 16.16–16.18 Describe and distinguish between the Excavata, Unikonta, and Archaeplastida groups. 16.18 Describe the life cycle of Ulva, noting each form in the alternation of generations and how each is produced. 16.19 Describe the protist ancestors of animals, plants, and fungi. Explain how each ancestral protist group is similar to its most likely descendants. Copyright © 2015 Pearson Education, Inc. 1 Outline I. Introduction 1. Microorganisms residing in and on your body outnumber your own cells 10 to 1—100 trillion bacteria, archaea, and protists call your body home. 2. Scientists hypothesize that disrupting our microbial communities may a. increase diseases, b. predispose us to certain cancers, and c. contribute to conditions such as asthma and other allergies, irritable bowel syndrome, Crohn’s disease, and autism. II. Prokaryotes A. 16.1 Prokaryotes are diverse and widespread 1. Prokaryotic cells are smaller than eukaryotic cells. a. Prokaryotes range from 1 to 5 µm in diameter. b. Eukaryotes range from 10 to 100 µm in diameter. 2. The collective biomass of prokaryotes is at least 10 times that of all eukaryotes. 3. Prokaryotes live in habitats a. too cold, b. too hot, c. too salty, d. too acidic, and e. too alkaline for eukaryotes to survive. 4. Some bacteria are pathogens causing disease. 5. But most bacteria on our bodies are benign or beneficial. 6. Our microbiota consists of the community of microorganisms that live in and on our bodies. 7. Each of us harbors several hundred different species and genetic strains of prokaryotes, including a few positive effects that are well studied. a. Some intestinal bacteria supply essential vitamins and enable us to extract nutrition from food mole- cules that we can’t otherwise digest. b. Many of the bacteria that live on our skin perform helpful housekeeping functions such as decom- posing dead skin cells. c. Other prokaryotes guard against pathogenic intruders. 8. Prokaryotes in soil help to decompose dead organisms and other organic waste material, which return vital chemical elements to the environment. 9. If prokaryotes were to disappear, a. the chemical cycles that sustain life would halt and b. all forms of eukaryotic life would be doomed. 10. There are two very different kinds of prokaryotes, which are classified in different domains. a. Archaea b. Bacteria B. 16.2 External features contribute to the success of prokaryotes 1. Prokaryotic cells have three common cell shapes. a. Cocci are spherical prokaryotic cells. They sometimes occur in chains that are called streptococci. b. Bacilli are rod-shaped prokaryotes. Bacilli may also be threadlike, or filamentous. c. Spiral prokaryotes are like a corkscrew. i. Short and rigid prokaryotes are called spirilla. ii. Longer, more flexible cells are called spirochetes. 2 Copyright © 2015 Pearson Education, Inc. 2. Nearly all prokaryotes have a cell wall. Cell walls a. provide physical protection and b. prevent the cell from bursting in a hypotonic environment. 3. When stained with Gram stain, cell walls of bacteria are either a. gram-positive, with simpler cell walls containing peptidoglycan, or b. gram-negative, with less peptidoglycan. These bacteria are more complex and more likely to cause disease. 4. The cell wall of many prokaryotes is covered by a capsule, a sticky layer of polysaccharides or protein. 5. The capsule a. enables prokaryotes to adhere to their substrate or to other individuals in a colony and b. shields pathogenic prokaryotes from attacks by a host’s immune system. 6. Some prokaryotes have external structures that extend beyond the cell wall. a. Flagella are adaptations that enable them to move about in response to chemical or physical signals in their environment. b. Hairlike projections called fimbriae enable prokaryotes i. to stick to a surface or each other or ii. latch onto the host cells they colonize. C. 16.3 Populations of prokaryotes can adapt rapidly to changes in the environment 1. Prokaryote population growth a. occurs by binary fission, b. can rapidly produce a new generation within hours, and c. can generate a great deal of genetic variation by spontaneous mutations, increasing the likelihood that some members of the population will survive changes in the environment. 2. The genome of a prokaryote typically a. has about one-thousandth as much DNA as a eukaryotic genome and b. is one long, circular chromosome packed into a distinct region of the cell. 3. Many prokaryotes also have additional small, circular DNA molecules called plasmids, which replicate independently of the chromosome. 4. Some prokaryotes form specialized cells called endospores that remain dormant through harsh condi- tions. 5. Endospores can survive extreme heat or cold. 6. When the endospore receives environmental cues that conditions have improved, it a. absorbs water and b. resumes growth. D. 16.4 Prokaryotes have unparalleled nutritional diversity 1. Prokaryotes exhibit much more nutritional diversity than eukaryotes, allowing them to inhabit almost every nook and cranny on Earth. 2. Two sources of energy are used. a. Phototrophs capture energy from sunlight. b. Chemotrophs harness the energy stored in chemicals. Copyright © 2015 Pearson Education, Inc. CHAPTER 16 Microbial Life: Prokaryotes and Protists 3 3. Two sources of carbon are used by prokaryotes. a. Autotrophs obtain carbon atoms from carbon dioxide. b. Heterotrophs obtain their carbon atoms from the organic compounds present in other organisms. 4. The terms that describe how prokaryotes obtain energy and carbon are combined to describe their modes of nutrition. a. Photoautotrophs obtain energy from sunlight and use CO2 for carbon. b. Photoheterotrophs obtain energy from sunlight but get their carbon atoms from organic sources. c. Chemoautotrophs harvest energy from inorganic chemicals and use carbon from CO2 to make or- ganic molecules. d. Chemoheterotrophs acquire energy and carbon from organic molecules. E. 16.5 CONNECTION: Biofilms are complex associations of microbes 1. Biofilms are highly organized colonies that attach to surfaces. 2. Biofilms consist of one or several species of prokaryotes and may also include protists and fungi. 3. Biofilms can form on most any support, including rocks, soil, organic material, or the surface of stag- nant water. 4. Biofilm formation begins when prokaryotes secrete signaling molecules that attract nearby cells into a cluster. 5. Once the cluster becomes sufficiently large, the cells produce a gooey coating that glues them to the support and to each other, making the biofilm extremely difficult to dislodge. 6. Biofilms a. communicate by chemical signals, b. coordinate a division of labor, and c. collectively defend against invaders. 7. Channels in the biofilm allow a. nutrients to reach cells in the interior, b. wastes to leave, and c. a variety of environments to develop within it. F. 16.6 CONNECTION: Prokaryotes help clean up the environment 1. Prokaryotes are useful for cleaning up contaminants in the environment because prokaryotes a. have great nutritional diversity, b. are quickly adaptable, and c. can form biofilms. 2. Bioremediation is the use of organisms to remove pollutants from a. soil, b. air, or c. water. 3. Prokaryotic decomposers are the mainstays of sewage treatment facilities. a. Raw sewage is first passed through a series of screens and shredders. b. Solid matter then settles out from the liquid waste, forming sludge. c. Sludge is gradually added to a culture of anaerobic prokaryotes, including bacteria and archaea. d. The microbes decompose the organic matter into material that can be placed in a landfill or used as fertilizer. 4. Liquid wastes are treated separately from the sludge. a. Liquid wastes are sprayed onto a thick bed of rocks. b. Biofilms of aerobic bacteria and fungi growing on the rocks remove much of the dissolved organic material. c. Fluid draining from the rocks is sterilized and then released, usually into a river or ocean. 4 Copyright © 2015 Pearson Education, Inc. 5. Bioremediation is becoming a useful tool for cleaning up toxic chemicals released into the soil and wa- ter. 6. Environmental workers may change the natural environment to accelerate the activity of naturally oc- curring prokaryotes capable of metabolizing pollutants. G. 16.7 Bacteria and Archaea are the two main branches of prokaryotic evolution 1. New studies of representative genomes of prokaryotes and eukaryotes strongly support the three-do- main view of life.