Chapter 27 Prokaryotes

Overview: They’re (Almost) Everywhere! Most prokaryotes are microscopic But what they lack in size they more than make up for in numbers The number of prokaryotes in a single handful of fertile soil Is greater than the number of people who have ever lived

Prokaryotes thrive almost everywhere Including places too acidic, too salty, too cold, or too hot for most other organisms Figure 27.1

Biologists are discovering That these organisms have an astonishing genetic diversity

Most prokaryotes are unicellular Concept 27.1: Structural, functional, and genetic adaptations contribute to prokaryotic success Most prokaryotes are unicellular Although some species form colonies

Prokaryotic cells have a variety of shapes The three most common of which are spheres (cocci), rods (bacilli), and spirals 1 m 2 m 5 m (a) Spherical (cocci) (b) Rod-shaped (bacilli) (c) Spiral Figure 27.2a–c

Cell-Surface Structures One of the most important features of nearly all prokaryotic cells Is their cell wall, which maintains cell shape, provides physical protection, and prevents the cell from bursting in a hypotonic environment

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 (a) Gram-positive. Gram-positive bacteria have a cell wall with a large amount of peptidoglycan that traps the violet dye in the cytoplasm. The alcohol rinse does not remove the violet dye, which masks the added red dye. (b) Gram-negative. Gram-negative bacteria have less peptidoglycan, and it is located in a layer between the plasma membrane and an outer membrane. The violet dye is easily rinsed from the cytoplasm, and the cell appears pink or red after the red dye is added. Figure 27.3a, b Peptidoglycan layer Cell wall Plasma membrane Protein Gram- positive bacteria 20 m Outer membrane Lipopolysaccharide negative

The cell wall of many prokaryotes Is covered by a capsule, a sticky layer of polysaccharide or protein 200 nm Capsule Figure 27.4

Some prokaryotes have fimbriae and pili Which allow them to stick to their substrate or other individuals in a colony 200 nm Fimbriae Figure 27.5

Motility Most motile bacteria propel themselves by flagella Which are structurally and functionally different from eukaryotic flagella Flagellum Filament Hook Cell wall Plasma membrane Basal apparatus 50 nm Figure 27.6

In a heterogeneous environment, many bacteria exhibit taxis The ability to move toward or away from certain stimuli

Internal and Genomic Organization Prokaryotic cells Usually lack complex compartmentalization

(a) Aerobic prokaryote (b) Photosynthetic prokaryote Some prokaryotes Do have specialized membranes that perform metabolic functions (a) Aerobic prokaryote (b) Photosynthetic prokaryote 0.2 m 1 m Respiratory membrane Thylakoid membranes Figure 27.7a, b

The typical prokaryotic genome Is a ring of DNA that is not surrounded by a membrane and that is located in a nucleoid region Figure 27.8 1 m Chromosome

Some species of bacteria Also have smaller rings of DNA called plasmids

Reproduction and Adaptation Prokaryotes reproduce quickly by binary fission And can divide every 1–3 hours

Many prokaryotes form endospores Which can remain viable in harsh conditions for centuries Endospore 0.3 m Figure 27.9

Rapid reproduction and horizontal gene transfer Facilitate the evolution of prokaryotes to changing environments

Examples of all four models of nutrition are found among prokaryotes Concept 27.2: A great diversity of nutritional and metabolic adaptations have evolved in prokaryotes Examples of all four models of nutrition are found among prokaryotes Photoautotrophy Chemoautotrophy Photoheterotrophy Chemoheterotrophy

Major nutritional modes in prokaryotes Table 27.1

Metabolic Relationships to Oxygen Prokaryotic metabolism Also varies with respect to oxygen

Facultative anaerobes Obligate aerobes Require oxygen Facultative anaerobes Can survive with or without oxygen Obligate anaerobes Are poisoned by oxygen

Prokaryotes can metabolize nitrogen Nitrogen Metabolism Prokaryotes can metabolize nitrogen In a variety of ways In a process called nitrogen fixation Some prokaryotes convert atmospheric nitrogen to ammonia

Metabolic Cooperation Cooperation between prokaryotes Allows them to use environmental resources they could not use as individual cells

In the cyanobacterium Anabaena Photosynthetic cells and nitrogen-fixing cells exchange metabolic products Photosynthetic cells Heterocyst 20 m Figure 27.10

In some prokaryotic species Metabolic cooperation occurs in surface-coating colonies called biofilms Figure 27.11 1 m

Until the late 20th century 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 Is leading to a phylogenetic classification of prokaryotes Is allowing systematists to identify major new clades

A tentative phylogeny of some of the major taxa of prokaryotes based on molecular systematics Domain Bacteria Domain Archaea Eukarya Alpha Beta Gamma Epsilon Delta Proteobacteria Chlamydias Spirochetes Cyanobacteria Gram-positive bacteria Korarchaeotes Euryarchaeotes Crenarchaeotes Nanoarchaeotes Eukaryotes Universal ancestor Figure 27.12

Diverse nutritional types Bacteria Diverse nutritional types Are scattered among the major groups of bacteria The two largest groups are The proteobacteria and the Gram-positive bacteria

Proteobacteria Figure 27.13 Rhizobium (arrows) inside a root cell of a legume (TEM) Nitrosomonas (colorized TEM) Chromatium; the small globules are sulfur wastes (LM) Fruiting bodies of Chondromyces crocatus, a myxobacterium (SEM) Bdellovibrio bacteriophorus Attacking a larger bacterium (colorized TEM) Helicobacter pylori (colorized TEM). 2.5 m 1 m 0.5 m Chromatium; the small globules are sulfur wastes (LM) 10 m 5 m Fruiting bodies of Chondromyces crocatus, a myxobacterium (SEM) Bdellovibrio bacteriophorus Attacking a larger bacterium (colorized TEM) 2 m Figure 27.13

Chlamydias, spirochetes, Gram-positive bacteria, and cyanobacteria Chlamydia (arrows) inside an animal cell (colorized TEM) 5 m Leptospira, a spirochete (colorized TEM) 5 m 1 m Hundreds of mycoplasmas covering a human fibroblast cell (colorized SEM) Streptomyces, the source of many antibiotics (colorized SEM) 50 m Figure 27.13 Two species of Oscillatoria, filamentous cyanobacteria (LM)

Archaea share certaintraits with bacteria And other traits with eukaryotes Table 27.2

Some archaea Extreme thermophiles Live in extreme environments Thrive in very hot environments

Extreme halophiles Live in high saline environments Figure 27.14

Methanogens Live in swamps and marshes Produce methane as a waste product

Concept 27.4: Prokaryotes play crucial roles in the biosphere Prokaryotes are so important to the biosphere that if they were to disappear The prospects for any other life surviving would be dim

Prokaryotes play a major role 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 Breaking down corpses, dead vegetation, and waste products Nitrogen-fixing prokaryotes Add usable nitrogen to the environment

Symbiotic Relationships Many prokaryotes Live with other organisms in symbiotic relationships such as mutualism and commensalism Figure 27.15

Other types of prokaryotes Live inside hosts as parasites

Some prokaryotes are human pathogens Concept 27.5: Prokaryotes have both harmful and beneficial impacts on humans Some prokaryotes are human pathogens But many others have positive interactions with humans

Pathogenic Prokaryotes Prokaryotes cause about half of all human diseases Lyme disease is an example 5 µm Figure 27.16

Pathogenic prokaryotes typically cause disease By releasing exotoxins or endotoxins Many pathogenic bacteria Are potential weapons of bioterrorism

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 Figure 27.17

Prokaryotes are also major tools in Mining The synthesis of vitamins Production of antibiotics, hormones, and other products