Prokaryotes and the Origins of Metabolic Diversity Chapter 27 Prokaryotes and the Origins of Metabolic Diversity

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

The Adaptive Ability of Prokaryotes Prokaryotes thrive almost everywhere including places too acidic, too salty, too cold, or too hot for most other organisms Figure 27.1

Bacteria and Archaea Bacteria and Archaea are the two main branches of prokaryotic evolution. Traditionally: Prokaryotes were classified as Monera New Scheme – 3 Domains: Bacteria & Archaea Archaea differ from bacteria structurally, biochemically, and physiologically. Archaea tend to live in extreme environments.

The Three Domains of Life

Genetic Diversity of Prokaryotes Biologists are discovering That these organisms have an astonishing genetic diversity Structural, functional, and genetic adaptations contribute to prokaryotic success Most prokaryotes are unicellular Although some species form colonies

Common Prokaryotic Shapes Prokaryotic cells have a variety of shapes The three most common of which are spheres (cocci), rods (bacilli), and spirals (spirilla). 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

Gram Positive v/s Gram Negative Bacteria 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

Pathogenic Bacteria Among pathogenic bacteria, gram-negative species are generally more threatening than gram-positive species. They are commonly more resistant to antibiotics than gram positive species, and the lipopolysaccharides on their walls are often toxic. Many antibiotics inhibit the synthesis of peptidoglycan in bacteria and thus prevent the formation of a functional cell wall – particularly in gram-positive species.

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

Attachment in Prokaryotes 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 Chemitaxis Phototaxis Magnetitaxis

Internal and Genomic Organization Prokaryotic cells usually lack complex compartmentalization although 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 Prokaryotic Genome 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

Plasmids Some species of bacteria also have smaller rings of DNA called plasmids Plasmids are separate from the bacterial chromosome

Reproduction and Adaptation Prokaryotes reproduce quickly by binary fission And can divide every 1–3 hours No mitosis or meiosis

Reproduction and Adaptation Prokaryotes have three mechanisms that transfer genes between individuals: Transformation Transduction Conjugation Rapid reproduction and horizontal gene transfer Facilitate the evolution of prokaryotes to changing environments

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

Major Nutritional Modes of Prokaryotes 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 of Prokaryotes Chemoautotrophs are primary producers of deep sea communities associated with hot water vents. Table 27.1

Nutritional Diversity Among Chemoheterotrophs The majority of known prokaryotes are chemoheterotrophs: Saprobes (decomposers that absorb their nutrients from dead organic matter) Parasites (absorb nutrients from the body fluids of living hosts)

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

Prokaryotes can metabolize nitrogen in a variety of ways Nitrogen Metabolism Prokaryotes can metabolize nitrogen in a variety of ways In a process called nitrogen fixation some prokaryotes convert atmospheric nitrogen (N2) to ammonia (NH4) Nitrogen fixation is the only biological mechanism that makes atmospheric nitrogen available to organisms for incorporation into organic compounds.

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

Halophiles Extreme halophiles Live in high saline environments Figure 27.14

Mehtanogens Methanogens Live in swamps and marshes Produce methane as a waste product Important decomposers in sewage treatment Can convert garbage and dung to methane

Prokaryotes and the Biosphere 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

Prokaryotic Interactions Other types of prokaryotes Live inside hosts as parasites 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

Pathogenenic Prokarytoes Pathogenic prokaryotes typically cause disease by releasing exotoxins or endotoxins Exotoxins are proteins secreted by prokaryotes that can produce disease symptoms even in the absence of the bacterium: Botulism Cholera Endotoxins are components of the outer membranes of gram-negative bacteria Salmonella 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 Sewage treatment, oil spill clean ups, commercial products Figure 27.17

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