Prokaryotes

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
Biologists are discovering
That these organisms have an astonishing genetic diversity
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
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
The cell wall of many prokaryotes
Is covered by a capsule, a sticky layer of polysaccharide or protein
Some prokaryotes have fimbriae and pili
Which allow them to stick to their substrate or other individuals in a colony
Motility
Most motile bacteria propel themselves by flagella
Which are structurally and functionally different from eukaryotic flagella
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
Some prokaryotes
Do have specialized membranes that perform metabolic functions
The typical prokaryotic genome
Is a ring of DNA that is not surrounded by a membrane and that is located in a nucleoid region
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
Rapid reproduction and horizontal gene transfer
Facilitate the evolution of prokaryotes to changing environments
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
Obligate aerobes
Require oxygen
Facultative anaerobes
Can survive with or without oxygen
Obligate anaerobes
Are poisoned by oxygen
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
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
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
Figure 27.12
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
Chromatium; the small
globules are sulfur wastes (LM)
Fruiting bodies of
Chondromyces crocatus,
a myxobacterium (SEM)
Bdellovibrio bacteriophorus
Attacking a larger bacterium
(colorized TEM)
2.5 m
1 m
0.5 m
10 m
5 m
2 m
Figure 27.13
Chlamydias, spirochetes, Gram-positive bacteria, and cyanobacteria
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
Archaea
Archaea share certaintraits with bacteria
And other traits with eukaryotes
Some archaea
Live in extreme environments
Extreme thermophiles
Thrive in very hot environments
Extreme halophiles
Live in high saline environments
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
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
Other types of prokaryotes
Live inside hosts as parasites
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
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
Prokaryotes are also major tools in
Mining
The synthesis of vitamins
Production of antibiotics, hormones, and other products