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Prokaryotes
Lecture Chapter Outline
Overview: They're (Almost) Everywhere! Prokaryotes were the earliest organisms on Earth. Today, they still dominate the biosphere. Their collective biomass outweighs all eukaryotes combined at least tenfold. More prokaryotes inhabit a handful of fertile soil or the mouth or skin of a human than the total number of people who have ever lived. 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. 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. Concept 27.1 Structural, functional, and genetic adaptations contribute to prokaryotic success
Prokaryotes are small.
Most prokaryotes are unicellular. Some species may aggregate transiently or form true colonies, showing division of labor between specialized cell types. Most prokaryotes have diameters in the range of 15 m, compared to 10100 m for most eukaryotic cells. The largest prokaryote discovered so far has a diameter of 750 m. The most common shapes among prokaryotes are spheres (cocci), rods (bacilli), and helices.
Nearly all prokaryotes have a cell wall external to the plasma membrane.
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In nearly all prokaryotes, a cell wall maintains the shape of the cell, affords physical protection, and prevents the cell from bursting in a hypotonic environment. In a hypertonic environment, most prokaryotes lose water and plasmolyze, like other walled cells. Severe water loss inhibits the reproduction of prokaryotes, which explains why salt can be used to preserve foods. Most bacterial cell walls contain peptidoglycan, a polymer of modified sugars cross-linked by short polypeptides. The walls of archaea lack peptidoglycan. The Gram stain is a valuable tool for identifying specific bacteria based on differences in their cell walls. Gram-positive bacteria have simple cell walls with large amounts of peptidoglycans. Gram-negative bacteria have more complex cell walls with less peptidoglycan. An outer membrane on the cell wall of gram-negative cells contains lipopolysaccharides, carbohydrates bonded to lipids. Among pathogenic bacteria, gram-negative species are generally more deadly than gram-positive species. The lipopolysaccharides on the walls of gram-negative bacteria are often toxic, and the outer membrane protects the pathogens from the defenses of their hosts. Gram-negative bacteria are commonly more resistant than grampositive species to antibiotics because the outer membrane impedes entry of the drugs. Many antibiotics, including penicillin, inhibit the synthesis of crosslinks in peptidoglycans, preventing the formation of a functional wall, especially in gram-positive species. These drugs cripple many species of bacteria, without affecting human and other eukaryote cells that do not synthesize peptidoglycans. Many prokaryotes secrete another sticky protective layer of polysaccharide or protein, the capsule, outside the cell wall. 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. Another way for prokaryotes to adhere to one another or to the substratum is by surface appendages called fimbriae and pili. 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.
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Many prokaryotes are motile.
About half of all prokaryotes are capable of directional movement. Some species can move at speeds exceeding 50 m/sec, about 100 times their body length per second. The beating of flagella scattered over the entire surface or concentrated at one or both ends is the most common method of movement. The flagella of prokaryotes differ in structure and function from those of eukaryotes. In a heterogeneous environment, many prokaryotes are capable of taxis, movement toward or away from a stimulus. Prokaryotes that exhibit chemotaxis respond to chemicals by changing their movement patterns. Solitary E. coli may exhibit positive chemotaxis toward other members of their species, enabling the formation of colonies. The cells of prokaryotes are simpler than those of eukaryotes in both internal structure and genomic organization. Prokaryotic cells lack the complex compartmentalization found in eukaryotic cells. Instead, prokaryotes use specialized infolded regions of the plasma membrane to perform many metabolic functions, including cellular respiration and photosynthesis. Prokaryotes have smaller, simpler genomes than eukaryotes. On average, a prokaryote has only about one-thousandth as much DNA as a eukaryote. In the majority of prokaryotes, the genome consists of a ring of DNA with few associated proteins. The prokaryotic chromosome is located in the nucleoid region. Prokaryotes may also have smaller rings of DNA called plasmids, which consist of only a few genes. Prokaryotes can survive in most environments without their plasmids because their chromosomes program all essential functions. Plasmid genes provide resistance to antibiotics, direct metabolism of unusual nutrients, and other special contingency functions. Plasmids replicate independently of the chromosome and can be transferred between partners during conjugation. Although the general processes for DNA replication and translation of mRNA into proteins are fundamentally alike in eukaryotes and prokaryotes, some of the details differ. For example, prokaryotic ribosomes are slightly smaller than the eukaryotic version and differ in protein and RNA content.
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The cellular and genomic organization of prokaryotes is fundamentally different from that of eukaryotes.
Lecture Outline for Campbell/Reece Biology, 7th Edition, Pearson Education, Inc.
These differences are great enough that selective antibiotics, including tetracycline and erythromycin, bind to prokaryotic ribosomes to block protein synthesis in prokaryotes but not in eukaryotes.
Populations of prokaryotes grow and adapt rapidly.
Prokaryotes have the potential to reproduce quickly in a favorable environment. Prokaryotes reproduce asexually via binary fission, synthesizing DNA almost continuously. While most prokaryotes have generation times of 13 hours, some species can produce a new generation in 20 minutes under optimal conditions. A single cell in favorable conditions will produce a large colony of offspring very quickly. Of course, prokaryotic reproduction is limited because cells eventually exhaust their nutrient supply, accumulate metabolic wastes, or are consumed by other organisms. Some bacteria form resistant cells called endospores when an essential nutrient is lacking in the environment. A cell replicates its chromosome and surrounds one chromosome with a durable wall to form the endospore. The original cell then disintegrates to leave the endospore behind. An endospore is resistant to all sorts of trauma. Endospores can survive lack of nutrients and water, extreme heat or cold, and most poisons. Most endospores can survive in boiling water. Endospores may be dormant for centuries or more. When the environment becomes more hospitable, the endospore absorbs water and resumes growth. Sterilization in an autoclave kills endospores by heating them to 120C under high pressure. Lacking meiotic sex, mutation is the major source of genetic variation in prokaryotes. With generation times of minutes or hours, prokaryotic populations can adapt very rapidly to environmental changes as natural selection favors gene mutations that confer greater fitness. As a consequence, prokaryotes are important model organisms for scientists who study evolution in the laboratory. Richard Lenski and his colleagues have maintained colonies of E. coli through more than 20,000 generations since 1988. The researchers regularly freeze samples of the colonies and later thaw them to compare their characteristics to those of their descendents.
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Such comparisons have revealed that the colonies in Lenski's laboratory can grow 60% faster than those that were frozen in 1988. Lenski's team is studying the genetic changes underlying the adaptation of the bacteria to their environment. By measuring RNA production, the researchers found that two separate colonies showed changes in expression of the same 59 genes, compared to the original colonies. The direction of change--increased or decreased expression-- was the same for every gene. This is an apparent case of parallel adaptive evolution. Horizontal gene transfer also facilitates rapid evolution of prokaryotes. 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. Concept 27.2 A great diversity of nutritional and metabolic adaptations have evolved in prokaryotes Organisms can be categorized by their nutrition, based on how they obtain energy and carbon to build the organic molecules that make up their cells. Nutritional diversity is greater among prokaryotes than among all eukaryotes. Every type of nutrition observed in eukaryotes is found in prokaryotes, along with some nutritional modes unique to prokaryotes. Organisms that obtain energy from light are phototrophs. Organisms that obtain energy from chemicals in their environment are chemotrophs. Organisms that need only CO2 as a carbon source are autotrophs. Organisms that require at least one organic nutrient--such as glucose--as a carbon source are heterotrophs. These categories of energy source and carbon source can be combined to group prokaryotes according to four major modes of nutrition. 1. 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.
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2. Chemoautotrophs need only CO2 as a carbon source but obtain energy by oxidizing inorganic substances. These substances include hydrogen sulfide (H 2S), ammonia (NH3), and ferrous ions (Fe2+) among others. This nutritional mode is unique to prokaryotes. 3. Photoheterotrophs use light to generate ATP but obtain their carbon in organic form. This mode is restricted to a few marine prokaryotes. 4. 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. Prokaryotic metabolism also varies with respect to oxygen. Obligate aerobes require O2 for cellular respiration. Facultative anaerobes will use O2 if present but can also grow by fermentation in an anaerobic environment. Obligate anaerobes are poisoned by O2 and use either fermentation or anaerobic respiration. In anaerobic respiration, inorganic molecules other than O2 accept electrons from electron transport chains. Nitrogen is an essential component of proteins and nucleic acids in all organisms. Eukaryotes are limited in the forms of nitrogen they can use. In contrast, diverse prokaryotes can metabolize a wide variety of nitrogenous compounds. Nitrogen-fixing prokaryotes convert N2 to NH3, making atmospheric nitrogen available to themselves (and eventually to other organisms) for incorporation into organic molecules. Nitrogen-fixing cyanobacteria are the most self-sufficient of all organisms. They require only light energy, CO2, N2, water, and some minerals to grow. Prokaryotes were once thought of as single-celled individualists. Microbiologists now recognize that cooperation between prokaryotes allows them to use environmental resources they cannot exploit as individuals. Cooperation may involve specialization in cells of a prokaryotic colony. For example, the cyanobacterium Anabaena forms filamentous colonies with specialized cells to carry out nitrogen fixation. Photosynthesis produces O2, which inactivates the enzymes involved in nitrogen fixation. Most cells in the filament are photosynthetic, while a few specialized cells called heterocysts carry out only nitrogen fixation.
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A heterocyst is surrounded by a thickened cell wall that restricts the entry of oxygen produced by neighboring photosynthetic cells. Heterocysts transport fixed nitrogen to neighboring cells in exchange for carbohydrates. In some prokaryotic species, metabolic cooperation occurs in surface-coating colonies known as biofilms. Cells in a colony secrete signaling molecules to recruit nearby cells, causing the colony to grow. Once the colony is sufficiently large, the cells begin producing proteins that adhere the cells to the substrate and to one another. Channels in the biofilms allow nutrients to reach cells in the interior and allow wastes to be expelled. In some cases, different species of prokaryotes may cooperate. For example, sulfate-consuming bacteria and methane-consuming archaea coexist in ball-shaped aggregates in the mud of the ocean floor. The bacteria use the archaea's waste products. In turn, the bacteria produce compounds that facilitate methane consumption by the archaea. Each year, these archaea consume an estimated 300 billion kg of methane, a major greenhouse gas. Concept 27.3 Molecular systematics is illuminating prokaryotic phylogeny Until the late 20th century, systematists based prokaryotic taxonomy on criteria such as shape, motility, nutritional mode, and Gram staining. These characteristics may not reflect evolutionary relationships. Applying molecular systematics to the of investigation prokaryotic phylogeny has been very fruitful. Microbiologists began comparing sequences of prokaryotic genes in the 1970s. Carl Woese and his colleagues used small-subunit ribosomal RNA (SSU-rRNA) as a marker for evolutionary relationships. They concluded that many prokaryotes once classified as bacteria are actually more closely related to eukaryotes and that they belong in a domain of their own--Archaea. Microbiologists have since analyzed larger amounts of genetic data, including whole genomes of some species. They found that a few traditional taxonomic groups, such as cyanobacteria, are monophyletic. Other groups, such as gram-negative bacteria, are scattered throughout several lineages.
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Two important lessons have already emerged from studies of prokaryotic phylogeny. One is that the genetic diversity of prokaryotes is immense. When researchers began to sequence the genes of prokaryotes, they could only investigate those species that can be cultured in the laboratory, a tiny minority of all prokaryotes. Norman Price of the University of Colorado pioneered methods that allow researchers to sample genetic material directly from the environment. Every year, new prokaryotes are identified that add major new branches to the tree of life. Some researchers suggest that certain branches represent new kingdoms. While only 4,500 prokaryotes have been fully characterized, a single handful of soil could contain 10,000 prokaryotic species, according to some estimates. Another important lesson is the significance of horizontal gene transfer in the evolution of prokaryotes. Over hundreds of millions of years, prokaryotes have acquired genes from distantly related species, and they continue to do so today. As a result, significant portions of the genomes of many prokaryotes are actually mosaics of genes imported from other species.
Researchers are identifying a great diversity of archaea in extreme environments and in the oceans.
Early on prokaryotes diverged into two lineages, the domains Archaea and Bacteria. The name bacteria was once synonymous with prokaryotes, but it now applies to just one of the two distinct prokaryotic domains. However, most known prokaryotes are bacteria. Bacteria include the vast majority of familiar prokaryotes, from pathogens causing strep throat to beneficial species making Swiss cheese. Every major mode of nutrition and metabolism is represented among bacteria. The major bacterial taxa are now accorded kingdom status by most prokaryotic systematists. Archaea share certain traits with bacteria and other traits with eukaryotes. Archaea also have many unique characteristics, as expected for a taxon that has followed a separate evolutionary path for so long. However, much of the research on archaea has focused not on phylogeny, but on their ecology--their ability to live where no other life can.
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The first prokaryotes to be classified in domain Archaea are species that can live in environments so extreme that few other organisms can survive there. Such organisms are known as extremophiles, or lovers of extreme environments. Extremophiles include extreme thermophiles, extreme halophiles, and methanogens. Extreme thermophiles thrive in hot environments. The optimum temperatures for most thermophiles are 60C80C. Sulfolobus oxidizes sulfur in hot sulfur springs in Yellowstone National Park. Another sulfur-metabolizing thermophile can survive at temperatures as high as 113C in water near deep-sea hydrothermal vents. Pyrococcus furiosus is an extreme thermophile that is used in biotechnology as the source of DNA polymerase for the polymerase chain reaction (PCR). Extreme halophiles live in such salty places as the Great Salt Lake and the Dead Sea. Some species merely tolerate elevated salinity; others require an extremely salty environment to grow. Colonies of certain extreme halophiles form a purple-red scum from bacteriorhodopsin, a photosynthetic pigment very similar to the visual pigment in the human retina. Methanogens obtain energy by using CO2 to oxidize H2, producing methane as a waste product. Methanogens are among the strictest anaerobes and are poisoned by O2. Some species live in swamps and marshes where other microbes have consumed all the oxygen. Marsh gas is actually methane produced by the archaea. Methanogens are important decomposers in sewage treatment. Other methanogens live in the anaerobic guts of animals, playing an important role in their nutrition. They contribute to the greenhouse effect through the production of methane. 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. Genetic prospecting has revealed that both Euryarchaeota and Crenarchaeota include many species of archaea that are not extremophiles. These species exist in habitats ranging from farm soils to lake sediments to the surface of the ocean water.
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New findings continue to update our understanding of archaean phylogeny. A new clade, Korarchaeota, has been identified that appears to be the oldest lineage in the domain Archaea. In 2002, researchers exploring hydrothermal vents off the cost of Iceland discovered archaean cells only 0.4 m in diameter attached to a much larger crenarchaeote. The genome of this tiny archaean is one of the smallest known of any organisms, containing only 500,000 base pairs. This prokaryote belongs to a fourth archaean clade called Nanoarchaeota. Three new nanoarchaeote species have since been found, one from Yellowstone's hot springs, one from hot springs in Siberia, and one from a hydrothermal vent in the Pacific. Concept 27.4 Prokaryotes play crucial roles in the biosphere If humans were to disappear from the planet tomorrow, life on Earth would go on for most other species. But prokaryotes are so important to the biosphere that if they were to disappear, the prospects for any other life surviving would be dim.
Prokaryotes are indispensable links in the recycling of chemical elements in ecosystems.
The atoms that make up the organic molecules in all living things were at one time part of inorganic compounds in the soil, air, and water. Life depends on the recycling of chemical elements between the biological and chemical components of ecosystems. Prokaryotes play an important role in this process. Chemoheterotrophic prokaryotes function as decomposers, breaking down corpses, dead vegetation, and waste products and unlocking supplies of carbon, nitrogen, and other elements essential for life. Prokaryotes also mediate the return of elements from the nonliving components of the environment to the pool of organic compounds. Autotrophic prokaryotes use carbon dioxide to make organic compounds, which are then passed up through food chains. Prokaryotes have many unique metabolic capabilities. They are the only organisms able to metabolize inorganic molecules containing elements such as iron, sulfur, nitrogen, and hydrogen. Cyanobacteria not only synthesize food and restore oxygen to the atmosphere, but they also fix nitrogen.
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This stocks the soil and water with nitrogenous compounds that other organisms can use to make proteins. When plants and animals die, other prokaryotes return the nitrogen to the atmosphere.
Many prokaryotes are symbiotic.
Prokaryotes often interact with other species of prokaryotes or eukaryotes with complementary metabolisms. An ecological relationship between organisms that are in direct contact is called symbiosis. If one of the symbiotic organisms is larger than the other, it is termed the host, and the smaller is known as the symbiont. In commensalism, one symbiotic organism benefits while the other is not harmed or helped by the relationship. In parasitism, one symbiotic organism, the parasite, benefits at the expense of the host. In mutualism, both symbiotic organisms benefit. Human intestines are home to an estimated 500 to 1,000 species of bacteria, which greatly outnumber all human cells in the body. Many of these species are mutualists, digesting food that our own intestines cannot. In 2003, scientists at Washington University in St. Louis published the first complete genome for a gut mutualist, Bacteroides thetaiotaomicron. The genome includes a large array of genes involved in synthesizing carbohydrates, vitamins, and other nutrients needed by humans. Signals from the bacterium activate human genes that build the network of intestinal blood vessels necessary to absorb food. Other signals induce human cells to produce antimicrobial compounds to which B. thetaiotaomicron is not susceptible, protecting the bacterium from its competitors.
Concept 27.5 Prokaryotes have both harmful and beneficial impacts on humans Pathogenic prokaryotes represent only a small fraction of prokaryotic species. Other prokaryotes serve as essential tools in agriculture and industry. Prokaryotes cause about half of human diseases. Between 2 and 3 million people a year die of the lung disease tuberculosis, caused by the bacillus Mycobacterium tuberculosis. Another 2 million die from diarrhea caused by other prokaryotes.
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Lyme disease, caused by a bacterium carried by ticks that live on deer and field mice, is the most widespread pest-carried disease in the United States. If untreated, Lyme disease can lead to debilitating arthritis, heart disease, and nervous disorders. Pathogens cause illness by producing poisons called exotoxins and endotoxins. Exotoxins are proteins secreted by prokaryotes. Exotoxins can produce disease symptoms even if the prokaryote is not present. An exotoxin produced by Vibrio cholerae causes cholera, a serious disease characterized by severe diarrhea. The exotoxin stimulates intestinal cells to release chloride ions (Cl-) into the gut; water follows by osmosis. Clostridium botulinum, which grows anaerobically in improperly canned foods, produces an exotoxin that causes botulism. Endotoxins are lipopolysaccharide components of the outer membrane of some gram-negative bacteria. In contrast to exotoxins, endotoxins are released only when the bacteria die and their cell walls break down. The endotoxin-producing bacteria in the genus Salmonella are not normally present in healthy animals. Salmonella typhi causes typhoid fever. Other Salmonella species, including some that are common in poultry, cause food poisoning. Since the discovery that germs cause disease, improved sanitation and improved treatments have reduced mortality and extended life expectancy in developed countries. Antibiotics have greatly reduced the threat of pathogenic prokaryotes and have saved a great many lives. However, resistance to antibiotics is currently evolving in many strains of prokaryotes. The rapid reproduction of prokaryotes enables genes conferring resistance to multiply quickly through prokaryotic populations as a result of natural selection. These genes can spread to other species by horizontal gene transfer. Horizontal gene transfer can also spread genes associated with virulence, turning harmless prokaryotes into fatal pathogens. E. coli is ordinarily a harmless symbiont in the human intestines. Pathogenic strains causing bloody diarrhea have arisen. One of the most dangerous strains is called O157:H7. Today, it is a global threat, with 75,000 cases annually in the United States alone.
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In 2001, an international team of scientists sequenced the genome of O157:H7 and compared it with the genome of a harmless strain of E. coli. 1,387 of the 5,416 genes in O157:H7 have no counterpart in the harmless strain. These 1,387 genes must have been incorporated into the genome of O157:H7 through horizontal gene transfer, most likely through the action of bacteriophages. Many of the imported genes are associated with the pathogen's invasion of its host. For example, some genes code for exotoxins that enable O157:H7 to attach itself to the intestinal wall and extract nutrients. Pathogenic prokaryotes pose a potential threat as weapons of bioterrorism. In October 2001, endospores of Bacillus anthracis, the bacterium that causes anthrax, were mailed to news media and the U.S. Senate. Other prokaryotes that could serve as weapons include C. botulinum and Yersinia pestis, which causes plague. This threat has stimulated intense research on pathogenic prokaryotes.
Humans use prokaryotes in research and technology.
Humans have learned to exploit the diverse metabolic capabilities of prokaryotes for scientific research and for practical purposes. Much of what we know about metabolism and molecular biology has been learned using prokaryotes, especially E. coli, as simple model systems. Increasingly, prokaryotes are used to solve environmental problems. The use of organisms to remove pollutants from air, water, and soil is bioremediation. The most familiar example is the use of prokaryote decomposers to treat human sewage. Anaerobic bacteria decompose the organic matter into sludge (solid matter in sewage), while aerobic microbes do the same to liquid wastes. Other bioremediation applications include breaking down radioactive waste and cleaning up oil spills. In the mining industry, prokaryotes help recover metals from ores. Bacteria assist in extracting more than 30 billion kg of copper from copper sulfides each year. Other prokaryotes can extract gold from ore. Through genetic engineering, humans can now modify prokaryotes to produce vitamins, antibiotics, hormones, and many other products.
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Craig Venter of the Human Genome Project has announced that he and his colleagues are attempting to build synthetic chromosomes for prokaryotes, producing new species form scratch. Venter hopes to design prokaryotes that can perform specific tasks, such as producing large amounts of hydrogen to reduce dependence on fossil fuels.
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