CHAPTER 27 PROKARYOTES AND THE ORIGINS OF

METABOLIC DIVERSITY

• Prokaryotes were the earliest organisms on Earth and evolved alone for 1.5 billion years.• Their collective biomass outweighs all eukaryotes

combined by 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.

They’re (almost) everywhere! An overview of prokaryotic life

• Live in diverse environments.• Most are benign or beneficial but others cause disease. • Diverse in structure and in metabolism.• About 5,000 species are known, but actual diversity may

range from about 400,000 to 4 million species.

Bacteria and archaea are the two main branches of prokaryote evolution

• Most prokaryotes are unicellular.• The most common

shapes among prokaryotes are spheres (cocci), rods (bacilli), and helices.

The Structure, Function, and Reproduction of Prokaryotes

• Most bacterial cell walls contain peptidoglycan, a polymer of modified sugars cross-linked by short polypeptides.• The walls of archaea lack peptidoglycan.

Nearly all prokaryotes have a cell wall external to the plasma membrane

Gram stain- used identify bacteria based on differences in their cell walls.

• Gram-positive bacteria have simpler cell walls, with large amounts of peptidoglycans.

• Gram-negative bacteria have more complex cell walls and less peptidoglycan.• Generally more threatening. The outer membrane

protects against host defenses. More resistant to antibiotics.

• Many prokaryotes secrete a sticky protective layer, the capsule, outside the cell wall.

• Pili- an appendage used to adhere to surfaces or other prokaryotes. Also used for conjugation.

• Flagella

• Taxis- movement toward or away from a stimulus (chemicals, light, magnetic fields).

Many prokaryotes are motile

• Prokaryotic cells lack a nucleus enclosed by membrane• smaller, simpler genomes than eukaryotes.

• small rings of DNA, plasmids, that consist of only a few genes.

The cellular and genomic organization of prokaryotes is fundamentally different from that of eukaryotes

• Prokaryotes reproduce only asexually via binary fission, synthesizing DNA almost continuously.

Populations of prokaryotes grow and adapt rapidly

Transformation- a cell can absorb and integrate fragments of DNA from their environment.Conjugation- one cell directly transfers genes to another cell.Transduction- viruses transfer genes between prokaryotes.

• Endospore- forms when a cell replicates its chromosome and surrounds one chromosome with a durable wall.

Can survive lack of nutrients and water, extreme heat or cold, and most poisons.May be dormant for centuries or more.When the environment becomes more hospitable, the endospore absorbs water and resumes growth.

• Species that use light energy are phototrophs.

• Species 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 as a carbon source are heterotrophs.

Nutrition and Metabolic Diversity-Prokaryotes can be grouped into four categories according to how they obtain energy and carbon

• Photoautotrophs- photosynthetic organisms that harness light energy to drive the synthesis of organic compounds from carbon dioxide.• Ex: cyanobacteria.

• Chemoautotrophs need only CO2 as a carbon source, but they obtain energy by oxidizing inorganic substances, rather than light.• These substances include hydrogen sulfide (H2S),

ammonia (NH3), and ferrous ions (Fe2+) among others.

• Photoheterotrophs use light to generate ATP but obtain their carbon in organic form.

• Chemoheterotrophs must consume organic molecules for both energy and carbon.

• Prokaryotes are responsible for the key steps in the cycling of nitrogen through ecosystems.• Some convert ammonium (NH4

+) to nitrite (NO2-).

• Others convert nitrite or nitrate (NO3-) to N2 (gas).

• Nitrogen fixation- convert N2 to NH4+, making

atmospheric nitrogen available to other organisms for incorporation into organic molecules.

• Obligate aerobes- require O2 for cellular respiration.

• Facultative anerobes- 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.

• The very first prokaryotes were heterotrophs, but photosynthesis capabilities soon followed.

Photosynthesis evolved early in prokaryotic life

• Carl Woese clustered prokarotes into taxonomic groups based on comparisons of nucleic acid sequences- small-subunit ribosomal RNA (SSU-rRNA)

A Survey of Prokaryotic DiversityMolecular systematics is leading to phylogenetic classification of prokaryotes

• Archaea are extremophiles, “lovers” of extreme environments.

Researchers are identifying a great diversity of archaea in extreme environments and in the oceans

• Methanogens obtain energy by using CO2 to oxidize H2 replacing methane as a waste.

• They live in swamps and marshes where other microbes have consumed all the oxygen.• Methanogens are important decomposers in sewage

treatment.

• Other methanogens live in the anaerobic guts of herbivorous animals, playing an important role in their nutrition.

• Extreme halophiles live in such saline places as the Great Salt Lake and the Dead Sea.

Extreme thermophiles thrive in hot environments.

60oC-80oC

Sulfolobus oxidizes sulfur in hot sulfur springs in Yellowstone National Park

Another sulfur-metabolizing thermophile lives at 105oC water near deep-sea hydrothermal vents

Most known prokarotes are bacteria

• Decomposers

• Metabolize inorganic molecules containing elements such as iron, sulfur, nitrogen, and hydrogen

• Restore oxygen to the atmosphere

• Fix nitrogen

The Ecological Impact of ProkaryotesProkaryotes are indispensable links in the recycling of chemical elements in ecosystems

• Commensalism, one symbiont receives benefits while the other is not harmed or helped by the relationship.

• Parasitism, one symbiont, the parasite, benefits at the expense of the host.

• Mutualism, both symbionts benefit.

Many prokaryotes are symbiotic

• Some pathogens are opportunistic.• These are normal residents of the host, but only cause

illness when the host’s defenses are weakened.

Exotoxins are proteins secreted by prokaryotes.

• C. botulinum, V. cholerae, E. coli

Endotoxins are components of the outer membranes of some gram-negative bacteria.

Salmonella

Pathogenic prokaryotes cause many human diseases

• The application of organisms to remove pollutants from air, water, and soil is bioremediation.• Decomposers treat human sewage.

Humans use prokaryotes in research and technology

• Soil bacteria have been developed to decompose petroleum products at the site of oil spills or to decompose pesticides.

The chemical industry produces acetone, butanol, and other products from bacteria.

The pharmaceutical industry cultures bacteria to produce vitamins and antibiotics.

The food industry used bacteria to convert milk to yogurt and various kinds of cheese.