PROKARYOTES © 2012 Pearson Education, Inc. Chapter 16 Microbial Life: Prokaryotes and Protists

PROKARYOTES

© 2012 Pearson Education, Inc.

Chapter 16 Microbial Life: Prokaryotes and Protists

Prokaryotic cells are smaller than eukaryotic cells.

– Prokaryotes range from 1–5 µm in diameter.

– Eukaryotes range from 10–100 µm in diameter.

The collective biomass of prokaryotes is at least 10 times that of all eukaryotes.

16.1 Prokaryotes are diverse and widespread


Figure 16.1 Bacteria on a pin point

Prokaryotes live in habitats

– too cold,

– too hot,

– too salty,

– too acidic, and

– too alkaline for eukaryotes to survive.

Some bacteria are pathogens, causing disease. But most bacteria on our bodies are benign or beneficial.



Several hundred species of bacteria live in and on our bodies,

– decomposing dead skin cells,

– supplying essential vitamins, and

– guarding against pathogenic organisms.

Prokaryotes in soil decompose dead organisms, sustaining chemical cycles.



Prokaryotic cells have three common cell shapes.

– Cocci are spherical prokaryotic cells. They sometimes occur in chains that are called streptococci.

– Bacilli are rod-shaped prokaryotes. Bacilli may also be threadlike, or filamentous.

– Spiral prokaryotes are like a corkscrew.

– Short and rigid prokaryotes are called spirilla.

– Longer, more flexible cells are called spirochetes.

16.2 External features contribute to the success of prokaryotes


Figure 16.2A

Cocci Bacilli Spirochete

Nearly all prokaryotes have a cell wall. Cell walls

– provide physical protection and

– prevent the cell from bursting in a hypotonic environment.

When stained with Gram stain, cell walls of bacteria are either

– Gram-positive, with simpler cell walls containing peptidoglycan, or

– Gram-negative, with less peptidoglycan, and more complex and more likely to cause disease.



Figure 16.2B Gram-positive (purple) and gram-negative (pink) bacteria

The cell wall of many prokaryotes is covered by a capsule, a sticky layer of polysaccharides or protein.

The capsule

– enables prokaryotes to adhere to their substrate or to other individuals in a colony and

– shields pathogenic prokaryotes from attacks by a host’s immune system.



Figure 16.2C

CapsuleTonsil cell

Bacterium

Some prokaryotes have external structures that extend beyond the cell wall.

– Flagella help prokaryotes move in their environment.

– Hairlike projections called fimbriae enable prokaryotes to stick to their substrate or each other.



Figure 16.2D

Flagella

Fimbriae

Prokaryote population growth

– occurs by binary fission,

– can rapidly produce a new generation within hours, and

– can generate a great deal of genetic variation

– by spontaneous mutations,

– increasing the likelihood that some members of the population will survive changes in the environment.

16.3 Populations of prokaryotes can adapt rapidly to changes in the environment


The genome of a prokaryote typically

– has about one-thousandth as much DNA as a eukaryotic genome and

– is one long, circular chromosome packed into a distinct region of the cell.

Many prokaryotes also have additional small, circular DNA molecules called plasmids, which replicate independently of the chromosome.



Figure 16.3A

Chromosome Plasmids

Some prokaryotes form specialized cells called endospores that remain dormant through harsh conditions.

Endospores can survive extreme heat or cold.



Figure 16.3b

Bacterium

Endospores

Prokaryotes exhibit much more nutritional diversity than eukaryotes.

Two sources of energy are used.

– Phototrophs capture energy from sunlight.

– Chemotrophs harness the energy stored in chemicals.

16.4 Prokaryotes have unparalleled nutritional diversity


Two sources of carbon are used by prokaryotes.

– Autotrophs obtain carbon atoms from carbon dioxide.

– Heterotrophs obtain their carbon atoms from the organic compounds present in other organisms.



The terms that describe how prokaryotes obtain energy and carbon are combined to describe their modes of nutrition.

– Photoautotrophs obtain energy from sunlight and use carbon dioxide for carbon.

– Photoheterotrophs obtain energy from sunlight but get their carbon atoms from organic molecules.

– Chemoautotrophs harvest energy from inorganic chemicals and use carbon dioxide for carbon.

– Chemoheterotrophs acquire energy and carbon from organic molecules.



Figure 16.4-0

Sunlight Chemicals

Photoautotrophs Chemoautotrophs

Oscillatoria Unidentified “rock-eating” bacteria

PhotoheterotrophsChemoheterotrophs

Rhodopseudomonas

Salmonella typhimuriumOrg

an

ic c

om

po

un

ds

CO

2

CA

RB

ON

SO

UR

CE

ENERGY SOURCE

Prokaryotes are useful for cleaning up contaminants in the environment because prokaryotes

– have great nutritional diversity,

– are quickly adaptable, and

– can form biofilms.

16.6 CONNECTION: Prokaryotes help clean up the environment


Bioremediation is the use of organisms to remove pollutants from

– soil,

– air, or

– water.



Prokaryotic decomposers are the mainstays of sewage treatment facilities.

– Raw sewage is first passed through a series of screens and shredders.

– Solid matter then settles out from the liquid waste, forming sludge.

– Sludge is gradually added to a culture of anaerobic prokaryotes, including bacteria and archaea.

– The microbes decompose the organic matter into material that can be placed in a landfill or used as fertilizer.



Liquid wastes are treated separately from the sludge.

– Liquid wastes are sprayed onto a thick bed of rocks.

– Biofilms of aerobic bacteria and fungi growing on the rocks remove much of the dissolved organic material.

– Fluid draining from the rocks is sterilized and then released, usually into a river or ocean.



Figure 16.6A

Rotatingspray arm

Rock bed coatedwith aerobicprokaryotesand fungi

OutflowLiquid wastes

Bioremediation is becoming an important tool for cleaning up toxic chemicals released into the soil and water by industrial processes.

Environmental engineers change the natural environment to accelerate the activity of naturally occurring prokaryotes capable of metabolizing pollutants.



Figure 16.6b

New studies of representative genomes of prokaryotes and eukaryotes strongly support the three-domain view of life.

– Prokaryotes are now classified into two domains:

– Bacteria and

– Archaea.

– Archaea have at least as much in common with eukaryotes as they do with bacteria.

16.7 Bacteria and archaea are the two main branches of prokaryotic evolution


Table 16.7

Archaeal inhabitants of extreme environments have unusual proteins and other molecular adaptations that enable them to metabolize and reproduce effectively.

– Extreme halophiles thrive in very salty places.

– Extreme thermophiles thrive in

– very hot water, such as geysers, and

– acid pools.

16.8 Archaea thrive in extreme environments—and in other habitats


Figure 16.8A Orange and yellow colonies of heat-loving archaea growing in a Nevada geyser

Methanogens

– live in anaerobic environments,

– give off methane as a waste product from

– the digestive tracts of cattle and deer and

– decomposing materials in landfills.

16.8 Archaea thrive in extreme environments—and in other habitats


The domain Bacteria is currently divided into five groups, based on comparisons of genetic sequences.

1. Proteobacteria

– are all gram negative,

– share a particular rRNA sequence, and

– represent all four modes of nutrition.

16.9 Bacteria include a diverse assemblage of prokaryotes


– Proteobacteria also include Rhizobium species that

– live symbiotically in root nodules of legumes and

– convert atmospheric nitrogen gas into a form usable by their legume host.

– Symbiosis is a close association between organisms of two or more species.

– Rhizobium is an endosymbiont, living within another species.



Figure 32.13B Root nodules on a soybean plant

Shoot

Nodules

Roots

Bacteria withinvesicle in aninfected cell

2. Gram-positive bacteria

– rival proteobacteria in diversity and

– include the actinomycetes common in soil.

– Streptomyces is often cultured by pharmaceutical companies as a source of many antibiotics.



Figure 16.9B Streptomyces, the source of many antibiotics

3. Cyanobacteria

– Cyanobacteria are the only group of prokaryotes with plantlike, oxygen-generating photosynthesis.

– Some species, such as Anabaena, have specialized cells that fix nitrogen.



Figure 16.9c

Photosyntheticcells

Nitrogen-fixing cells

Capsule

4. Chlamydias

– Chlamydias live inside eukaryotic host cells.

– Chlamydia trachomatis

– is a common cause of blindness in developing countries and

– is the most common sexually transmitted disease in the United States infecting urethral cells.



Figure 16.9D Chlamydia cells (arrows) inside an animal cell

5. Spirochetes are

– helical bacteria and

– notorious pathogens, causing

– syphilis and

– Lyme disease.



Figure 16.9E Treponema pallidum, the spirochete that causes syphilis

All organisms are almost constantly exposed to pathogenic bacteria.

Most bacteria that cause illness do so by producing a poison or toxins.

16.10 CONNECTION: Some bacteria cause disease


Koch’s postulates are four essential conditions used to establish that a certain bacterium is the cause of a disease. They are

1. find the bacterium in every case of the disease,

2. isolate the bacterium from a person who has the disease and grow it in pure culture,

3. show that the cultured bacterium causes the disease when transferred to a healthy subject, and

4. isolate the bacterium from the experimentally infected subject.

16.11 Koch’s postulates are used to prove that a bacterium causes a disease


Koch’s postulates were used to demonstrate that the bacterium Helicobacter pylori is the cause of most peptic ulcers.

The 2005 Nobel Prize in Medicine was awarded to Barry Marshall and Robin Warren for this discovery.

16.11 SCIENTIFIC DISCOVERY: Koch’s postulates are used to prove that a bacterium causes a disease


PROTISTS


Protists

– are a diverse collection of mostly unicellular eukaryotes,

– may constitute multiple kingdoms within the Eukarya, and

– refer to eukaryotes that are not

– plants,

– animals, or

– fungi.

16.12 Protists are an extremely diverse assortment of eukaryotes


Protists obtain their nutrition in many ways. Protists include

– autotrophs, called algae, producing their food by photosynthesis,

– heterotrophs, called protozoans, eating bacteria and other protists,

– heterotrophs, called parasites, deriving their nutrition from a living host, and

– mixotrophs, using photosynthesis and heterotrophy.



Figure 16.12A

Autotrophy Heterotrophy

Caulerpa, a green alga Giardia, a parasite

Mixotrophy

Euglena

Protists are found in many habitats including

– anywhere there is moisture and

– the bodies of host organisms.



Figure 16.12B A protist (excavate) from a termite gut covered by thousands of flagella

Recent molecular and cellular studies indicate that nutritional modes used to categorize protists do not reflect natural groups and that endosymbiosis has occurred.

Protist phylogeny remains unclear.

One hypothesis, used here, proposes four monophyletic supergroups.

16.13 Endosymbiosis of unicellular algae is the key to much protist diversity


Stramenopiles include

– diatoms, unicellular algae with a glass cell wall containing silica,

– brown algae, large complex algae with characteristic brown pigments in their chloroplasts like seaweed and kelp

– water molds, unicellular heterotrophs that are usually freshwater decomposers

16.14 The SAR supergroup represents the range of protist diversity


Figure 16.14A Diatom, a unicellular alga that is a stramenopile

Foraminiferans

– are found in the oceans and in fresh water,

– have porous shells, called tests, composed of calcium carbonate, and

– have pseudopodia that function in feeding and locomotion.

16.17 Rhizarians include a variety of amoebas


Figure 16.14B Brown algae: a kelp “forest”, a stamenopile

Figure 16.14C Water mold, a stramenopile

Alveolata includes

– dinoflagellates, unicellular autotrophs, heterotrophs, and mixotrophs that are common components of marine plankton,



Figure 16.14D A red tide caused by Gymnodinium, a dinoflagellate

Alveolata include

– dinoflagellates, unicellular autotrophs, heterotrophs, and mixotrophs that are common components of marine plankton,

– ciliates, unicellular heterotrophs and mixotrophs that use cilia to move and feed,

– a group including parasites, such as Plasmodium, which causes malaria.



Figure 16.14E A freshwater ciliate showing cilia distributed over the cell surface and around the mouth

MouthCell mouth

The two largest groups of Rhizaria, foramniferans and radiolarians, are among the organisms referred to as amoebas.

Amoebas move and feed by means of pseudopodia, temporary extensions of the cell.

Foramniferans have porous shells called tests and are both freshwater and marine



Figure 16.14F A foraminiferan (inset SEM shows a foram test of calcium carbonate)

Radiolarians

– are mostly marine and

– produce a mineralized internal skeleton made of silica.



Figure 16.14G A radiolarian skeleton of silica

Fossil fuels

– are the organic remains of organisms that lived hundreds of millions of years ago and

– primarily consist of

– diatoms and

– primitive plants.

16.15 Can algae provide a renewable source of energy?


Lipid droplets in diatoms and other algae may serve as a renewable source of energy.

If unicellular algae could be grown on a large scale, this oil could be harvested and processed into biodiesel.

Numerous technical hurdles remain before industrial-scale production of biofuel from algae becomes a reality.

16.15 Can algae provide a renewable source of energy?


Figure 16.15 Green algae in a bioreactor

Excavata has recently been proposed as a group on the basis of molecular and morphological similarities.

The name refers to an “excavated” feeding groove possessed by some members of the group.

Excavates

– have modified mitochondria that lack functional electron transport chains and

– use anaerobic pathways such as glycolysis to extract energy.

16.16 Some excavates have modified mitochondria


Excavates include

– heterotrophic termite endosymbionts



Figure 16.12B A protist from a termite gut covered by thousands of flagella

Excavates include

– heterotrophic termite endosymbionts,

– autotrophic species,

– mixotrophs such as Euglena



Figure 16.12A

Mixotrophy

Euglena

Excavates include



– mixotrophs such as Euglena,

– the common waterborne parasite Giardia intestinalis,



Figure 16.12A

Autotrophy Heterotrophy

Caulerpa, a green alga Giardia, a parasite

Mixotrophy

Euglena

Excavates include





– the parasite Trichomonas vaginalis, which causes 5 million new infections each year of human reproductive tracts,



Figure 16.16A A parasitic excavate: Trichomonas vaginalis

Flagella

Undulatingmembrane

Excavates include





– the parasite Trichomonas vaginalis, which causes 5 million new infections each year of human reproductive tracts, and

– the parasite Trypanosoma, which causes sleeping sickness in humans.



Figure 16.16B A parasitic excavate: Trypanosoma (with blood cells)

Unikonta is a controversial grouping joining

– Amoebozoans, which are protists and

– a group that includes animals and fungi.

16.17 Unikonts include protists that are closely related to fungi and animals


Amoebozoans have lobe-shaped pseudopodia and include

– many species of free-living amoebas,

– some parasitic amoebas cause diseases like dysentary, and

– slime molds.



Figure 16.17A An amoeba beginning to ingest an algal cell

Plasmodial slime molds

– are common where there is moist, decaying organic matter and

– consist of a single, multinucleate mass of cytoplasm undivided by plasma membranes, called a plasmodium.



Figure 16.17B A plasmodial slime mold: Physarum

Cellular slime molds

– are common on rotting logs and decaying organic matter and

– usually exist as solitary amoeboid cells, but when food is scarce, amoeboid cells

– swarm together, forming a slug-like aggregate that wanders around for a short time and then

– forms a stock supporting an asexual reproductive structure that produces spores.



Figure 16.17C An aggregate of amoeboid cells (left) and the reproductive structure of a cellular slime mold, Dictyostelium

Archaeplastids include:

– red algae,

– green algae, and

– land plants.

16.18 Archaeplastids include red algae, green algae, and land plants


Red algae

– are mostly multicellular,

– contribute to the structure of coral reefs, and

– are commercially valuable.



Figure 16.18A An encrusted red alga

Green algae may be unicellular, colonial, or multicellular.

– Volvox is a colonial green algae, and

– Chlamydomonas is a unicellular alga propelled by two flagella.



Figure 16.18B Green algae, colonial (left) and unicellular (right)

Volvox Chlamydomonas

Ulva, or sea lettuce, is

– a multicellular green alga with

– a complex life cycle that includes an alternation of generations that consists of

– a multicellular diploid (2n) form, the sporophyte, that alternates with

– a multicellular haploid (1n) form, the gametophyte.



Figure 16.18C

Mitosis

Spores

Mitosis

Femalegametophyte

Gametes

Malegametophyte

KeyHaploid (n)Diploid (2n)

Figure 16.18C

Mitosis

Spores

Mitosis

Femalegametophyte

Gametes

Malegametophyte

Fusion ofgametes

Zygote

KeyHaploid (n)Diploid (2n)

Figure 16.18C

Mitosis

Spores

Meiosis

Mitosis

Femalegametophyte

Gametes

Malegametophyte

Fusion ofgametes

ZygoteSporophyte

Mitosis KeyHaploid (n)Diploid (2n)

16.19 EVOLUTION CONNECTION: Multicellularity evolved several times in eukaryotes

The origin of the eukaryotic cell led to an evolutionary radiation of new forms of life.

Unicellular protists are much more diverse in form than simpler prokaryotes.


Multicellular organisms (seaweeds, plants, animals, and most fungi) are fundamentally different from unicellular organisms.

– All of life’s activities occur within a single cell in unicellular organisms.

– A multicellular organism has various specialized cells that perform different functions and are interdependent.


Multicellular organisms have evolved from three different lineages:

– SAR-stramenopiles, alveolata, rhizaria (brown algae),

– unikonts (fungi and animals), and

– archaeplastids (red algae, green algae, and plants).

Figure 16.19a A hypothesis for the phylogeny of plants, fungi, and animals

Red algae

Other green algae

Charophytes

Land plants

Amoebozoans

Nucleariid amoebas

Fungi

Choanoflagellates

Animals

Green

algae

Arch

aeplastid

sU

niko

nts

An

cestral eukaryo

te

KeyAll unicellularBoth unicellularand multicellularAll multicellular


One hypothesis states that two separate unikont lineages led to fungi and animals, which diverged more than 1 billion years ago.

A combination of morphological and molecular evidence suggests that choanoflagellates are the closest living protist relative of animals.

Figure 16.19b The closest living protist relatives of fungi (top) and animals (bottom)16.19b-0

Nucleariids

Fungi

Choanoflagellates

Animals

1 billionyears ago

Sponge

Spongecollar cell

Colonialchoanoflagellate

Individualchoanoflagellate

A nucleariid (type of amoeba) closest living protistanrelative of fungi

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PROKARYOTES © 2012 Pearson Education, Inc. Chapter 16 Microbial Life: Prokaryotes and Protists