Chapter 16

Chapter 16
The Origin and Evolution of Microbial Life: Prokaryotes and Protists
Introduction: How Ancient Bacteria Changed the World
Virtually all metabolic pathways on Earth evolved in prokaryotic
cells, before the evolution of eukaryotes
The products generated by prokaryotic metabolism changed
the Earth’s atmosphere and rocks
Fossilized stromatolites from 3 billion years ago contain the
fossils of photosynthetic cyanobacteria
– These bacteria produced O2 and made Earth’s atmosphere aerobic
Introduction: How Ancient Bacteria Changed the World
Living stromatolites still form in the warm, shallow waters of
Shark Bay, western Australia
16.1 Prokaryotes are diverse and widespread
A.) Prokaryotes lived alone on Earth for over 1 billion years
– They remain the most numerous and widespread organisms on Earth
– The total biomass of prokaryotes is ten times that of eukaryotes
Most prokaryotes are 1–5 µm in diameter (vs. 10–100 µm for
eukaryotic cells)
More prokaryotes live in your mouth than the total number of
humans that have ever lived
There are ten times as many prokaryotes living in and on your
body as the number of cells in your body
Prokaryotes live in cold, hot, salty, acidic, and alkaline habitats
Although some bacteria are pathogenic and cause disease,
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
16.2 Bacteria and archaea are the two main branches of
prokaryotic evolution
The two prokaryotic domains, Bacteria and Archaea,
diverged soon after life on Earth arose
Present day Archaea and Eukarya evolved from a common
ancestor, complicated by gene transfer between prokaryotic
Some genes of Archaea are similar to bacterial genes, some are
similar to eukaryotic genes, and some are unique to Archaea
16.4 Various structural features contribute to the success of
A.) Prokaryotic cell walls maintain cell shape, provide physical
protection, and prevent the cell from bursting in a hypotonic
– In a hypertonic environment, most prokaryotes lose water and shrink
away from their wall
B.) The cell walls of Archaea and Bacteria differ
Bacterial cell walls can be distinguished with a gram stain
– Gram-positive bacteria have simple walls with a thick layer of
– Gram-negative bacteria have complex walls with less peptidoglycan and
an outer membrane of lipids bonded to carbohydrates
D.) Some prokaryotes stick to the substrate or each other with
hair-like appendages called pili
Sex pili join prokaryotes during conjugation
The flagella of Bacteria and Archaea allow them to move in
response to chemical and physical signals in their environment
The prokaryotic flagellum is a naked protein without
– The flagellum rotates like a propeller
Are prokaryotic flagella homologous or analogous to
eukaryotic flagella?
Some prokaryotes can withstand harsh conditions by forming
endospores within an outer cell
– The endospore has a thick protective coat
– It can dehydrate and is tolerant of extreme heat or cold
When conditions improve, the endospore absorbs water and
resumes growth, sometimes after centuries
K.) Some prokaryotic cells have specialized membranes that
perform metabolic functions
– Aerobic prokaryotes carry out cellular respiration on infoldings of the
plasma membrane
– Where is cellular respiration carried out in a eukaryote?
– Cyanobacteria carry out photosynthesis on infolded thylakoid
– Where is photosynthesis carried out in a photosynthetic eukaryote?
L.) Prokaryotic DNA forms a circular chromosome
– Smaller rings of DNA called plasmids carry genes that may provide
resistance to antibiotics or metabolize rare nutrients, among other
metabolic activities
M.) Many prokaryotes can transfer genes, such as antibiotic
resistance genes, within or between species
16.5 Prokaryotes obtain nourishment in a variety of ways
A.) In some prokaryotes, metabolic cooperation occurs in surfacecoating colonies called biofilms
– Biofilms form when cells in a colony secrete signaling molecules that
recruit nearby cells
– Channels in the biofilm allow nutrients and wastes to move inside and
outside the biofilm
Biofilms cause ear and urinary tract infections and the dental
plaque that produces tooth decay
16.6 Archaea thrive in extreme environments—and in other
Archaea are among the most abundant cells on Earth
They are a major life-form in the oceans
16.7 Bacteria include a diverse assemblage of prokaryotes
Clades of gram-negative bacteria
– Alpha proteobacteria
– Rhizobium species live in legume nodules and fix atmospheric N2
– Photosynthetic gamma proteobacteria
– Sulfur bacteria oxidize H2S
Clades of gram-negative bacteria
– Delta proteobacteria
– Myxobacteria form elaborate colonies and congregate into fruiting bodies that
release resistant spores
– Chlamydias live inside eukaryotic host cells
– Chlamydias cause blindness and sexually transmitted disease
– Spirochetes are helical bacteria
– Spirochetes cause syphilis and Lyme disease
Clades of gram-positive bacteria
– Actinomycetes are common soil bacteria that decompose organic matter
– Streptomyces is a source of many antibiotics
– Mycoplasmas lack cell walls
– They are the tiniest of all known cells, with diameters as small as 0.1 µm (about
5 times the size of a ribosome)
– Cyanobacteria carry out oxygen-generating photosynthesis
– Ancient cyanobacteria formed stromatolites that made the atmosphere aerobic
16.8 CONNECTION: Some bacteria cause disease
A.) Pathogenic bacteria cause disease by producing poisonous
exotoxins or endotoxins
– Exotoxins are proteins secreted by bacterial cells
– Some of the most powerful toxins known are exotoxins, including the
toxin that causes lockjaw
– Staphylococcus aureus produces several exotoxins, including one that
causes deadly toxic shock syndrome
B.) Endotoxins are components of the outer membrane of gramnegative bacteria, released when the cell dies or is digested by a
defensive cell
– Endotoxins produce septic shock, bacterial meningitis, and food
– The most widespread pest-carried disease in the United States is Lyme
disease, caused by the spirochaete Borrelia burgdorferi
16.9 CONNECTION: Bacteria can be used as biological
The bacterium that causes anthrax can be used as biological
– Bacillus anthracis forms hardy endospores
– Weaponizing anthrax involves manufacturing endospores that disperse
easily in air, where they are inhaled and germinate in the lungs
The Biological Weapons Convention has been signed by 103
nations, who have pledged never to develop or store biological
16.10 CONNECTION: Prokaryotes help recycle chemicals and
clean up the environment
Prokaryotes are key participants in chemical cycles, making
nitrogen available to plants and thus animals
They also decompose organic wastes and dead organisms to
inorganic chemicals
Bioremediation is the use of organisms to remove pollutants
from soil, air, or water
– Prokaryotes are decomposers in sewage treatment and can clean up oil
spills and toxic mine wastes
16.11 Protists are an extremely diverse assortment of
Protists constitute several kingdoms within the domain Eukarya
Protists obtain their nutrition in a variety of ways
– Algae are autotrophic protists
– Protozoans are heterotrophic protists, eating bacteria and other protists
– Fungus-like protists obtain organic molecules by absorption
Symbiosis is a close association between organisms of two or
more species
– Endosymbiosis—living within another
– Termite endosymbionts digest cellulose in the wood eaten by the host
– The protists have endosymbiotic prokaryotes that metabolize the
Protists are eukaryotes, with
– Membrane-bound chromosomes
– Multiple chromosomes
– Flagella or cilia with 9 + 2 pattern of microtubules
Some protists have a very high level of cellular complexity
endosymbiosis is the key to protist diversity
What is the origin of the enormous diversity of protists?
– Complex eukaryotic cells evolved when prokaryotes took up residence
within larger prokaryotes
16.13 A tentative phylogeny of eukaryotes includes multiple
clades of protists
The taxonomy of protists remains a work in progress
– The names, boundaries, and placement of clades will continue to change
as genomes of more protists are sequenced and compared
16.14 Diplomonads and parabasalids have modified
Diplomonads may be the most ancient surviving lineage of
– They have modified mitochondria without DNA or electron transport
– Most are anaerobic
Parabasalids are heterotrophic protists with modified
mitochondria that generate some energy anaerobically
– The parasite Trichomonas vaginalis is sexually transmitted, feeding on
white blood cells and bacteria living in the cells lining the vagina
16.15 Euglenozoans have flagella with a unique internal
Euglenozoans are a diverse clade of protists
– Their common feature is a crystalline rod of unknown function inside
their flagella
Euglenozoans include heterotrophs, photosynthetic autotrophs,
and pathogenic parasites
16.16 Alveolates have sacs beneath the plasma membrane
A.) Alveolates have membrane-enclosed sacs or alveoli beneath
the plasma membrane
B.) Dinoflagellates are important members of marine and
freshwater phytoplankton
– Some live within coral animals, feeding coral reef communities
– Dinoflagellate blooms cause red tides
C.) Ciliates use cilia to move and feed.
D.) Apicomplexans are animal parasites such as Plasmodium,
which causes malaria
16.17 Stramenopiles have “hairy” and smooth flagella
A.) Stramenopiles are named for their “hairy” flagellum, usually
paired with a “smooth” flagellum
– Water molds are fungus-like and decompose dead organisms in
freshwater habitats
– Diatoms are unicellular, with silicate cell walls
– Brown algae are large, complex algae called seaweeds; all are
multicellular and most are marine
16.18 Amoebozoans have lobe-shaped pseudopodia
Amoebas move and feed by means of pseudopodia
B.) Members of the clade amoebozoans include many free-living
amoebas, some parasitic amoebas, and slime molds
– All have lobe-shaped pseudopodia
A plasmodial slime mold is an amoebozoan that forms a
plasmodium, a multinucleate mass of cytoplasm
– The plasmodium extends pseudopodia through soil and rotting logs,
engulfing food by phagocytosis as it grows
– Under adverse conditions, the plasmodium forms reproductive structures
that produce spores
Cellular slime molds live as solitary amoeboid cells
– When food is scarce, the amoeboid cells swarm together, forming a
slug-like aggregate that migrates, before forming a fruiting body borne
on a stalk
16.19 Foraminiferans and radiolarians have threadlike
Foraminiferans and radiolarians move and feed by means of
threadlike pseudopodia
Foraminiferans live in marine and freshwater
– They have porous tests composed of calcium carbonate, with small
pores through which pseudopodia extend
Radiolarians produce an internal silicate skeleton
– The test is composed of organic materials
16.20 Red algae and green algae are the closest relatives of land
Red algae are typically soft-bodied, but some have cell walls
encrusted with hard, chalky deposits
Green algae split into two groups, the chlorophytes and the
– The charophytes are the closest living relatives of land plants
The life cycles of most green algae involve the alternation of
generations, in which a haploid (n) gametophyte alternates
with a diploid (2n) sporophyte generation
Multicellularity evolved several times in eukaryotes
Multicellularity evolved in several different lineages, probably by
specialization of the cells of colonial protists.
– Stramenopile lineage brown algae
– Unnamed lineage red algae, green algae, land plants
– Opisthokont lineage fungi and animals
Multicellular life arose over a billion years ago.
By 543 million years ago, diverse animals and multicellular
algae lived in aquatic environments; plants and fungi colonized
land 500 million years ago
You should now be able to
§ Compare the characteristics of the three domains of life; explain
why biologists consider Archaea to be more closely related to
Eukarya than to Bacteria
§ Describe the structures and functions of the diverse features of
prokaryotes; explain how these features have contributed to their
§ Describe the nutritional diversity of prokaryotes; explain the
significance of biofilms
§ Describe the diverse types of Archaea living in extreme and
moderate environments
You should now be able to
§ Distinguish between the subgroups of the domain Bacteria, noting
the particular structure, special features, and habitats of each
§ Distinguish between bacterial exotoxins and endotoxins, noting
examples of each
§ Describe the positive natural roles of prokaryotes
§ Describe the basic types of protists; explain why biologists
currently think that they represent many clades
You should now be able to
Explain how primary endosymbiosis and secondary endosymbiosis
led to further cellular diversity
Describe the major protist clades noting characteristics and
examples of each
Describe the life cycle of Ulva, noting each form in the alternation
of generations and how each is produced
Explain how multicellular life may have evolved in eukaryotes