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Prokaryote diversity
Level 1 Biological Diversity
Jim Provan
Campbell: Chapter 27
Prokaryotes are (almost) everywhere
Prokaryotes dominate the
biosphere:
Collective mass of
prokaryotes outweighs
eukaryotes tenfold
More prokaryotes in the
average human mouth
than all the humans that
ever lived
Prokaryote diversity:
About 5,000 species known
Actual numbers believed to
be 400,000 to 4 million
The three domains of life
Domain
Bacteria
Domain
Archaea
Domain
Eukarya
Diversity of prokaryotic cell shapes
Prokaryote cell walls
Prokaryote cell walls
Present in nearly all prokaryotes:
Provides physical protection
Prevents cells from bursting in hypotonic environment
Reason prokaryotes were sometimes grouped with
plants in early classification systems
Cell wall contains peptidoglycan, rather than cellulose:
Gram-negative bacteria generally more pathogenic than
gram-positive bacteria:
– Lipopolysaccharides can be toxic
– Outer membrane less permeable to antibiotics
Antibiotics inhibit cross-linking of peptidoglycan
Prokaryote organisation
Prokaryotes have smaller, simpler genomes than
eukaryotes:
On average, about one-thousandth as much DNA
Concentrated in twisted nucleoid region
Single, circular chromosome
Also have “accessory” DNA – plasmids
General mechanism of DNA replication and protein
translation is largely the same as in eukaryotes
No organelles, but some have specialised membranes
which carry out similar functions
Prokaryote organisation
Prokaryote reproduction
Prokaryotes reproduce only asexually by binary fission:
DNA synthesis is almost continuous
No mitosis or meiosis
Gene transfer is possible:
Transformation – take up of genes from surroundings
Conjugation – direct transfer of genes between cells
Transduction – gene transfer by viruses
Mutation is the major source of genetic variation
“Growth” is actually increase in numbers, which
effectively proceeds geometrically
Prokaryote metabolic diversity
Mode
Energy source
Carbon source
Example
Photoautotroph
Light
CO2
Cyanobacteria, plants, protists
Chemoautotroph
Inorganic compounds CO2
Some prokaryotes (Sulfolobus)
Photoheterotroph
Light
Some prokaryotes
Chemoheterotroph Organic compounds
Organic compounds
Organic compounds
Prokaryotes, protists, fungi,
animals, parasitic plants
Proteobacteria
a
b
g
d
e
DOMAIN
ARCHAEA
DOMAIN EUKARYA
Crenarchaeota
DOMAIN BACTERIA
Euryarchaeota
Cyanobacteria
Gram positive
bacteria
Spirochetes
Chlamydias
A survey of prokaryote diversity
The three domains of life
DOMAIN
CHARACTERISTIC
Bacteria
Archaea
Eukarya
Nuclear envelope
x
x

Organelles
x
x

Peptidoglycan cell wall

x
x
Membrane lipids
Unbranched
Some branched
Unbranched
RNA polymerase
One
Several
Several
Initiator amino acid
f-Met
Met
Met
x


Inhibited
Not inhibited
Not inhibited
x


Introns
Antibiotic response
Histones
Domain Archaea
Inhabit most extreme
environments:
Methanogens - strict
anaerobes which use H2 to
reduce CO2 to methane (CH4)
Extreme halophiles - live in
saline environments five to
ten times saltier than sea
water
Extreme thermophiles - live
in very hot environments
such as sulphur springs of
deep-sea hydrothermal vents
Domain Bacteria: Proteobacteria
Domain Bacteria: others
Ecological impact of prokaryotes
Prokaryotes are crucial
components of
ecosystems:
Decomposition of waste
products
Recycling of elements from
inorganic sources (soil, air
etc.)
Many prokaryotes are
symbiotic:
Rhizobium – N2 fixation
Bioluminescence
Some prokaryotes are pathogenic
Pathogenic prokaryotes
cause half of all human
disease
Some pathogens are
opportunistic: they cause
illness when the host’s
defences are weakened
Mostly produce toxins:
Exotoxins secreted by
prokaryotes e.g. in food or
water
Endotoxins are components
of bacterial membranes
Humans use prokaryotes
Used as model systems
to study genetics e.g.
Escherichia coli
Used in bioremediation:
Sewage treatment
Decomposition of oil
spills by soil bacteria
Metabolic “factories”:
Organic solvents
Antibiotic production
Food industry