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Archaea, Bacteria, and Viruses
Chapter 19
Prokaryotes and Eukaryotes
• Terms introduced by
Edouard Chatton in
1920s
• Based on microscopic
observations
Prokaryotes
Eukaryotes
“primitive
nucleus”
“true nucleus”
Lack clear
Clear nucleus
nucleus and other and other
inclusions
inclusions
All organisms
with cells that
lack a nucleus
All organisms
with cells that
have a nucleus
Prokaryotes
• Carl Woese
– Late 1970s
– Proposed using the rRNA gene to create
universal tree of life
• rRNA critical for proteins synthesis
• Useful in determining evolutionary relationships
Three Domains
Domain
Cell Type
Description
Eukaryotic
Membrane bounded organelles, linear
chromosomes
Archaea
Prokaryotic
Found in extreme environments, cell
structure differs from members of Domain
Bacteria
Bacteria
Prokaryotic
Ordinary bacteria, found in every habitat
on earth, play major role as decomposers
Eukarya
Why Should A Botanist Study
Prokaryotes?
• Reasons for studying prokaryotes
– Many of the biochemical compounds,
enzymes, and metabolic pathways of plants
also are found in prokaryotes.
– The evolutionary ancestors of plants were
prokaryotes.
– Plants form ecological associations with
prokaryotes.
Viruses
• Consist of
– Either DNA or RNA
– Protein coat
• Not prokaryotes
• Noncellular  cannot live independently
• Discoveries obtained studying viruses can
be used to guide plant research
Prokaryotic Cell Structure
• Lacks internal membrane-enclosed
organelles
• Surrounded by plasma membrane
– Bacteria plasma membrane lipids similar to
those of eukaryotes
– Archaeal plasma membrane lipids very
different
• Held together by stronger bonds
Categories of Bacterial Cells
• Divided on basis of differential staining
technique developed by Gram
– Gram-negative cells
– Gram-positive cells
Gram-positive Cells
• Capsule
– Waxy polysaccharide
– Protects some human pathogens from being
engulfed by immune system cells
• Penicillin very effective against Grampositive cells
– Inhibits formation of cell wall
– Causes lysis of cells in hypotonic solutions
Comparison of Gram-positive and
Gram-negative Cells
Gram-positive
Gram-negative
Thick peptidoglycan cell wall
Thin peptidoglycan cell wall
No second membrane – some
have waxy polysaccharide
capsule
Second membrane outside
cell wall –
lipopolysaccharide layer
composed of phospholipids,
polysaccharide, and protein
No periplasmic space
Periplasmic space between
cell wall and
lipopolysaccharide layer
Shapes of Bacteria
• Determined by cell wall
– Also functions to keep cell from bursting in hypotonic
solution
– Composed of peptidoglycan
• Shapes
–
–
–
–
–
Cocci  small, round cells
Bacilli  rods
Vibrios  bent or hooked rods
Spirilla  helical forms
Stalked forms
Archaea
• Most have paracrystalline surface layer (S
layer)
– Composed of protein or glycoprotein
– Sensitive to proteases and surfactants
• Some have outer covering of
pseudopeptidoglycan
• Some have thick walls of polysaccharide
• Typically lack an outer membrane
Bacterial DNA
• Typically a single, circular chromosome
– Size in Escherichia coli
• 1.4 mm in length
• Contains 4.6 million nucleotide pairs
• Not surrounded by nuclear envelope
• Complexed with specific structural proteins
that organize it into loops
• Localized in area called nucleoid
Archaeal DNA
• Chromosome complexed with histone
proteins, similar to chromosomes of
eukaryotes
Plasmids
• Accessory genes
• Small circles of DNA approximately 2,000
to 200,000 nucleotide pairs in length
• Can replicate independently of main
chromosome
Plasmids
• Examples of information carried by
plasmid genes
– Antibiotic resistance
– Enzymes and structural proteins that transfer
copies of plasmid to Bacteria that do not have
any
Plasmids
– F plasmid
• Can incorporate itself into main chromosome
• Contains genes for making tube called F pilus
– Connects its cell with another that lacks F plasmid
– Transfers plasmid or chromosomal DNA from donor (cell
with pilus) to receiver cell
– Transfer of DNA is called conjugation
Prokaryotic Ribosomes
• General composition and structure similar
to those of eukaryotes
– Two subunits made of RNA and protein
• Smaller than eukaryotic ribosomes
• rRNAs of Archaea
– More similar to those of eukaryotes than
Bacteria
Binary Fission
• Method of reproduction in prokaryotic cells
• Differs from mitosis
– Prokaryotes lack microtubules therefore do
not have spindle apparatus
Flagella
• Used by many Bacterial and some Archaeal
cells for swimming
• Formed of subunits of protein flagellin
• Parts of flagellum
– Filament
– Hook
– Basal body
• Powered by basal body
• In some instances can reverse swimming
direction
Pilus
• Extracellular organelle
• Thin, hollow, nonmotile projection from cell
• Proteins at ends of structure attach cell to
solid surfaces or to receptors on other
cells
Cisternae
• Cisternae or thylakoid membranes
• Found in some prokaryotic cells
• Consist of flattened bladders than enclose
separate compartments within cytoplasm
• Function
– in light reactions of photosynthesis in
photosynthetic prokaryotes
– Energy storage
Endospores
• Means of survival for some prokaryotic
cells
• Small, desiccated cells in condition of
suspended animation
• Contain complete genome and needed
chemicals for germination and growth
when conditions improve
Endospores
• Resistant to many things such as boiling,
oxidizing agents, antibiotic compounds
• Formation involves activation of special
genes
– In Bacteria such as Clostridium tetani
• Nucleoid and ribosomes surrounded by spore wall
• Rest of cell degenerates
Endospores
– Actinobacteria
• Form spores on vertical stalk
• Spores blown to new sites by air currents
– Myxobacteria
• Form sacs of endospores
• Spores released when sac is hydrated
Nutritional Requirements of
Prokaryotes
• Methods of obtaining carbon
– Autotroph (“self-feeding)  incorporate
carbon into organic molecules from inorganic
sources
– Heterotroph (“other feeding”)  derive carbon
from breakdown of organic compounds
Nutritional Requirements of
Prokaryotes
• Methods of deriving energy
– Chemotroph (“chemical feeding”)  obtain
energy from catalyzing inorganic reactions
– Phototroph (“light feeding”)  obtain energy
by absorbing light photons
Nutritional Requirements of
Prokaryotes
Carbon source
Energy source
Chemoautotroph
Carbon from inorganic source
incorporated into organic
molecules
Catalyze inorganic
reactions
Chemoheterotroph
Carbon from breakdown of
organic compounds
Catalyze inorganic
reactions
Photoautotroph
Carbon from inorganic source
incorporated into organic
molecules
Derive energy by
absorbing light photons
Photoheterotroph
Carbon from breakdown of
organic compounds
Derive energy by
absorbing light photons
Groups of Archaea
• Methanogens
• Halophiles
• Thermoacidophiles
Groups of Archaea
• Methanogens
– Chemoautotrophs
– Require anoxic environment to obtain energy
– Produce methane
Reaction used by methanogens to derive energy:
CO2 + 4H2  CH4 + 2H2O
Groups of Archaea
• Halophiles
– Live in saturated salt solutions
– Some have little or no cell wall
• Will burst if moved from its normal environment
– Example: Halobacterium halobium
•
•
•
•
•
Unique type of photosynthesis
Photoreceptor – bacteriorhodopsin
No electron transport chain
Cannot make carbohydrates by reducing CO2
Photoheterotroph
Groups of Archaea
• Thermoacidophiles
– Live in hot, acidic environments
– Optimum temperature is 70 to 75ºC with
maximum of 88ºC
– Optimum pH is 2-3 (minimum pH 0.9)
Chemoheterotrophs
• Live on organic compounds of living or
dead tissue or on excretions of other
organisms
• Roles
– May be harmful parasites
Chemoheterotrophs
– Can be beneficial
• Compete for niches with potential pathogens
• Gut chemoheterotrophs provide humans with
vitamin K
• In dead tissue and on excretions, play role of
recycling carbon, nitrogen, and other elements
Chemoheterotrophs
• Some undergo fermentation
– Lactobacillus
• Extensively studied bacterium, E. coli, is
chemoheterotroph
– Group Proteobacteria
– Family Enterobacteriaceae
• Members live in soil and in intestines of animals
• Often called enteric or coliform Bacteria
Chemoheterotrophs
– Presence of coliform Bacteria in water
supplies indicates contamination with sewage
• Humans sewage carries pathogenic Bacteria and
viruses
– Some strains of E. coli are not harmful, others
produce toxins that cause severe infections
Examples of Chemoheterotrophs
Genus
Phylum
Comment
Rhizobium
Proteobacteria
N2-fixing plant symbionts
Frankia
Actinobacteria
N2-fixing plant symbionts
Erwinia, Agrobacterium,
Pseudomonas syringae
Proteobacteria
Plant pathogens
Desulfovibrio, Desulfomonas
Proteobacteria
Sulfate-reducing bacteria
Stigmatella, Chondromyces
Proteobacteria
Myxobacteria; colonial spore formers
Streptomyces
Actinobacteria
Antibiotic producers
Spirochaeta, Treponema
Spirochaetes
Long, thin, spiral-shaped; some
pathogenic
Bacillus
Firmicutes
Aerobic endospore-formers
Escherichia
Proteobacteria
Enteric; model organism
**Rhodopseudomonas,
Chromatium
Proteobacteria
Anaerobic phototrophs; purple nonsulfur
and purple sulfur groups
All of the above examples are in the Domain Bacteria.
**Can be photoautotroph or chemoheterotroph
Chemoautotrophs
• Examples
– Lithotrophs
• Specialize in oxidation of inorganic compounds
• Recycle nitrogen and sulfur
– Nitrogen and methane oxidizers
– Methanogenic Archaea
– Thermophilic and thermoacidophilic Archaea
Examples of Chemoautotrophs
Domain
Genus
Phylum
Comment
Archaea
Methanococcus,
Methanospirillum
Euryarchaeota
Methanogenic
CO2 + 4H2  CH4 + 2H2O
Archaea
Archaea
Bacteria
Thermoplasma
Euryarchaeota
Thermoacidophile; grows at pH
1-4 and 33-67ºC
Pyrolobus
Crenarchaeota
Extreme thermophile; grows
up113ºC
Nitrosomonas,
Nitrobacter,
Methylomonas
Proteobacteria
Nitrogen, methane oxidizers
Photoautotrophs
• Includes
– Green sulfur Bacteria
– Purple nonsulfur Bacteria
– Cyanobacteria
• Light absorbing pigments
– Bacteriochlorophyll
• Anaerobic phototrophs
– Chlorophyll
• Cyanobacteria
• All reduce carbon to CO2
Examples of Photoautotrophs
Domain
Genus
Phylum
Comment
Bacteria
**Rhodopseudomonas,
Chromatium
Anaerobic phototrophs;
Proteobacteria purple nonsulfur and
purple sulfur groups
Bacteria
Chlorobium
Chlorobi
Bacteria
Anabaena, Nostoc,
Prochloron
Cyanobacteria Oxygen producers
Anaerobic phototrophs;
green sulfur group
**Can be either photoautotroph or chemoheterotroph
Photoautotrophs and
Endosymbiosis
• Primitive cyanobacteria and
chloroxybacteria thought to be
evolutionary precursors of plastids of
photosynthetic eukaryotes
• Strong evidence on similarities between
light-harvesting complexes of
– Cyanobacteria and red algae
– Chloroxybacteria and green algae
Symbiotic Relationships Between
Prokaryotes and Plants
• Rhizobium lives in soil
– Synthesizes enzyme nitrogenase
• Converts N2 to ammonium (NH4+)
• Forms close mutualistic relationship with
legumes
– Plant contributes high energy carbohydrates and a
protected environment
– Bacterium contributes nitrogenase and other
enzymes
– Both partners benefit from supply of fixed nitrogen
Symbiotic Relationships Between
Prokaryotes and Plants
• Association occurs in special organs
called root nodules
• Sequence of events in establishment of
relationship
– Root secretes attractive chemical
– Chemical induces Rhizobium in vicinity to
swim toward root and begins induction of
nitrogen fixation genes in Rhizobium
Symbiotic Relationships Between
Prokaryotes and Plants
– Rhizobium enters at a root hair and moves
inward through infection thread
– Rhizobium loses its cell wall and begins
synthesizing nitrogen-fixing enzymes as it
moves inward
– Bacteria reach root cortex
– Bacteria are released from infection thread
into several cells
– Bacteria without cell walls are now called
bacteroids
Symbiotic Relationships Between
Prokaryotes and Plants
– Bacteroids become surrounded by special
membrane called the peribacteroid membrane
– Chemicals secreted by Bacteria (or
bacteroids) during formation of infection
thread
• Induce cell division in root cortex and pericycle,
forming nodule
• Induce synthesis of nodule proteins including
leghemoglobin that buffers oxygen concentration in
part of nodule where nitrogen is fixed
Symbiotic Relationships Between
Prokaryotes and Plants
• Other examples of symbiotic nitrogen
fixing Bacteria
– Frankia – lives within cells of root nodules of
alder trees and other plants
– Anabaena – association with water fern,
Azolla
– Nostoc – invades cavities in gametophytes of
hornworts and specialized cells of cycads
Bacterial Parasites
• Parasitism
– Symbiotic relationship in which one organism
benefits at the expense of the other
• Plant pathogens divided into subgroups
called pathovars according to plants they
infect
Bacterial Parasites
• Pathovars of Pseudomonas syringae
cause
– Wildfire disease of tobacco
– Blights of beans, peas, and soybeans
• Pathovars of Erwinia amylovora cause
– Fire blight of apple and pear
Bacterial Parasites
• Vectors for carrying Bacteria to uninfected
plants include water, insects, humans, or
other animals
• Bacteria enter plants through natural
openings
– Stomata
– Lenticels
– Hydathodes
– Nectarthodes
Bacterial Parasites
• Some Bacterial pathogens can overwinter
in dead tissue
– Return to infect new plant tissue during next
growing season
• Plant defenses against infecting bacteria
– Hypersensitivity response
• Produce antibiotic compounds
– Phytoalexins  directly kill some pathogenic cells
– Hydrogen peroxide  may restrict spread of infection by
causing necroses of adjacent plant cells
Viruses
• Subcellular parasites
– Lack internal structures found in prokaryotic
and eukaryotic cells
– Cannot reproduce on their own
• Invade host cells and use host’s
metabolism to reproduce themselves
• Simple structure
– Either DNA or RNA surrounded by protein
coat
Viruses
• Shape varies
• Well-studied plant virus – tobacco mosaic virus
• Plant viruses
– Too large to pass through cell wall
– In nature, almost always spread by insects that pick
up viral particles as they chew or suck on infected
plants and then transmit them to uninfected plants
– Mites and fungi can also infect plants with viruses
when they enter plant cells
Viruses
• Viral infections usually do not kill plants
• Infected plants usually stunted compared
to uninfected plants
• Infection often causes changes in color or
shape of foliage