Download 5/2/12 Microbial symbioses

25.1 Lichens
• Lichens
– Leafy or encrusting microbial symbioses
– Often found growing on bare rocks, tree trunks,
house roofs, and the surfaces of bare soils
(Figure 25.1)
– A mutalistic relationship between a fungus and an
alga (or cyanobacterium)
• Alga is photosynthetic and produces organic
matter
• The fungus provides a structure within which the
phototrophic partner can grow protected from
erosion (Figure 25.2)
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Figure 25.1
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Figure 25.2
Algal layer
Fungal
hyphae
Rootlike
connection
to substrate
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25.2 “Chlorochromatium aggregatum”
• In freshwater there are microbial mutualisms
called consortia
• Consist of green sulfur bacteria (called epibionts)
and a flagellated rod-shaped bacterium
(Figure 25.3 and 25.4)
– Consortium given a “genus species” name
– Green sulfur bacteria are obligate anaerobic
phototrophs
– Flagellated rod allows for movement
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Figure 25.3
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Figure 25.4
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II. Plants as Microbial Habitats
• 25.3 The Legume–Root Nodule Symbiosis
• 25.4 Agrobacterium and Crown Gall Disease
• 25.5 Mycorrhizae
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25.3 The Legume–Root Nodule Symbiosis
• The mutalistic relationship between
leguminous plants and nitrogen-fixing
bacteria is one of the most important
symbioses known
• Examples of legumes include soybeans,
clover, alfalfa, beans, and peas
• Rhizobia are the best-known nitrogen-fixing
bacteria engaging in these symbioses
Animation: Root Nodule Bacteria and Symbioses with Legumes
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25.3 The Legume–Root Nodule Symbiosis
• Infection of legume roots by nitrogen-fixing
bacteria leads to the formation of root
nodules that fix nitrogen (Figure 25.7)
– Leads to significant increases in combined
nitrogen in soil
• Nodulated legumes grow well in areas where
other plants would not (Figure 25.8)
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Figure 25.7
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Figure 25.8
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25.3 The Legume–Root Nodule Symbiosis
• Nitrogen-fixing bacteria need O2 to generate
energy for N2 fixation, but nitrogenases are
inactivated by O2
• In the nodule, O2 levels are controlled by the
O2-binding protein leghemoglobin (Figure 25.9)
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Figure 25.9
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25.3 The Legume–Root Nodule Symbiosis
• Cross-inoculation group
– Group of related legumes that can be infected
by a particular species of rhizobia
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25.3 The Legume–Root Nodule Symbiosis
• Critical steps in root nodule formation
(Figure 25.10):
– Step 1: Recognition and attachment of bacterium
to root hairs (Figure 25.11)
– Step 2: Excretion of nod factors by the bacterium
– Step 3: Bacterial invasion of the root hair
– Step 4: Travel to the main root via the infection
thread
– Step 5: Formation of bacteroid state within plant
cells
– Step 6: Continued plant and bacterial division,
forming the mature root nodule
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Figure 25.10
Root hair
Recognition and attachment
(rhicadhesin-mediated)
Rhizobial cell
Excretion of nod factors
by bacterium causing
root hair curling
Invasion. Rhizobia penetrate
root hair and multiply
within an “infection thread”
Bacteria in infection
thread grow toward
root cell
Infection thread
Invaded plant cells
and those nearby are
stimulated to divide
Formation of
bacteroid state within
plant root cells
Soil
Nodules
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Continued plant and
bacterial cell division
leads to nodules
25.3 The Legume–Root Nodule Symbiosis
• Bacterial nod genes direct the steps in
nodulation
• nodABC gene encodes proteins that produce
oligosaccharides called nod factors
(Figure 25.12)
• Nod factors
– Induce root hair curling
– Trigger plant cell division
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25.3 The Legume–Root Nodule Symbiosis
• The legume–bacteria symbiosis is
characterized by several metabolic reactions
and nutrient exchange (Figure 25.14)
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Figure 25.14
Plant cytoplasm
Sugars
Symbiosome
membrane
Organic acids
Bacteroid
membrane
Bacteroid
Citric
acid
cycle
e Proton
motive
force
Electron transport
chain
Lb  Leghemoglobin
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Photosynthesis
Succinate
Malate
Fumarate
Pyruvate
e
Nitrogenase
Glutamine
Asparagine
25.3 The Legume–Root Nodule Symbiosis
• A few legume species form nodules on their
stems (Figure 25.15)
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Figure 25.15
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25.5 Mycorrhizae
• Mycorrhizae
– Mutualistic associations of plant roots and fungi
– Two classes:
• Ectomycorrhizae
• Endomycorrhizae
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25.5 Mycorrhizae
• Ectomycorrhizae
– Fungal cells form an extensive sheath around
the outside of the root with only a little
penetration into the root tissue (Figure 25.21)
– Found primarily in forest trees, particularly
boreal and temperate forests
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Figure 25.21
Fungal
filament
Forked
root
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25.5 Mycorrhizae
• Endomycorrhizae
– Fungal mycelium becomes deeply embedded
within the root tissue (Figure 25.22)
– Are more common than ectomycorrhizae
– Found in >80% of terrestrial plant species
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Figure 25.22
Epidermis
S
Mycelium
A
S
HP
A
HP
Outer
cortex
Inner
cortex
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25.5 Mycorrhizae
• Mycorrhizal fungi assist plants (Figure 25.23)
– Improve nutrient absorption
• This is due to the greater surface area
provided by the fungal mycelium
– Helping to promote plant diversity
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Figure 25.23
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III. Mammals as Microbial Habitats
• 25.6 The Mammalian Gut
• 25.7 The Rumen and Ruminant Animals
• 25.8 The Human Microbiome
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25.6 The Mammalian Gut
•
•
•
•
Herbivores – animals that consume plants
Carnivores – animals that consume meat
Omnivores – animals that consume both
Phylogenetics suggests that different lineages
evolved a herbivorous lifestyle (Figure 25.24)
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Figure 25.24
Sheep and cow
Herbivores
Carnivores
Omnivores
Pig
Horse
Brown bear
Giant panda
Dog
Lion
Rabbit
Human
Gorilla
Orangutan
Baboon
Spider monkey
Lemur
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25.6 The Mammalian Gut
• Microbial associations with certain animals led to
ability to catabolize plant fibers
– Plant fibers composed of insoluble
polysaccharides.
• Cellulose most abundant component
– Two digestive plans have evolved in herbivorous
animals (Figure 25.25)
– Foregut fermentation – fermentation chamber
precedes the small intestine
– Hindgut fermentation – uses cecum and/or large
intestine
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Figure 25.25
Foregut fermenters Examples:
Ruminants (photo 1), colobine monkeys,
macropod marsupials, hoatzin (photo 2)
Foregut
fermentation
chamber
1.
2.
Acidic
stomach
Hindgut fermenters Examples: Cecal
Small
intestine
animals (photos 3 and 4), primates,
some rodents, some reptiles
3.
Large
intestine
(colon)
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Hindgut
Cecum fermentation
chambers
4.
25.7 The Rumen and Ruminant Animals
• Microbes form intimate symbiotic relationships
with higher organisms
• Ruminants
– Herbivorous mammals (e.g., cows, sheep, goats)
– Possess a special digestive organ (the rumen)
• Cellulose and other plant polysaccharides are
digested with the help of microbes (Figure 25.26)
– Rumen well studied because of implanted
sampling port
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Figure 25.26
Food
Esophagus
Small intestine
Cud
Reticulum
Rumen
Smaller
food
particles
Omasum
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Abomasum
25.7 The Rumen and Ruminant Animals
• The rumen contains 1010–1011 microbes/g of
rumen constituents
• Fermentation in the rumen is mediated by
cellulolytic microbes that hydrolyze cellulose to
free glucose that is then fermented, producing
volatile fatty acids (e.g., acetic, propionic, butyric)
and CH4 and CO2 (Figure 25.27)
• Fatty acids pass through rumen wall into
bloodstream and are utilized by the animal as its
main energy source
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Figure 25.27
FEED, HAY, etc.
Cellulose, starch, sugars
Cellulolysis, amylolysis
Fermentation
SUGARS
Pyruvate
Fermentation
Formate
Succinate
Acetate
Propionate
Acetate
Butyrate
Rumen wall
Ruminant bloodstream
Lactate Propionate  CO2
VFAs
Removed by
eructation to
atmosphere
Overall stoichiometry of rumen fermentation:
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25.7 The Rumen and Ruminant Animals
• Rumen microbes also synthesize amino acids
and vitamins for their animal host
• Rumen microbes themselves can serve as a
source of protein to their host when they are
directly digested
• Anaerobic bacteria dominate in the rumen
(Figure 25.28)
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25.7 The Rumen and Ruminant Animals
• Abrupt changes in an animal’s diet can result in
changes in the rumen flora
• Rumen acidification (acidosis) is one
consequence of such a change
• Anaerobic protists and fungi are also abundant in
the rumen
• Many perform metabolisms similar to those of
their prokaryotic counterparts
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25.10 Termites
• Termites decompose cellulose and hemicellulose
• Termites classified as higher or lower based on
phylogeny
• Termite gut consists of foregut, midgut, and
hindgut (Figure 25.34)
– Posterior alimentary tract of higher termites
(Termitidae)
• Diverse community of anaerobes including
cellulolytic anaerobes (Figure 25.35)
– Lower termites
• Anaerobic bacteria and cellulolytic protists
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Figure 25.34
Foregut
Midgut
Hindgut
Paunch
Hindgut compartments
Cellulose
Glucose
Anoxic
Microoxic
2 mm
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Acetate
0.5 mm