25.1 lichens lichens –leafy or encrusting microbial symbioses –often found growing on bare...
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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
Rootlikeconnectionto substrate
Fungalhyphae
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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
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Animation: Root Nodule Bacteria and Symbioses with LegumesAnimation: Root Nodule Bacteria and Symbioses with Legumes
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 25.10Root hair
Rhizobial cell
Infection thread
Invaded plant cellsand those nearby arestimulated to divide
Nodules
Soil
Continued plant andbacterial cell divisionleads to nodules
Formation ofbacteroid state withinplant root cells
Bacteria in infectionthread grow towardroot cell
Invasion. Rhizobia penetrateroot hair and multiplywithin an “infection thread”
Excretion of nod factorsby bacterium causingroot hair curling
Recognition and attachment(rhicadhesin-mediated)
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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 25.14Plant cytoplasm Photosynthesis
Symbiosomemembrane
Bacteroidmembrane
Sugars
Organic acids
Bacteroid SuccinateMalate
Fumarate
Pyruvate
ee
Nitrogenase
Citricacidcycle
Protonmotiveforce
Electron transportchain
Lb Leghemoglobin
GlutamineAsparagine
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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
Fungalfilament
Forkedroot
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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
Mycelium
S
S
HP
HP
A
A
Outercortex
Innercortex
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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
HerbivoresCarnivoresOmnivores
Sheep and cow
Horse
Giant panda
Rabbit
Gorilla
Orangutan
Dog
Lion
Pig
Brown bear
Human
Baboon
Spider monkey
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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
Foregutfermentationchamber
Hindgutfermentationchambers
Hindgut fermenters
Foregut fermenters
Examples: Cecalanimals (photos 3 and 4), primates,some rodents, some reptiles
Examples:Ruminants (photo 1), colobine monkeys,macropod marsupials, hoatzin (photo 2)
Acidicstomach
Smallintestine
Largeintestine(colon)
Cecum
1. 2.
3. 4.
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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 EsophagusFood
Cud
Reticulum
Smaller foodparticles
Omasum Abomasum
Small intestine
Rumen
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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
Fermentation Fermentation
Cellulolysis, amylolysis
Formate
Pyruvate Succinate
Lactate Propionate CO2
Removed byeructation toatmosphere
Acetate
Ru
min
an
t b
loo
ds
tre
am
Ru
me
n w
all
VFAs
Acetate
Propionate
Butyrate
Overall stoichiometry of rumen fermentation:
SUGARS
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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
Hindgut compartments
Paunch
2 mm
Foregut
Midgut
Hindgut
Anoxic
Microoxic
Glucose
0.5 mm
Cellulose
Acetate
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