1. 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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2. Figure 25.1
Figure 25.1 Lichens.
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3. Rootlike connection to substrate
Figure 25.2
Algal layer
Fungal hyphae
Figure 25.2 Lichen structure.
Rootlike connection to substrate
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4. 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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5. Figure 25.3
Figure 25.3 Drawings of some motile phototrophic consortia found in freshwater lakes.
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6. Figure 25.4
Figure 25.4 Phase-contrast micrograph of “Pelochromatium roseum” from Lake Dagow (Brandenburg, Germany).
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7. 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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8. 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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9. 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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10. Figure 25.7 Figure 25.7 Soybean root nodules.
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11. Figure 25.8 Figure 25.8 Effect of nodulation on plant growth.
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12. 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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13. Figure 25.9 Figure 25.9 Root nodule structure.
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14. 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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15. 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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16. 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
Figure Steps in the formation of a root nodule in a legume infected by Rhizobium.
Soil
Nodules
Continued plant and bacterial cell division leads to nodules
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17. 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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18. 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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19. Succinate Malate Fumarate
Figure 25.14
Plant cytoplasm
Photosynthesis
Sugars
Symbiosome membrane
Organic acids
Bacteroid membrane
Bacteroid
Succinate Malate Fumarate
Citric acid cycle
Pyruvate e e
Proton motive force
Nitrogenase
Figure The root nodule bacteroid.
Electron transport chain
Glutamine Asparagine
Lb  Leghemoglobin
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20. 25.3 The Legume–Root Nodule Symbiosis
A few legume species form nodules on their stems (Figure 25.15)
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21. Figure 25.15
Figure Stem nodules formed by stem-nodulating Azorhizobium.
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22. 25.5 Mycorrhizae Mycorrhizae
Mutualistic associations of plant roots and fungi
Two classes:
Ectomycorrhizae
Endomycorrhizae
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23. 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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24. Fungal filament Forked root Figure 25.21 Figure 25.21 Mycorrhizae.
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25. 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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26. Epidermis S Mycelium A HP S Outer cortex HP A Inner cortex
Figure 25.22
Epidermis S
Mycelium A HP S
Outer cortex
Figure Arbuscular mycorrhizae root colonization. HP A
Inner cortex
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27. 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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28. Figure 25.23 Figure 25.23 Effect of mycorrhizal fungi on plant growth.
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29. 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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30. 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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31. Sheep and cow Herbivores Pig Carnivores Omnivores Horse Brown bear
Figure 25.24
Sheep and cow
Herbivores
Carnivores
Pig
Omnivores
Horse
Brown bear
Giant panda
Dog
Lion
Rabbit
Human
Figure Phylogenetic tree showing multiple origins of herbivory among mammals.
Gorilla
Orangutan
Baboon
Spider monkey
Lemur
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32. 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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33. Foregut fermentation chamber
Figure 25.25
Examples:
Ruminants (photo 1), colobine monkeys, macropod marsupials, hoatzin (photo 2)
Foregut fermenters 1. 2.
Foregut fermentation chamber
Acidic stomach
Small intestine
Hindgut fermenters
Examples: Cecal
animals (photos 3 and 4), primates, some rodents, some reptiles 3. 4.
Figure Variations on vertebrate gut architecture.
Hindgut fermentation chambers
Large intestine (colon)
Cecum
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34. 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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35. Rumen Esophagus Food Small intestine Cud Reticulum
Figure 25.26
Esophagus
Food
Small intestine
Cud
Reticulum
Rumen
Smaller food particles
Omasum
Abomasum
Figure The rumen.
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36. 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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37. VFAs Acetate Propionate Butyrate Figure 25.27 FEED, HAY, etc.
Cellulose, starch, sugars
Cellulolysis, amylolysis
Fermentation
Fermentation
SUGARS
Formate
Pyruvate
Succinate
Propionate  CO2
Lactate
Acetate
Acetate
Propionate
Figure Biochemical reactions in the rumen.
Ruminant bloodstream
Butyrate
Removed by eructation to atmosphere
Rumen wall
VFAs
Overall stoichiometry of rumen fermentation:
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38. 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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39. 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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40. 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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41. Figure 25.34 Cellulose Figure 25.34 Termite gut anatomy and function.
Foregut
Midgut
Hindgut
Paunch
Hindgut compartments
Cellulose
Glucose
Figure Termite gut anatomy and function.
Anoxic
Acetate
Microoxic
0.5 mm
2 mm
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