25 1 Lichens Lichens Leafy or encrusting microbial

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) © 2012 Pearson Education, Inc.

Figure 25. 1 © 2012 Pearson Education, Inc.

Figure 25. 2 Algal layer Fungal hyphae Rootlike connection to substrate © 2012 Pearson

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 © 2012 Pearson Education, Inc.

Figure 25. 3 © 2012 Pearson Education, Inc.

Figure 25. 4 © 2012 Pearson Education, Inc.

II. Plants as Microbial Habitats • 25. 3 The Legume–Root Nodule Symbiosis • 25.

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 © 2012 Pearson Education, Inc.

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) © 2012 Pearson Education, Inc.

Figure 25. 7 © 2012 Pearson Education, Inc.

Figure 25. 8 © 2012 Pearson Education, Inc.

25. 3 The Legume–Root Nodule Symbiosis • Nitrogen-fixing bacteria need O 2 to generate energy for N 2 fixation, but nitrogenases are inactivated by O 2 • In the nodule, O 2 levels are controlled by the O 2 -binding protein leghemoglobin (Figure 25. 9) © 2012 Pearson Education, Inc.

Figure 25. 9 © 2012 Pearson Education, Inc.

25. 3 The Legume–Root Nodule Symbiosis • Cross-inoculation group – Group of related legumes

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc. Continued plant and bacterial cell division leads to nodules

25. 3 The Legume–Root Nodule Symbiosis • Bacterial nod genes direct the steps in nodulation • nod. ABC gene encodes proteins that produce oligosaccharides called nod factors (Figure 25. 12) • Nod factors – Induce root hair curling – Trigger plant cell division © 2012 Pearson Education, Inc.

25. 3 The Legume–Root Nodule Symbiosis • The legume–bacteria symbiosis is characterized by several

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 © 2012 Pearson Education, Inc. Photosynthesis Succinate Malate Fumarate Pyruvate e Nitrogenase Glutamine Asparagine

25. 3 The Legume–Root Nodule Symbiosis • A few legume species form nodules on

Figure 25. 15 © 2012 Pearson Education, Inc.

25. 5 Mycorrhizae • Mycorrhizae – Mutualistic associations of plant roots and fungi –

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 © 2012 Pearson Education, Inc.

Figure 25. 21 Fungal filament Forked root © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

Figure 25. 22 Epidermis S Mycelium A S HP A HP Outer cortex Inner

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 © 2012 Pearson Education, Inc.

Figure 25. 23 © 2012 Pearson Education, Inc.

III. Mammals as Microbial Habitats • 25. 6 The Mammalian Gut • 25. 7

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) © 2012 Pearson Education, Inc.

Figure 25. 24 Sheep and cow Herbivores Carnivores Omnivores Pig Horse Brown bear Giant

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 © 2012 Pearson Education, Inc.

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) © 2012 Pearson Education, Inc. 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 © 2012 Pearson Education, Inc.

Figure 25. 26 Food Esophagus Small intestine Cud Reticulum Rumen Smaller food particles Omasum

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 CH 4 and CO 2 (Figure 25. 27) • Fatty acids pass through rumen wall into bloodstream and are utilized by the animal as its main energy source © 2012 Pearson Education, Inc.

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 CO 2 VFAs Removed by eructation to atmosphere Overall stoichiometry of rumen fermentation: © 2012 Pearson Education, Inc.

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) © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.