B

commensalism) or negative (predation, parasitism, amensalism, and competition) as shown in figure 28.1. These interactions will be discussed next.

1. Define the terms symbiosis and microbial ecology. How are they similar and different?

2. In what ways can different microorganisms be in physical contact?

3. Define the terms population, community, and ecosystem.

4. List several important diseases that involve cyclic and intermittent symbioses.

Mutualism

Mutualism [Latin mutuus, borrowed or reciprocal] defines the relationship in which some reciprocal benefit accrues to both partners. This is an obligatory relationship in which the mutual-ist and the host are metabolically dependent on each other. Several examples of mutualism are presented next.

The protozoan-termite relationship is a classic example of mutualism in which the flagellated protozoa live in the gut of termites and wood roaches. (figure 28.2a). These flagellates exist on a diet of carbohydrates, acquired as cellulose ingested by their host (figure 28.2b). The protozoa engulf wood particles, digest the cellulose, and metabolize it to acetate and other products. Termites oxidize the acetate released by their flagellates. Because the host is almost always incapable of synthesizing cellulases (enzymes that catalyse the hydrolysis of cellulose), it is dependent on the mutualistic protozoa for its existence.

This mutualistic relationship can be readily tested in the laboratory if wood roaches are placed in a bell jar containing wood chips and a high concentration of O2. Because O2 is toxic to the flagellates, they die. The wood roaches are unaffected by the high O2 concentration and continue to ingest wood, but they soon die of starvation due to a lack of cellulases.

Lichens are another excellent example of mutualism (figure 28.3). Lichens are the association between specific ascomycetes (the fungus) and certain genera of either green algae or cyanobacteria. In a lichen, the fungal partner is termed the my-cobiont and the algal or cyanobacterial partner, the phycobiont.

Figure 28.2 Mutualism. Light micrographs of (a) a worker termite of the genus Reticulitermes eating wood (X10), and (b) Trichonympha, a multiflagellated protozoan from the termite's gut (X135). Notice the many flagella that occur over most of its length. The ability of Trichonympha to break down cellulose enables termites to use wood as a food source.

Figure 28.2 Mutualism. Light micrographs of (a) a worker termite of the genus Reticulitermes eating wood (X10), and (b) Trichonympha, a multiflagellated protozoan from the termite's gut (X135). Notice the many flagella that occur over most of its length. The ability of Trichonympha to break down cellulose enables termites to use wood as a food source.

The remarkable aspect of this mutualistic association is that its morphology and metabolic relationships are so constant that lichens are assigned generic and species names. The characteristic morphology of a given lichen is a property of the mutualistic association and is not exhibited by either symbiont individually. Ascomycetes (pp. 560-61); cyanobacteria (pp. 471-76)

Because the phycobiont is a photoautotroph—dependent only on light, carbon dioxide, and certain mineral nutrients—the fungus can get its organic carbon directly from the alga or cyanobacterium. The fungus often obtains nutrients from its partner by haustoria (projections of fungal hyphae) that penetrate the phycobiont cell wall. It also uses the O2 produced during phyco-biont photophosphorylation in carrying out respiration. In turn the fungus protects the phycobiont from excess light intensities, provides water and minerals to it, and creates a firm substratum within which the phycobiont can grow protected from environmental stress.

Many marine invertebrates (sponges, jellyfish, sea anemones, corals, ciliates) harbor endosymbiotic, spherical algal cells called zooxanthellae within their tissue (figure 28.4a). Because the degree of host dependency on the mutualistic alga is somewhat variable, only one well-known example is presented.

The hermatypic (reef-building) corals (figure 28.4b) satisfy most of their energy requirements using their zooxanthellae. Pigments produced by the coral protect the algae from the harmful effects of ultraviolet radiation. Clearly the zooxanthellae also benefit the coral because the calcification rate is at least 10 times greater in the light than in the dark. Hermatypic corals lacking zooxanthellae have a very low rate of calcification. Based on their stable carbon isotopic composition, it has been determined that most of the organic carbon in the tissues of the hermatypic corals has come from the zooxanthellae. Because of this coral-algal mu-tualistic relationship—capturing, conserving, and cycling nutrients and energy—coral reefs are among the most productive and successful of known ecosystems.

Figure 28.3 Lichens. Crustose (encrusting) lichens growing on a granite post.

Sulfide-Based Mutualisms

Tube worm-bacterial relationships exist several thousand meters below the surface of the ocean, where the Earth's crustal plates are spreading apart (figure 28.5). Vent fluids are anoxic, contain high concentrations of hydrogen sulfide, and can reach a temperature of 350°C. The seawater surrounding these vents has sulfide

600 Chapter 28 Microorganism Interactions and Microbial Ecology

Figure 28.4 Zooxanthellae. (a) Zooxanthellae (green) within the tip of a hydra tentacle (X150). (b) The green color of this rose coral (Manilina) is due to the abundant zooxanthellae within its tissues.

Metal sulfides, iron-manganese oxides, hydroxides and iron silicates from — rising vent fluid precipitate in seawater forming white and black "smoke."

Direction of bottom current

Copper-iron-zinc sulfides precipitate inside vent and chimney

Seawater seeps through cracks and fissures in crust

Metal sulfides, iron-manganese oxides, hydroxides and iron silicates from — rising vent fluid precipitate in seawater forming white and black "smoke."

Seawater seeps through cracks and fissures in crust

Fractured Basalt

From crust seawater leaches:

Copper

Iron

Manganese Zinc Sulfur Silicon

Magma heating seafloor

Basalt

Figure 28.5 Basic Structure of a Hydrothermal Vent with its Mutualistic Microbe-Animal Associations.

Reduced chemicals including sulfide are released as seawater penetrates the fractured basaltic ocean floor, is heated, and returns as vent fluid to the ocean, creating environments for growth of the tube worms and their procaryotic mutualists.

Prescott-Harley-Klein: Microbiology, Fifth Edition

VIII. Ecology and Symbiosis

28. Microorganism Interactions and Microbial Ecology

© The McGraw-Hill Companies, 2002

28.2 Microbial Interactions

Gill plume O,

Gill plume O,

Figure 28.6 The Tibe Worm—Bacterial Relationship. (a) A community of tube worms (Riftiapachyptila) at the Galápagos Rift hydrothermal vent site (depth 2,550 m). Each worm is more than a meter in length and has a 20 cm gill plume. (b, c) Schematic illustration of the anatomical and physiological organization of the tube worm. The animal is anchored inside its protective tube by the vestimentum. At its anterior end is a respiratory gill plume. Inside the trunk of the worm is a trophosome consisting primarily of endosymbiotic bacteria, associated cells, and blood vessels. At the posterior end of the animal is the opisthosome, which anchors the worm in its tube. (d) Oxygen, carbon dioxide, and hydrogen sulfide are absorbed through the gill plume and transported to the blood cells of the trophosome. Hydrogen sulfide is bound to the worm's hemoglobin (HSHbO2) and carried to the endosymbiont bacteria. The bacteria oxidize the hydrogen sulfide and use some of the released energy to fix CO2 in the Calvin cycle. Some fraction of the reduced carbon compounds synthesized by the endosymbiont is translocated to the animal's tissues.

Gill plume O,

Trophosome cell

Bacteria

Capillary

Capillary Nutrients

Endosymbiotic bacteria

CO2 H,S

Gill plume O,

Trophosome cell

Bacteria

Capillary

Capillary Nutrients

Endosymbiotic bacteria

CO2 H,S

Sulfide oxidation

Calvin cycle

Figure 28.6 The Tibe Worm—Bacterial Relationship. (a) A community of tube worms (Riftiapachyptila) at the Galápagos Rift hydrothermal vent site (depth 2,550 m). Each worm is more than a meter in length and has a 20 cm gill plume. (b, c) Schematic illustration of the anatomical and physiological organization of the tube worm. The animal is anchored inside its protective tube by the vestimentum. At its anterior end is a respiratory gill plume. Inside the trunk of the worm is a trophosome consisting primarily of endosymbiotic bacteria, associated cells, and blood vessels. At the posterior end of the animal is the opisthosome, which anchors the worm in its tube. (d) Oxygen, carbon dioxide, and hydrogen sulfide are absorbed through the gill plume and transported to the blood cells of the trophosome. Hydrogen sulfide is bound to the worm's hemoglobin (HSHbO2) and carried to the endosymbiont bacteria. The bacteria oxidize the hydrogen sulfide and use some of the released energy to fix CO2 in the Calvin cycle. Some fraction of the reduced carbon compounds synthesized by the endosymbiont is translocated to the animal's tissues.

Sulfide oxidation

Calvin cycle concentrations around 250 ^M and temperatures 10 to 20°C above the normal seawater temperature of 2.1°C.

The giant (>1 m in length), red, gutless tube worms (Riftia spp.) near these hydrothermal vents provide an example of a unique form of mutualism and animal nutrition in which chemolithotrophic bacterial endosymbionts are maintained within specialized cells of the tube worm host (figure 28.6). To date all attempts to culture these microorganisms have been unsuccessful.

The tube worm takes up hydrogen sulfide from the seawater and binds it to hemoglobin (the reason the worms are bright red). The hydrogen sulfide is then transported in this form to the bacteria, which use the sulfide-reducing power to fix carbon dioxide in the Calvin cycle (see figure 10.4). The CO2 required for this cycle is transported to the bacteria in three ways: freely dissolved in the blood, bound to hemoglobin, and in the form of organic acids such as malate and succinate. These acids are decarboxy-lated to release CO2 in the trophosome, the tissue containing bacterial symbionts. Using these mechanisms, the bacteria syn thesize reduced organic material from inorganic substances. The organic material is then supplied to the tube worm through its circulatory system and serves as the main nutritional source for the tissue cells.

Methane-Based Mutualisms

Other unique food chains involve methane-fixing microorganisms as the first step in providing organic matter for consumers. Methanotrophs, bacteria capable of using methane, occur as in-tracellular symbionts of methane-vent mussels. In these mussels the thick fleshy gills are filled with bacteria. In addition, methan-otrophic carnivorous sponges have been discovered in a mud volcano at a depth of 4,943 m in the Barbados Trench. Abundant methanotrophic symbionts were confirmed by the presence of enzymes related to methane oxidation in sponge tissues. These sponges are not satisfied to use bacteria to support themselves; they also trap swimming prey to give variety to their diet.

602 Chapter 28 Microorganism Interactions and Microbial Ecology

Microorganism-Insect Mutualisms

Mutualistic associations are common in the insects. This is related to the foods used by insects, which often include plant sap or animal fluids lacking in essential vitamins and amino acids. The required vitamins and amino acids are provided by bacterial symbionts in exchange for a secure physical habitat and ample nutrients. The aphid is an excellent example of this mutualistic relationship. This insect contains Buchnera aphidicola in its cytoplasm, and a mature insect contains literally millions of these bacteria in its body. The Buchnera provides its host with amino acids, particularly tryptophan, and if the insect is treated with antibiotics, it dies. Wolbachia pipientis, a rickettsia, is a cytoplasmic endosymbiont found in 15 to 20% of insect species and can control the reproduction of its host. This microbial association is thought to be a major factor in the evolution of sex and speciation in the parasitic wasps. Wolbachia also can cause cytoplasmic incompatibility in insects, parthenogenesis in butterflies, and the feminization of genetic males in isopods. What could be the advantage to the Wolbachia? By limiting sexual variability, the bacterium might benefit by creating a more stable asexual environment for its own longer-term maintenance. Our understanding of microbe-insect mutualisms, including the role of Wolbachia in insects, is constantly expanding with the increased use of molecular techniques.

1. What is a lichen? Discuss the benefits the phycobiont and mycobiont provide each other.

2. What is the critical characteristic of a mutualistic relationship?

3. How do tube worms obtain energy and organic compounds for their growth?

4. What is the source of the waters released in a deep hydrothermal vent, and how is it heated?

5. What are important roles of bacteria, such as Buchnera and Wolbachia, in insects?

The Rumen Ecosystem

Ruminants are a group of herbivorous animals that have a stomach divided into four compartments and chew a cud consisting of regurgitated, partially digested food. Examples include cattle, deer, elk, camels, buffalo, sheep, goats, and giraffes. This feeding method has evolved in animals that need to eat large amounts of food quickly, chewing being done later at a more comfortable or safer location. More importantly, by using microorganisms to degrade the thick cellulose walls of grass and other vegetation, ruminants digest vast amounts of otherwise unavailable forage. Because ruminants cannot synthesize cellulases, they have established a mutualistic relationship with anaerobic microorganisms that produce these enzymes. Cellulases hydrolyze the p (1^ 4) linkages between successive D-glucose residues of cellulose and release glucose, which is then fermented to organic acids such as acetate, butyrate, and propionate (see figure 9.10). These organic acids are the true energy source for the ruminant.

The upper portion of a ruminant's stomach expands to form a large pouch called the rumen (figure 28.7) and also a smaller

Initial food

Small intestine

Initial food

Small intestine

Figure 28.7 Ruminant Stomach. The stomach compartments of a cow. The microorganisms are active mainly in the rumen. Arrows indicate direction of food movement.

honeycomblike reticulum. The bottom portion of the stomach consists of an antechamber called the omasum, with the "true" stomach (abomasum) behind it.

The insoluble polysaccharides and cellulose eaten by the ruminant are mixed with saliva and enter the rumen. Within the rumen, food is churned in a constant rotary motion and eventually reduced to a pulpy mass, which is partially digested and fermented by microorganisms. Later the food moves into the reticulum. It is then regurgitated as a "cud," which is thoroughly chewed for the first time. The food is mixed with saliva, reswallowed, and reen-ters the rumen while another cud is passed up to the mouth. As this process continues, the partially digested plant material becomes more liquid in nature. The liquid then begins to flow out of the reticulum and into the lower parts of the stomach: first the omasum and then the abomasum. It is in the abomasum that the food encounters the host's normal digestive enzymes and the digestive process continues in the regular mammalian way.

The rumen contains a large and diverse microbial community (about 1012 organisms per milliliter), including procaryotes, anaerobic fungi such as Neocallimastix, and ciliates and other protozoans. Food entering the rumen is quickly attacked by the cellulolytic anaerobic procaryotes, fungi, and protozoa. Although the masses of procaryotes and protozoa are approximately equal, the processing of rumen contents is carried out mainly by the procaryotes. Microorganisms break down the plant material, as illustrated in figure 28.8. Because the reduction potential in the rumen is — 30 mV, all indigenous microorganisms engage in anaerobic metabolism. The bacteria ferment carbohydrates to fatty acids, carbon dioxide, and hydrogen. The archaea (methanogens) produce methane (CH4) from acetate, CO2, and H2.

Dietary carbohydrates degraded in the rumen include soluble sugars, starch, pectin, hemicellulose, and cellulose. The largest percentage of each carbohydrate is fermented to volatile fatty acids (acetic, propionic, butyric, formic, and valeric), CO2, H2, and methane. Fatty acids produced by the rumen organisms are absorbed into the bloodstream and are oxidized by the animal as its

Prescott-Harley-Klein: VIII. Ecology and

Microbiology, Fifth Edition Symbiosis

28. Microorganism Interactions and Microbial Ecology

© The McGraw-Hill Companies, 2002

Mouth

Plant material"

Chewing

Rumen

Saliva"

Eructation

Regurgitation of"cud"

. Cellulose and other complex carbohydrates

ß (1^ 4) linkage hydrolyzed by microorganisms

D-glucose + cellobiose

Microbial fermentation

(Acetic, propionic,! butyric, valeric, /" formic acids I

Bloodstream to cells

„ Major energy source

Figure 28.8 Rumen Biochemistry.

(a) An overview of the biochemical-physiological processes occurring in various parts of a cow's digestive system. (b) More specific biochemical pathways involved in rumen fermentation of the major plant carbohydrates. The top boxes represent substrates and the bottom boxes some of the end products.

Pectin

Galacturonic acid

Xylose -

Crotonyl-CoA

Butyrate

Hemicellulose

Cellulose

Xylobiose b

Xylose

phosphate pathway

Cellobiose

Glucose

Glucose-6-P

Fructose-6-P

Was this article helpful?

0 0
Oplan Termites

Oplan Termites

You Might Start Missing Your Termites After Kickin'em Out. After All, They Have Been Your Roommates For Quite A While. Enraged With How The Termites Have Eaten Up Your Antique Furniture? Can't Wait To Have Them Exterminated Completely From The Face Of The Earth? Fret Not. We Will Tell You How To Get Rid Of Them From Your House At Least. If Not From The Face The Earth.

Get My Free Ebook


Post a comment