Lecture 25

Question 1

1. What is symbiosis?
2. What are the benefits to the symbiont?
3. What are the benefits to the host?
4. Give some examples:

Answer 1

1. The living together of two different organisms, a specific symbiotic bacterium and a specific microbial, animal, or plant host. Microbial symbiosis with animals and plants is very common in nature. Symbiosis is generally used to mean a mutually beneficial association.
2. Food; a habitat to colonize, reproduce in, and disperse from; and protection from predators.
3. A beneficial metabolic activity (i.e., a specific nutrient or enzymatic activity).
4. Examples:
   
   1. synthesis of vitamins (enteric bacteria in the human gut).
   2. production of digestive enzymes (cellulase by Teredinibacter in gland of deShayes of wood-boring bivalve molluscs).
   3. production of light (Photobacterium in light organs of fish).

Question 2

1. What are most symbiotic interactions based on?
2. What other ways can bacterial inteact with animals or plants?
3. What is a parasite?
4. What is a saprophyte?
5. What us the nature of the relationship?

Answer 2

1. Based on nutritional benefit provided by the symbiont to the host, and all depend on provision of nutrients by the host to the symbiont.
2. Bacteria can also associate and interact with plants and animals by being a commensal enteric symbiont (living in the intestine of an animal, using the nutrients in the intestine, beneficial or neutral.
3. Living in or on an animal or plant and causing disease.
4. Living on the surface of an animal or plant, using nutrients excreted to the surface, beneficial or neutral.
5. Relationship:
   
   1. Obligate - either partner (or both) is dependent on the symbiosis for its survival.
   2. Facultative - on of the partners (or both) can survive without the other.

Question 3

What are the two modes of symbiont transmission from one host generation to the next:

1. What is vertical transmission?
2. What is horizontal transmission?

Answer 3

1. From parent to offspring, usually maternally (via the egg). Example: Intracellular bacterial symbionts of insects.
2. Each new generation of the host picks up the symbiont from the environment. Example: Rhizobium in legumes.

Question 4

1. What is chemoautotrophic (sulfur-based) symbiosis correlated with?
2. Where were hydrothemal vent communities discovered?
3. What big invertebrate animals live in rift zones?
4. What were mussels and clams doing at these sites? What were crabs and shrimp doing at these sites?
5. How is there density and size supported at these sites, in other words what is the nutritional foundation of these invertabrate communities?

Answer 4

1. Plate tectonics and rift formation.
2. First discovered in 1977 off the coastline of Oregon; now known from several sea-floor plate spreading areas, rift zones.
3. Tube worms, mussels, giant white clams, crabs, shrimp, and some fishes.
4. Mussels and clams are filter feeders, and crabs and shrimp were seen to be scraping materials from the rocks and shells of the clams and mussels.
5. Chemolithautotrophic, sulfur-oxidizing bacter. They obtained energy from the oxidation of reduced inorganic compounds, carbon from fixation of carbon dioxide. Filter-feeders consume these bacteria from the water.

Question 5

1. How does a Riftia (a tube worm) feed with no mouth or gut tract?
2. What is the trophosome packed with?

Answer 5

1. The GI tract of Riftia is highly modified - mostly a spongy tissue, called a trophosome (feeding body; 50% of weight). Whitish, semi-transparent vestiment or sheath (these worms are also called vestimentiferans).
2. Packed with bacteria (10⁹ cells/g of tissue) and sulfur granules (S⁰) - greenish gold in color. Bacteria have been found to have high levels of enzymes for carbon dioxide fixation and sulfur metabolims (H₂S oxidation)

Question 6

1. What does the blood of the Riftia transport?
2. How is the hemoglobin in the Riftia unique?
3. How does the process of hemoglobin binding work?

Answer 6

1. Transports O₂ and H₂S from the surrounding vent water, via the plume, to the trophosome.
2. Hemoglobin in Riftia can bind H₂S in addition to being able to bind O₂, normally H₂S is highly toxic because it prevents the binding of oxygen but this is not so in Riftia.
3. The gill plume, up in the water column, binds O₂ and H₂S from seawater and vent water, transports these molecules to the trophosome. The bacteria then oxidize the H₂S to elemental sulfur (S⁰), and use the energy and electrons to fix CO₂. The CO₂ comes from seawater; it is transported by diffusion and via the animal’s circulatory system to the trophosome. The bacteria then release some of the fixed carbon, as carbohydrate, to the animal.

Question 7

1. What kind of animal’s are tube worms?
2. Does this kind of symbiosis occur only in the deep-sea?

Answer 7

1. Tube worms are autotrophic, in a sense, formed through this sulfur-based mutualism, allowing the energy available in the deep-sea hydrothermal vent areas to be exploited through a symbiotic mutualism with chemolithoautotrophs (sulfur oxidizing CO₂ fixing bacteria.
2. No, it varies.

Question 8

1. What is nitrogen-fixing symbiosis?
2. How do plants usually use nitrogen?
3. What do the bacteria in this symbiotic relationship do?
4. For the host, is the symbiosis obligate or facultative?
5. In symbiosis with legumes what do the bacteria form? What occurs in these structures?
6. What is a symbiosome?

Answer 8

1. Symbiosis of certain plants with nitrogen-fixing bacteria (rhizobia) allows these plants to colonize soils with very low nitrogen and grow well.
2. Plants typically use nitrogen in the form of ammonia, converting it to amino acids.
3. The bacteria convert N₂ from the atmosphere to ammonia via nitrogenase.
4. Facultative
5. Root nodules, nitrogen fixation occur in these structure.
6. Bacteroids surrounded by a plant cell cytoplasmic membrane.

Question 9

Bacterial colonization of the plant

1. What is the recognition/attachment step?
2. What is the excretion step?
3. What is the invasion/colonization step?

Answer 9

1. contact between the surface of the bacterium and the plant root cell, mediated by chemical signaling between plant and its specific type of symbiotic bacteria.
2. plant releases specific chemical factors to which the bacterial cells respond by growing and in turn producing nodulation factors, that stimulate formation by the plant of an infection thread (biochemical communication).
3. the bacterial cells travel through the infection thread as it forms, travel to the main root, stimulate the growth and division of cortical cells, which the bacteria colonize as bacteroids in a symbiosome.

Question 10

1. What does the plants transfer to the bacteroids?
2. How do bacteroids use these molecules?
3. What does the bacteroids eventually do with these molecules?

Answer 10

1. Transfers organic carbon molecules from photosynthesis.
2. Use those molecules as a source of energy for fixing atmospheric nitrogen.
3. Bacteroids eventually release ammonia to the plant, which the plant converts to amino acids.

Question 11

Cooperativity

1. Why does Rhizobium not fix nitrogen under aerobic conditions?
2. So, how is oxygen levels kept low for every generation?
3. What are cross-inoculation groups and bacterial specificity?

Answer 11

1. Becuase nitrogenase is inactivated by oxygen (and the nif genes are repressed). However, in the lab under microscopic conditions, it does fix nitrogen. Furthermore, the bacterium needs some oxygen for energy generation.
2. In the root nodule, a protein is made - leghemoglobin. Leghemoglobin, binds oxygen in the nodule and keeps its concentration low enough so that the bacterial nitrogenase can function. Genes from both the plant and the bacterium are necessary for the synthesis of leghemoglobin.
3. A paricular rhizobium species can infect and form root-nodule symbiosis with certain species of legumes but not with other legumes. A group of related legumes can be infected particular rhizobial species.

Question 12

1. What are Nod factors?
2. What do nod factors induce?
3. What are nod factors structurally?

Answer 12

1. Nod factors are signaling molecules produced by the bacterium during initiation of the symbiosis, and specific different nod factor are produced by different species of the bacteria.
2. Induce root hair curling and trigger cell division in the plant, leading to formation of the nodule.
3. Structurally are lip-chito-oligosaccharides; an acylated chitin oligomer with substitutions, e.g., R1 or R2 in the picture. Different molecules are recognized by different kinds of legumes.