quiz 4

More than 80 phyla have been found, but few have been studied in the lab. More than 90% of characterized bacterial species belong to four phyla: Proteobacteria, Actinobacteria, Firmicutes, and Bacterioidetes. Three divisions based on cell wall structure: Tenericutes, Gracilicutes, and Firmicutes.

Bacteria WITHOUT cell walls. One phylum: Mycoplamatota (latin for "flexible"). One class of note is Mollicutes: very small, membranes stablized with sterols, very reduced genome, must live in close association with eukaryotic cells, plant animal pathogens living in/on host cells!

GRAM NEGATIVE bacteria, with thin peptidoglycan layer, and LPS outer membrane. Cyanobacteria form symbiosis with many plants, fixing nitrogen.

GRAM POSITIVE bacteria, with thick peptidoglycan layer and embedded teichoic acids. Genus Frankia (an actinomycete) forms actinorhizal symbiosis with plants, fixes nitrogen through root nodules.

Largest + most diverse bacterial phyla, more than 1/3rd of characterized bacterial species are in this phyla, including majority of medical/industrial/agricultural signifcant species. Also contains pathogens, species involved in decomposition, nutrient tranformations and nitrogen fixation! All species are GRAM-NEG. Six classes, named after first 6 letters of greek alphabet: Alphaproteobacteria, Beta.., Gamma.., Delta.., Epsilon.., Zetaproteobacteria.

One family of note is Rhizobiaceae, aerobic chemoheterotrophic rod-shaped bacteria that contains several genera that form symbiotic relationships with plants in the legume family by forming nodules on their roots and fixing nitrogen! Most well known genus is Rhizobium.

Broad defintion: Any intimate and prolonged association between two or more dissimilar organisms
Narrow definition: As above, but implies mutalism, i.e. that each organism depends on the other for increased survival or fitness.

Takes dinitrogen (N2, a gas) to ammonium (NH4+). It takes a lot of energy to break N2, natural occurences of this include lightning, volcanic activity and combustion (forest fires). However, this can also occur biologically (though only in prokaryotes), and is a widespread characteristic in Bacteria and Archaea. Nitrogen fixation relies on the enzyme nitrogenase, and paradoxically needs a lot of aerobic cell repiration to generate ATP but aerobic conditions inactivate nitrogenase (Lhb helps with this paradox)

Enzyme composed of two subunits, the MoFe protein, which contains all machinery to perform reaction, but requires a steady source of electrons. Reaction requires six electrons for every nitrogen molecule. The other component, the Fe protein, uses the breakage of ATP to pump these electrons into the MoFe protein.

Rhizobium species were originally classifed according to the legume species they could nodulate, but these boundaries weren't absolute, which led to the concept of a Cross Inoculation Group: "legume + Rhizobium species that result in nodulation when grown together"

- Increasing flavonoid concentration near the root surface stimulates the expression of "nod" genes by Rhizobium, and nod genes cause the production of lectins by the roots.
-Bacteria attatch to root hairs, and attachment may be aided by lectins.
-Bacteria relase signals that cuase the root hair to curl around Rhizobium cells. Part of the cell wall of the root hair is degraded, allowing bacteria to enter.
- Root hair cell produces an infection thread consisting of a modified plasma membrane. New cellulose is formed on the inside of the root hair cell membrane, and at the same time some cortical cells divide in the root, forming a nodule meristem!
- Bacteria divide within the thread, the thread extends inwards penetrating through + between cells of the cortex. In the inner cortex the bacteria are released into the cytoplasm of dividing tetraploid cells.

In some legumes, like the groundnut, Rhizobium instead enters through a crack between epidermal cells and grows inside the root cortex, between root cortex cells.

Once inside root cortex cells, bacteria differentiate into their N2 fixing state (called "bacteroid"). Bacterial cells lose their cell walls, grow several times larger, usually occur in groups of about 4. Surrounded by a peribacteroid membrane formed by the host cell, membrane also encolses the peribacteroid space. Up to 20,000 bacteroid per infected root nodule cell.

Oxygen binding protein similar to hemoglobin. Very high affinity for O2, can function in O2 environments low enough for nitrogenase. Dual function:
- Lhb keeps O2 levels low enough for nitrogenase to function AND
- delivers enough O2 to the appropriate nodule microsites for adequate cell respiration to generate ATP

Lhb is present in nodule cell cytoplasm, outer layers of the nodule cell restrict gaseous diffusion, high rates of bacterial + mitochondrial respiration within the central nodule tissues to further reduce O2 concentrations.

Symbiosis between actinorhizal plant species (such as Red Alder) and nitrogen fixing bacteria in the genus Frankia (gram +, filamentous, aerobic, chemoheterotroph). Frankia infect plants either through root hair infection (Alnus) or through direct inercellular penetration of root epidermis cells and cortex, where root hairs either not present or not infected.

Frankia species have 3 cell types:
- Vegetative hyphae, prokaryotic filaments, "normal" growth state
- Vesicles, where all N2 fixation occurs, vesicle walls also regulate O2 levels (thickness + # of wall layers changes in response to external O2 concentration)
- Reproductive spores

Nitrogen-fixing blue green bacteria that are involved with many plants, such as:
- Azolla/Anabaena, heterocyst cells formed under leaves fix N2
- Gunnera, only angiosperm with stem nodules
- Cycads, caralloid roots can be infected
- Lichens, Lobaria/Nostoc

Azolla, a small water fern, is infected by Anabaena, a genus of cyanobacteria that fixes N2 by colonizing chambers underneath it's leaves, and has historically been used to fix N2 in rice paddies (grow azolla in rice paddies before planting rice, rice slowly overtakes azolla, as azolla die their nitrogen deposited into rice paddies). The cyanobacteria Anabaena forms specialized heterocyst cells to fix nitrogens! Heterocyst cells (fix N2 -> NH3) arise from the differntiation of vegetative cells (fix CO2, photosynthesis).