What are some strategies to protect nitrogenase from irreversible O2 damage?
- Anaerobic N2-fixation
- strict anaerobes
- Facultative anaerobes
- REspiratory and conformational protection
- Microaerophilic N2-fixation
- Compartmentation
- Heterocysts
- Root nodules
Describe anaerobic N2 fixation strategy
- strict anaerobes
- Obligate anaerobes such as Clostridium sp. that can fix N2 only in live anaerobic niche - Facultative anaerobes
- Species such as K. pneumoniae only fix N2 under anaerobic conditions- Do not fix N2 under aerobic conditions
Explain conformation protection N2 fixing strategy
FeSII protein
- protects nitrogenase from oxidative damage by binding to it
- Binds nitrogenase when oxidized
- Releases nitrogenase when reduced
The protein will sense oxygen before nitrogenase=> and change conformation
- Only found in AZOBACTER
Explain respiratory protection N2 fixation strategy
High rate of respiration uses up O2 at cell surface results in low enough intracellular [O2] for nitrogenase to function
- Occurs at high [O2] if energy source not limiting
Simple story:
- Oxygen is needed near the membrane for electron transport chain
- No need of oxygen in cytoplasm…
- Use very high rate of respiration to burn the oxygen
- Keeps the cytoplasm anaerobic
=> A. vinelandii respiratory chain adapts to [O2] => they have 2 electron transport chain
Explain microaerophilic N2-fixation strategy
Microaerophiles (AZOSPIRILLUM) possess terminal oxidases with high affinity for O2 to allow aerobic respiration at low [O2]
- Azospirillum is commonly used as a plant-growth promoting bacterium (PGPR), inoculated onto cereal crops etc.
- High affinity terminal oxidase “burns up” the O2 at the cell surface, resulting in a very low intracellular [O2]
=> REspiratory protection of nitrogenase - Aerobic respiration allows cells to generate ATP very efficiently to support N2-fixation
- Microaerophilic diazotrophs perform a delicate balancing act between incompatible processes: N2 fixation and aerobic respiration
- Can use aerotaxis to move in optimal [O2] (low) zones
What are the 2 compartmentation strategies?
- Heterocysts
- Root nodules
Describe heterocysts
- Single-celled cyanobacteria that can fix N2 only do so at night, when oxygenic photosynthesis is not active
- When N-demand is sensed in cyanbacteria that grow in chains and that can fix N2, some of the vegetative cells differentiate into heterocysts which can fix N2 day or night
- Heterocyst supplied by vegetative cells with disaccharides as carbon and energy source
- Exchance of C and N via microplasmadesmata!!!!
How can heterocysts be oxygen free?
Spatial separation of N2-fixation and oxygenic photosynthesis
- NO PS2 (oxygenic) in heterocysts
- Thick cell wall=O2 barrier - Rapid NH4+ assimilation and movement of fixed N out of heterocyst
How do heterocyst and vegetative cells exchange C and N ?
Via microplasmadesmata
What organisms can form root nodules? In what plant?
Rhizobia (in legumes)
Frankia sp ( alder)
How is O2 concentration regulated in nodules?
- Root nodule O2 barrier
- Tighly linked, water filled polant cells on periphery
- O2 transport and binding within the root nodule
- Leghemoglobin
- Maintains low [dissolved O2] , high flux from outside
- HIgh rate of respiration by bacteroids
- Terminal oxidase has high affinity for O2
- Also functions as a form of respiratory protection
What are examples of symbiotic N2-Fixation?
- Cyanobacteria with plants
- Aquatic fern Azolla with cyanobacterium Anabaena azollae
- Cyanobacteria with fungi and lichens
- Actinomycetes with woody plants
- Frankia (filamentous bacteria) with plants such as Alnus (alder, common in bogs etc. )
- N2-fixing form (diazovesicles) have thick cell walls => O2 barriers - Rhizobium-legume symbiosis
- Interaction between plant and bacteria species is usually very specific
Describe legumes?
Plant family with 15 0000 species
- Most are nodulated by Rhizobia
Very diverse morphology, ecology
Arctic annuals, tropical trees, crop, plants
- Peas, beans, soybean, clover, alfalfa, peanut etc
Legumes and N:
- LEgumes can grow in N-poor soil if nodulated
- Rhizobia can supply plant with all N needed
- Selective advantage over non-legumes in that niche
The plant supplies bacteroids within the nodules an energy sourc in the form of **C4-dicarboxylic acids **
- Plants is in control, and does not invest energy in nodulation and symbiotic N2-fixation if N is not needed
What are the steps in root nodule formation?
- species-specific plant flavonoid signals
- Rhizobial response
a. colonization
b. induction of nod genes and species-specific Nod signals - Plant response to specific Nod signal
- Infection
a. Infection thread
b. Contact recognition - Nodule and bacteroid development
- Nutrient exchange
