43. Biogeochemical cycling II Flashcards
1
Q
what is specific to nitrogen fixation?
A
- enzyme complex
– nitrogenase
2
Q
what is nitrogenase?
A
- two subunits
– iron-sulphur protein
– molybdenum-iron-sulphur protein - inactivated when O reacts with Fe in protein
3
Q
how is nitrogenase protexted from O2?
A
- secretion of extracellular polysaccharides/slime production
- maye change conformation in presence of O2
- high levels of respiration
4
Q
what is the action of nitrogenase?
A
- enzyme binds to molecule of N2 gase
- reducing agent (ferrodoxin) transfers successive pairs of H+ atoms to N2
- final products released freeing nitrogenase to bind to another N2
– 2 NH3
5
Q
how is nitrogenase regulated?
A
- encoded by nif genes
– also encode regulatory proteins in N-fixation
– found in free-living and symbiotic N-fixing bacteria
– chromosomal/plasmid DNA - regulation fo nif genes perfomed by nitrogen sensitive NifA protein
– insufficient N, NifA expression triggered
– sufficient N / O presence, NifL protein activated inhibiting NifA activity (protein-protein interaction) and then inhibition of nitrogenase formation
6
Q
how is nitrogen fixed in the root nodule?
A
- symbiotic relationship with N-fixing bacteria
– provide plant with source of fixed N - specialised root nodules housing symbiont (Rhizobium bacteria)
– inside root nodule symbiont assumed form of bacteroids, contained in vesicles formed by root cells
7
Q
how are root nodules formed?
A
- roots emit chemical signals attracting symbiont
– bacteria emit signals stimulating root hairs to elongate
– form infection thread by invagination of plasma membrane - bacteria penetrate root cortex within infection thread
– cells of root cortex and pericycle of stele begin dividing
– vesicles containing bacteria bud into cortical cells
– vesicle membranes derived by invagination from plasma membrane of root cells - growth continues in affected refions of cortex and pericycle
– two masses dividing cells fuse
– form nodule - nodule continues to grow
– vascular tissue connecting nodule to xylem and phloem of stele develops
– vascular tissue carries nitrogenous compounds from nodule into stele for distribution to rest of plant
8
Q
what is the formation of root nodules?
A
- symbiont attach to root hair
- infection thread forms
– symbiont enters root cells - bacteria change into bacteroids
– packed root cells enlarge - enlarged root cells form nodule
9
Q
how does the root nodule work for legumes?
A
- legumes produce haemoglobon
– leghaemoglobin - retains oxygen and acts as slow releas ebuffer source of oxygen
– for metabolically active N-fixers as make use of large ATP amounts to fix N
– protects nitrogenase enzyme
10
Q
what is nitrification?
A
- biological oxidation process
– sequentially converts N compounds into nitrites and nitrates - conversion of ammonia tonitrate performed by soil-living bacteria and other nitrifying bacter
11
Q
what are the processess of nitrification?
A
- 2 steps
– ammonia –> nitrite
– nitrite –> nitrate - first step
– NH3 + O2 = NO2- + 3H+ + 2e-
– nitrosomonas identified genus (also nitrosococcus and nitrosospira) - second step
– NO2- + H2O = NO3- + 2H+ + 2e-
– nitrobacter identified genus (also nitrospina, nitrococcus, nitrospira)
12
Q
what is dentrification?
A
- reduction of nitrates back to N2 gas
– completes cycle - anaerobic conditions
- loss of N for agricultural purposes
– but essential in current wastewater treatment to prevent eutophication - key microorganisms:
– Thiobacillus dentrificans
– micrococcus dentrificans etc…
13
Q
how is nitrogen a limiting nutrient?
A
- shortest supply
– limits growth of organisms/population - nutrient is limiting because:
– adding more, it will increase growth
– non-limiting added instead, growth unaffected - eg. adding N to half plants grew taller than intreated plants.
14
Q
how does human activity affect nitrogen cycling?
A
- huma activity released N in environment
– combustion of fossil fuels
– use of nitrogen-containing fertilisers - increase levels of N-containing compounds in atmpshere
– high levels of atm. N associated with ahrmful effects (acid rain) and greenhouse pollutants - artificial fertilisers containg N and P
– excess washed to lakes, streams and rivers
– effect saltwater and freshwater eutrophication
– causes overgrowth of algae
– leads to depletion of O2 in environment (death of ecosystem)
15
Q
what is eutrophication?
A
- body of water becomes enriched in dissolved nutrients
– phosphates / nitrates
– stimulates growth of aquatic plant life resulting in depletion of dissolved oxygn
