Lecture 24: Nitrogen Cycle Flashcards
1
Q
Why do we study the nitrogen cycle?
A
- Nitrogen is essential to all life
- Large impacts on the environment
- Large nitrogen reserves are inert
(‘hidden’), small ones are actively
recycled (air; 80% N2)
2
Q
What are the different forms of nitrogen
A
- Based on oxidation states
2. Forms are transformed by microbes
3
Q
What are the steps of the nitrogen cycle?
A
- di-nitrogen (N2) in the atmosphere
- N-fixation via microbes to organic N in soil
- This can the be mineralised by microbes
to ammonia (NH3) - Ammonia can undergo nitrification
from NH3 to NO2- (nitrite) to NO3-
(nitrate) - Nitrate can be reduced via
denitrification to NO2-, to NO, to
N2O, back to N2 in the atmosphere - “Anammox” (anaerobic ammonia
oxidation) also occurs, where NO2- and
NH3 is converted directly back to N2
4
Q
what is the relevance of nitrogen to plants and agriculture?
A
- Both ammonia (NH3) and nitrate
(NO3-) can be up-taken by plants - Big challenge in agriculture
- For good crop yields, fertilizers provide
additional NH3 and NO3- - NO3- (nitrate) can leak into oceans/rivers
via “leaching”, which is bad for aquatic
ecosystems - N2O (nitrous oxide) is a powerful green
house gas (300x more than CO2, so
if isn’t restored to N2 can be harmful
a. can be prod from denitrification and
nitrification
5
Q
What is leaching?
A
- Nitrate (NO3-) is very mobile in soil so
can leak into nearby rivers/oceans via a
process called “leaching” - Bad for aquatic ecosystems
6
Q
What is nitrogen fixation?
A
- Done in two ways:
a. Biological
b. Anthropogenic (industrial) - Only process where atmospheric N2 is
incorporated into biological matter - Two different types of organisms able to
carry out this process:
a. Free living
b. Symbiotic
7
Q
What are symbiotic nitrogen fixators?
A
- Rhizobium - root nodules of plants
2. Cyanobacteria - lichen
8
Q
What are free living nitrogen fixators?
A
- Cyanobacteria
2. Azotobacter
9
Q
What is the key enzyme in nitrogen fixation? BIOLOGICALLY
A
- Nitrogenase (nif): catalyses conversion of
N2 into NH3
a. N2 + 3H2 -> 2NH3 -> AAs -> proteins - V. energy-costly: triple bond of N2 hard
to break - Tightly regulated
- Oxygen sensitive
10
Q
How is nitrogen fixed industrially?
A
- “Haber-Bosch process”
a. how N fertilisers are made - Req. temps of 300-500C & pressures of
150-250 bars - ~50% of worlds human population would
not exist without this process
11
Q
What happens after nitrogen is fixed and is now bio available for other organisms?
A
"Mineralisation"
a. Organic N to NH3 (ammonia)
b. Makes it available for plants and
microbes
c. a lot less specialized; done by many
plants and microbes
12
Q
What is nitrification?
A
Ammonia to NO2- (nitrite) to NO3- (nitrate)
1. Important for fertiliser loss because NO3-
isn't stable in soil so can be lost
2. Two steps:
a. NH3 -> NO2-
i. carried out by AMO (ammonia
oxidisers)
ii. e.g., Nitrosomonas
b. NO2- -> NO3-
i. carried out by nitrite oxidisers
ii. e.g., Nitrobacter
3. Can lead to leaching; causing
eutrophication/environmental
pollution
4. Produces NO and N2O which are
climate active gases
5. Enzymes involved require:
a. O2 as an e- acceptor
b. CO2 as carbon source
c. NH3 & NO2- as e- donors
13
Q
What are ammonia oxidising bacteria?
A
- First discovered by Grace Frankland and
isolated by Sergei Winogradsky - Low cell densities and growth rates -
difficult to study - Typical soil AOB (nitrosospira and
nitrosomonas) belong to beta-
proteobacteria, typical marine AOB
(nitrosococcus) to gamma-proteobacteria
14
Q
What are AOA and what was their significance?
A
- AOA = Ammonia oxidising archaea
- Soil fosmid 54d9 contained 16S and 23S
ribosomal subunits, enabling
identification of the fosmid as an
archaeon - amoA and amoB encode for subunits
of “ammonia monooxygenase” - a key
enzyme in the global nitrogen cycle - Some ammonia oxidiser can also use
urea and cyanate as substrates - Significance? A WHOLE DIFFEERNT
DOMAIN OF LIFE! - AOA can nitrify in conditions where AOB
struggle (acidic soils, open ocean) -
THEIR NICHE
a. N. devanaterra
15
Q
What is comammox?
A
- Stands for “complete ammonia oxidation
into nitrate” - Single organism that can do both
ammonia and nitrite oxidation - Nitrospira sp. - nitrite oxidiser which
acquired ammonia oxidation genes via
horizontal gene transfer
16
Q
What is anammox?
A
1. stands for "anaerobic ammonium
oxidation"
2. especially significant to N turnover in
ocean and is large single source of
dinitrogen gas on earth
3. Chemolithoautotrophic bacteria use
NH4+ as e- donor and NO2- as acceptor
4. Uses "ladderane" membrane lipids that
form acellular "anammoxosome"
organelle
a. site of membrane-bound enzyme
combining nitrite and ammonia to N2.
Hydrazine is an intermediate
5. New low-cost method for N-removal in wastewater treatment
6. at least 3 genera of bacteria:
a. Brocadia
b. Kuenenia
c. Scalindua
7. Process patented:
NH4+ + NO2- -> N2 + 2H2O
17
Q
What is denitrification?
A
- Diverse; carried out by all three domains of life
- sequential formation of N2
- anaerobic
- Nitrogen is the electron acceptors -
essentially like our oxygen in respiration
a. lots of organisms can switch between
the two for energy - Performed primarily by heterotrophic bacteria. E.g.,
a. Paracoccus denitrificans
b. various pseudomonads - Some autrophic found:
a. Thiobacillus denitrificans - nirS and nirK encode nitrite reductase
(key marker gene for population
studies)
a. never present together
18
Q
What is the role of denitrification in agriculture and waste water treatment
A
- agriculture: remove nitrogen from field (BAD THING)
2. Wastewater treatment: removes nitrogen from system. Prevents eutrophication of receiving waters
19
Q
What is the importance of the nitrogen cycle to climate change?
A
- N2O and NO crucial to troposphere
a. N2O powerful greenhouse gas in
lower atmosphere
b. NO reacts in lower atmosphere and
produces NO2 (nitrogen dioxide) - Prod. is result of different processes:
a. Denitrification
b. Nitrification
20
Q
How can we ensure the nitrogen cycle doesnt impede on climate change?
A
1. Sustainable intensification of food
production
2. Improving soil N use efficiency
a. slow release fertilisers
b. Commercial nitrification inhibitors (e.g.,
DCD, nitrapyrin)
c. Demand driven fertilisation
