1. Nutrient Recycling – The Nitrogen Cycle
Explain the significance of microorganisms in the recycling of nutrients within an ecosystem
Explain the purpose of recycling nitrogen
Describe what happens during the nitrogen cycle

2. The Nitrogen Cycle

3. The nitrogen cycle All life requires nitrogen-compounds, e.g.,
proteins and nucleic acids.
78% Nitrogen in air
Nitrogen gas is INERT

4. The nitrogen cycle
Plants must secure their nitrogen in "fixed" form, i.e., incorporated in compounds such as:
nitrate ions (NO3-)
ammonia (NH3)/ammonium (NH+4)

5. The nitrogen cycle
Animals secure their nitrogen (and all other) compounds from plants (or animals that have fed on plants).
i.e. animals digest, absorb and assimilate forms of nitrogen from their food

6. The nitrogen cycle
Four biological processes participate in the cycling of nitrogen through the biosphere:
nitrogen fixation
decomposition
nitrification
denitrification

7. N in excretory products
NH+4
(ammonium)
N in animals
decomposition
excretion
decomposition
N in excretory products
Nitrogen fixation
Feeding/assimilation
Nitrification
decomposition
NO-2 (nitrites) N2
N in plants
denitrification
Nitrification
Absorption and assimilation
NO-3
(nitrates)

8. Nitrogen fixation
The nitrogen molecule (N2) is quite inert. To break it apart so that its atoms can combine with other atoms requires the input of substantial amounts of energy.
Three processes are responsible for most of the nitrogen fixation in the biosphere:

9. Nitrogen fixation biological fixation by certain microbes
atmospheric fixation by lightning
industrial fixation – Haber process

10. Nitrogen fixation Free-living bacteria such as Azotobacter
Symbiotic (mutualistic) bacteria Rhizobium in root nodules of legumes
Root nodules on clover plant

11. Short video clip

12. Decomposition
The proteins made by plants enter and pass through food webs
Nitrogen compounds return to the environment, chiefly in excretions.
The final beneficiaries of these materials are microorganisms of decay.
They break down the molecules in excretions and dead organisms into ammonia. (NH3)

13. Nitrification
Ammonia can be taken up directly by plants - usually through their roots.
ammonia converted into nitrates.
oxidise NH3 to nitrites (NO2-).
oxidise the nitrites to nitrates (NO3-).

14. Nitrification
These two groups or autotrophic bacteria are called nitrifying bacteria.

15. Denitrification
The three processes above remove nitrogen from the atmosphere and pass it through ecosystems.
Denitrification reduces nitrates to nitrogen gas, thus replenishing the atmosphere.
Thus it closes the nitrogen cycle.

16. Denitrification
Once again, bacteria such as Pseudomonas are the agents. They live deep in soil and in aquatic sediments where conditions are anaerobic.
They use nitrates as an alternative to oxygen for the final electron acceptor in their respiration.

17. N in excretory products
N in animals
decomposition
NH+4
excretion
decomposition
N in excretory products
Nitrogen fixation
Feeding/assimilation
Nitrification
decomposition
NO-2 N2
N in plants
denitrification
Nitrification
Absorption and assimilation
NO-3

18. Perfect flash card question
Your turn to work
Outline the roles of bacteria in the nitrogen cycle [6 marks]
Perfect flash card question

19. 3. Rhizobium / nitrogen-fixing bacteria lives in root nodules / symbiotic relationship with legumes; fixes nitrogen into ammonia; using energy from ATP;
Azotobacter / nitrogen-fixing bacteria lives free in soil; fixes nitrogen into ammonia;
Nitrosomonas / nitrifying bacteria converts ammonia to nitrite;
Nitrobacter / nitrifying bacteria converts nitrite to nitrate; Nitrosomonas and Nitrobacter live in well aerated soil / require good supply of oxygen;
Pseudomonas denitrificans / denitrifying bacteria converts nitrate to nitrogen; nitrate is used as terminal electron acceptor instead of oxygen; carries this out in low oxygen / badly aerated soil; 6 max
Responses must include at least three types of bacteria to achieve full marks. Award [0] for responses with just the name of bacterium.
[6]

20. Model answer – everyone must contribute!

21. Quick Test

22. Q.1
Which process describes the two step oxidation of ammonia to nitrate?
Denitrification
Nitrification
Nitrogen fixation

23. Q.2 Which bacterial species is responsible for denitrification?
Rhizobium
Nitrosomonas
Pseudomonas

24. Q.3 Which process describes the conversion of nitrogen gas to ammonia
Denitrification
Nitrification
Nitrogen fixation

25. Q.4
Which process describes the conversion of nitrogen containing compounds in plants to ammonia
Denitrification
Assimilation
Decomposition

26. Q.5 Which of these bacterial species is responsible for nitrification?
a) Nitrosomonas
Pseudomonas
Rhizobium

27. Answers

28. Q.1
Which process describes the two step oxidation of ammonia to nitrate?
Denitrification
Nitrification
Nitrogen fixation b)

29. Q.2 c) Which bacterial species is responsible for denitrification?
Rhizobium
Nitrosomonas
Pseudomonas c)

30. Q.3
Which process describes the conversion of nitrogen gas to ammonia
Denitrification
Nitrification
Nitrogen fixation c)

31. Q.4
Which process describes the conversion of nitrogen containing compounds in plants to ammonia
Denitrification
Assimilation
Decomposition c)

32. Q.5
Which of these bacterial species is responsible for nitrification?
a) Nitrosomonas
Pseudomonas
Rhizobium a)

33. Soil Bacteria DNA Nitrogen Protein N2 Legumes Lightning Root nodules
RNA
Legumes