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

The Nitrogen Cycle

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

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)

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

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

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)

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:

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

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

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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)

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-).

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

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.

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.

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

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

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]

Model answer – everyone must contribute!

Quick Test

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

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

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

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

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

Answers

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

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

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

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

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

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