Lecture 26: The Nitrogen Cycle

Lecture 26: The Nitrogen Cycle

• The nitrogen cycle involves several key processes, including nitrogen fixation, nitrification, and denitrification.
• These processes are mediated by various organisms, primarily bacteria.
• Cycling replenishes reactants over time, coupling the oxidation and reduction reactions, typically requiring a coupling of aerobic and anaerobic processes.
• Specific examples of the nitrogen cycle include fish tanks and agriculture.

Overview of Biogeochemical Cycles

• Each step in the biogeochemical cycle is mediated by sets of organisms.
• Cycling replenishes reactants over time.
• Cycling couples the oxidation and reduction of reactants, requiring the coupling of aerobic and anaerobic processes.

Key Nitrogen Cycle Processes

• Nitrogen Fixation: Converting atmospheric nitrogen (N₂) into usable forms like ammonia (NH₃) or amino acids. Crucial for living organisms.
• Nitrification: Transforming ammonia (or ammonium) into nitrites (NO₂⁻) and then nitrates (NO₃⁻). Carried out by bacteria.
• Denitrification: Converting nitrates back into atmospheric nitrogen (N₂). Essential for returning nitrogen to the atmosphere. This process occurs under anaerobic conditions.

Bacteria and ATP Production

• Bacteria involved in the nitrogen cycle use energy-releasing processes to generate ATP.

Focus Processes in the Nitrogen Cycle

• Nitrogen fixation: Converting N₂ (atmospheric) into amino acids.
• Nitrification: Creating nitrate (NO₃⁻) from ammonia (via biomass or nitrogen fixation), through aerobic respiration.
• Denitrification: Transforming nitrate (NO₃⁻) to N₂ (atmospheric nitrogen) through anaerobic respiration.

Oxidation States of Nitrogen

• Nitrates (NO₃⁻): (+5) Highly mobile, reactive.
• Nitrites (NO₂⁻): (+3) Reactive.
• Nitric Oxide (NO): (+2) Reactive gas, a greenhouse gas.
• Nitrous Oxide (N₂O): (+1) Greenhouse gas (anesthetic).
• Dinitrogen (N₂): (0) An inert gas.
• Ammonia (NH₃): (-3) Gas; + water forms ammonium (NH₄⁺).
• Ammonium (NH₄⁺): (-3). Low mobility in soil, common in organic nitrogen compounds.
• Organic N: (-3). Found in proteins, urea, etc.; in living organisms/organic matter.

Nitrogen Cycle in Animals & Plants

• Animals primarily contain organic N (proteins), and sometimes ammonium.
• Plants contain organic N, and nitrate too.

Rhizobia & Symbiotic Nitrogen Fixation

• Rhizobia are symbiotic bacteria associated with legumes (bean, clover, alfalfa).
• They play a critical role in nitrogen fixation, providing nitrogen to plants.
• Plants provide carbon and energy to the microbes, and a safe environment.
• Leghemoglobin is a protein which has a role in this process.

Cyanobacterial Heterocysts

• Cyanobacteria heterocysts are involved in nitrogen fixation.

Nitrification: Specific details

• Proteobacteria (Nitrospira, Nitrococcus, Nitrosomonas) and Thaumarchaeaota (chemo-litho-autotrophs) are involved in nitrification.
• Ammonia monooxygenase (amoA) is a key enzyme.
• Oxidizing agents are ammonia (NH₄⁺) and nitrite (NO₂⁻).
• Oxygen (O₂) is an electron acceptor.
• Carbon dioxide (CO₂) is a common carbon source.

Denitrification: Specific details

• Denitrification is a type of anaerobic respiration.
• Chemo-organo-heterotrophs and facultative anaerobes are involved.
• Electron acceptors are nitrates (NO₃⁻) and nitrites (NO₂⁻).
• Organic carbon is a typical carbon source.

Agricultural Applications & Issues

• Fertilizer use dramatically increases NH₄⁺ into the biosphere.
• Nitrification releases NO₃⁻, which is mobile in water runoff polluting aquatic environments.
• Denitrification produces N₂O, which is a potent greenhouse gas.

Eutrophication

• Eutrophication is nutrient enrichment in water bodies caused by runoff of nutrients like Nitrogen (N) and Phosphorus (P).
• It results in algae blooms, followed by oxygen depletion, thus creating "dead zones."

Ocean Dead Zones

• "Dead zones" are areas of low or no oxygen in the ocean caused by eutrophication (primarily from excessive nutrients).

Fish Tank Example

• Excess fish waste introduces ammonium (NH₄⁺) into the water.
• Beneficial bacteria convert this to nitrite (NO₂⁻) and finally to nitrate (NO₃⁻).
• Cycling a new fish tank establishes these bacteria.
• Excessive levels of ammonia, nitrites are toxic to fish.

Fertilizer Use & Crop Problems

• In wet conditions, denitrification of nitrates is common as anaerobic conditions and denitrifying bacteria are much more prevalent.
• Too much nitrate in the soil can lead to reduced crop development.

This quiz covers the essential processes of the nitrogen cycle, including nitrogen fixation, nitrification, and denitrification. It highlights the roles of various organisms, primarily bacteria, in these processes, as well as their importance in agriculture and ecosystems. Test your understanding of how nitrogen cycling replenishes essential nutrients over time.