Nutrient Cycles

Carbon is present in all organic molecules (carbohydrates, lipids, proteins, nucleic acids). The carbon cycle describes how carbon moves between the atmosphere, organisms, oceans, and Earth's crust.

Key Processes

The Role of Decomposers

• Saprophytic bacteria and fungi secrete enzymes onto dead organic matter (extracellular digestion)
• They absorb the soluble products and respire, releasing $CO_2$
• Decomposition rate depends on: temperature, moisture, oxygen availability, pH
• Optimal conditions: warm, moist, aerobic, neutral pH

Human Impact on the Carbon Cycle

• Burning fossil fuels releases carbon stored for millions of years → increased atmospheric $CO_2$
• Deforestation reduces photosynthesis (less $CO_2$ absorbed) and releases stored carbon when trees decay or burn
• Result: Enhanced greenhouse effect → global warming → climate change

Nitrogen is essential for making amino acids (proteins), nucleotides (DNA/RNA), and ATP. Although nitrogen gas ($N_2$) makes up 78% of the atmosphere, most organisms cannot use it directly — it must be converted to usable forms.

Key Processes

Nitrogen Fixation

Converting atmospheric $N_2$ into ammonia ($NH_3$) or ammonium ($NH_4^+$):

• Biological fixation: Carried out by nitrogen-fixing bacteria
  • Free-living in the soil (e.g., Azotobacter)
  • Mutualistic — living in root nodules of leguminous plants (e.g., Rhizobium in clover, peas, beans)
  • The bacteria convert $N_2$ into $NH_3$ using the enzyme nitrogenase
  • In return, the plant provides sugars from photosynthesis
• Industrial fixation: The Haber process ($N_2 + 3H_2 \rightarrow 2NH_3$) to make fertilisers
• Lightning: Provides energy to combine $N_2$ with $O_2$ → nitrogen oxides → dissolve in rain as nitrates

Nitrification

Conversion of ammonium ions to nitrites then to nitrates by nitrifying bacteria (aerobic process):

$NH_4^+ \xrightarrow{\textit{Nitrosomonas}} NO_2^- \xrightarrow{\textit{Nitrobacter}} NO_3^-$

• Nitrates ($NO_3^-$) are the main form of nitrogen absorbed by plant roots
• Requires aerobic conditions (well-aerated soil)

Assimilation

• Plants absorb nitrates from the soil by active transport
• Nitrates are used to make amino acids, which are assembled into proteins
• Animals obtain nitrogen by eating plants (or other animals)

Ammonification

• When organisms die or produce waste (urine, faeces), decomposers break down the nitrogen-containing compounds
• Proteins and nucleic acids are converted to ammonium ions ($NH_4^+$)
• This returns nitrogen to the soil in a usable form

$\text{Organic N (proteins)} \xrightarrow{\text{decomposers}} NH_4^+$

Denitrification

• Denitrifying bacteria (e.g., Pseudomonas) convert nitrates back to nitrogen gas ($N_2$)
• This occurs in anaerobic (waterlogged) conditions
• Removes usable nitrogen from the soil → reduces soil fertility

$NO_3^- \xrightarrow{\text{denitrifying bacteria}} N_2 \uparrow$

Summary of Nitrogen Cycle Bacteria

• Phosphorus is essential for ATP, DNA, RNA, and phospholipids
• It cycles between rocks/sediments, soil/water, and organisms
• Weathering of rocks releases phosphate ($PO_4^{3-}$) into soil
• Plants absorb phosphate from soil; animals obtain it from food
• Decomposers return phosphate to the soil
• Phosphorus does NOT have a gaseous phase — it cycles mainly through soil and water

• Microorganisms are essential for recycling nutrients
• Decomposers break down dead organisms and waste, releasing carbon, nitrogen, and other elements back into the abiotic environment
• Nitrogen-fixing bacteria make atmospheric nitrogen available to plants
• Nitrifying bacteria convert ammonium to nitrates (usable by plants)
• Without microorganisms, nutrients would be locked in dead organic matter and ecosystems would run out of essential elements

Question: Explain the role of bacteria in the nitrogen cycle. (6 marks)

Solution:

Nitrogen-fixing bacteria (e.g., Rhizobium in root nodules of leguminous plants) convert atmospheric nitrogen gas ($N_2$) into ammonium ions ($NH_4^+$) using the enzyme nitrogenase. This makes nitrogen available to plants.

Nitrifying bacteria (e.g., Nitrosomonas and Nitrobacter) carry out nitrification in aerobic conditions: they convert ammonium ions first into nitrites ($NO_2^-$) and then into nitrates ($NO_3^-$), which plants can absorb from the soil.

Saprophytic bacteria (decomposers) carry out ammonification — they break down nitrogen-containing molecules (proteins, nucleic acids) in dead organisms and waste products, releasing ammonium ions back into the soil.

Denitrifying bacteria (e.g., Pseudomonas) convert nitrates back into nitrogen gas in anaerobic conditions. This removes usable nitrogen from the ecosystem.

• Carbon cycle: photosynthesis (removes CO₂) ↔ respiration/combustion/decomposition (releases CO₂); fossil fuels store carbon long-term.
• Nitrogen cycle: N₂ → NH₄⁺ (fixation) → NO₃⁻ (nitrification) → absorbed by plants (assimilation) → decomposition (ammonification) → N₂ (denitrification).
• Key bacteria: Rhizobium (fixation), Nitrosomonas/Nitrobacter (nitrification), Pseudomonas (denitrification).
• Decomposers are essential — they recycle nutrients from dead organic matter back to the abiotic environment.
• Human activities (fossil fuel combustion, deforestation) have disrupted the carbon cycle, increasing atmospheric CO₂.