In any ecosystem, resources are finite. Organisms constantly take in nutrients and elements from their surroundings to grow and survive, but these materials must eventually be returned to the environment. Without the continuous recycling of these elements, life on Earth would quickly run out of the raw materials required to exist.

What you'll learn

• How carbon, nitrogen, and water cycle continuously between abiotic (non-living) and biotic (living) environments.
• The vital roles of decomposers and specific bacteria in recycling elements.
• Separate Biology: How indicator species are used to assess pollution, and how to control and calculate the rate of decay in composting and food preservation.

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Abiotic and Biotic Materials

An ecosystem is made up of all the living organisms interacting with each other and with the non-living parts of their environment. To understand how materials cycle, we must first distinguish between these two components.

Material cycles describe how elements move continuously between these biotic and abiotic components.

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The Carbon Cycle

Carbon is the fundamental building block of life. It forms the basis of all organic molecules, including carbohydrates, lipids, proteins, and DNA. Because there is a fixed amount of carbon on Earth, it must be recycled.

The carbon cycle relies on several key biological and chemical processes:

1. Photosynthesis: Green plants and algae absorb carbon dioxide (${CO}_2$) from the abiotic atmosphere. They use light energy to react it with water, forming carbon-containing organic molecules like glucose. Photosynthesis is the only process that removes carbon dioxide from the atmosphere.
2. Feeding: Carbon is transferred along food chains. When primary consumers (herbivores) eat plants, and secondary consumers (carnivores) eat those herbivores, carbon compounds are passed into their bodies as biotic biomass.
3. Respiration: All living organisms—plants, animals, and decomposers—respire to release energy. Aerobic respiration releases ${CO}_2$ back into the atmosphere as an abiotic waste product.
4. Decomposition: When organisms die or release waste (such as faeces), they are broken down by decomposers (microorganisms like bacteria and fungi). Decomposers secrete digestive enzymes onto dead material, absorb the digested nutrients, and respire. Their aerobic respiration releases ${CO}_2$ back into the atmosphere.
5. Combustion: Over millions of years, some dead plant and animal remains were buried under high pressure and temperature, forming fossil fuels (coal, oil, and natural gas). When these fossil fuels are burned (combusted) in factories or vehicles, locked-up carbon is rapidly released back into the atmosphere as ${CO}_2$.

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The Water Cycle

All living organisms require water to survive. Water acts as a solvent for chemical reactions, a medium for transport, and a key reactant in photosynthesis.

The water cycle describes the continuous movement of water on, above, and below the surface of the Earth:

• Evaporation: Heat from the Sun causes liquid water from oceans, lakes, and rivers to turn into water vapour (a gas) and rise into the atmosphere.
• Transpiration: Plants absorb water through their roots and transport it to their leaves. Water evaporates from the cells inside the leaves and diffuses out through the stomata as water vapour.
• Condensation: As water vapour rises, it cools and turns back into liquid water droplets, forming clouds.
• Precipitation: When the water droplets in clouds become too heavy, they fall back to the ground as rain, snow, hail, or sleet.

Producing Potable Water in Areas of Drought

In areas where freshwater is scarce, people must produce potable water (water that is clean and safe to drink) from salty seawater. This process is called desalination.

There are two primary methods of desalination:

1. Distillation: Seawater is boiled in a closed chamber. The water evaporates, leaving the heavy salts behind. The steam is directed into a condenser, where it cools down and condenses back into pure, potable liquid water. This method requires a vast amount of energy, making it expensive.
2. Reverse Osmosis: Seawater is forced under extremely high pressure through a semi-permeable membrane. The pores in the membrane are large enough to let water molecules pass through, but too small to let dissolved salt ions pass. The salt is trapped on one side, and pure drinking water emerges on the other.

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The Nitrogen Cycle

Nitrogen is essential for living organisms to build amino acids, proteins, and nucleic acids (DNA and RNA). While nitrogen gas ($N_2$) makes up about 78% of the Earth's atmosphere, it is highly unreactive because of its strong triple covalent bond. Plants and animals cannot absorb $N_2$ gas directly from the air.

Plants must absorb nitrogen in the form of soluble nitrate ions (${NO}_3^{-}$) from the soil. They take these up through active transport against a concentration gradient using energy from respiration. Animals then obtain their nitrogen by eating plants.

The nitrogen cycle depends entirely on four specific groups of bacteria to convert nitrogen into different chemical forms.

The Four Types of Nitrogen Bacteria

How Farmers Manage Nitrogen Levels

To keep crops growing productively, farmers must ensure high soil nitrate levels using two main strategies:

1. Fertilisers: Farmers add artificial chemical fertilisers (often containing nitrogen, phosphorus, and potassium - NPK) or natural organic fertilisers (like manure or compost) directly to the soil.
2. Crop Rotation: Instead of planting the same crop every year, farmers rotate crops. Every few years, they plant a legume crop (such as clover or peas). The nitrogen-fixing bacteria in the root nodules enrich the soil with nitrogen. When these crops are harvested or ploughed back into the soil, they decompose, releasing high amounts of nitrates for the next year’s crop.

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Pollution Indicators

(Separate Biology / Higher Tier only)

We can assess levels of environmental pollution by chemical testing, or by observing indicator species.

1. Water Pollution Indicators

When organic waste (such as sewage or agricultural runoff) enters rivers and lakes, aerobic bacteria multiply rapidly as they decompose the waste. These bacteria consume the dissolved oxygen in the water, causing oxygen levels to plummet.

• Highly Polluted Water (Low Oxygen): Bloodworms and sludgeworms are adapted to survive in low-oxygen conditions. They contain special hemoglobin-like pigments that allow them to extract oxygen even at extremely low concentrations. Finding many of these species indicates poor water quality.
• Clean Water (High Oxygen): Freshwater shrimps and stonefly nymphs are highly sensitive to drops in dissolved oxygen. They can only survive in clean, fast-flowing, well-oxygenated water. Their presence indicates excellent water quality.

2. Air Quality Indicators

Sulfur dioxide (${SO}_2$) is an acidic air pollutant released when fossil fuels containing sulfur impurities are burned.

• Lichens: Lichens are symbiotic partnerships between algae and fungi that grow on trees and rocks. Because they have no root systems, they absorb nutrients directly from rainwater and the air, making them highly sensitive to sulfur dioxide.
  • Crusty lichens are highly tolerant of pollution and can grow in heavily built-up cities.
  • Leafy (foliose) lichens can tolerate moderate levels of pollution.
  • Bushy (fruticose) lichens are extremely sensitive and will only grow in areas with very clean, unpolluted rural air.
• Blackspot Fungus: This is a fungal disease that infects rose leaves. The spores of blackspot fungus are highly sensitive to sulfur dioxide. Therefore, if your garden roses suffer from blackspot disease, it is paradoxically a sign that your local air is very clean!

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Decomposition and Decay

(Separate Biology only)

Decomposition is the process where organic matter is broken down into simpler inorganic substances by decomposers (bacteria and fungi).

Factors Affecting the Rate of Decomposition

The speed at which decomposers break down material is heavily dependent on abiotic conditions:

1. Temperature:
   • As temperature increases, the rate of decay increases because the enzymes in decomposers have more kinetic energy, leading to more frequent successful collisions.
   • However, if the temperature gets too high (typically above 45 °C), the enzymes denature, their active sites change shape, and decay stops completely.
   • If the temperature is too low (e.g., in a freezer), enzyme activity slows down drastically, halting decay.
2. Water Content (Moisture):
   • Decomposers require water to survive, carry out metabolic reactions, and secrete extracellular enzymes to digest food.
   • In completely dry conditions, decay stops.
3. Oxygen Availability:
   • Most decomposers break down materials via aerobic respiration, which requires oxygen.
   • High oxygen levels lead to rapid decay.
   • In anaerobic (oxygen-free) conditions, decay is much slower because decomposers are forced to respire anaerobically, which yields far less energy.

Composting vs. Food Preservation

We can apply these factors either to speed up decay (for composting) or prevent decay (for preserving food):

• Composting: Farmers and gardeners want to speed up decay. They use compost bins that are warm (insulated walls trap heat from respiration), moist (water is added if dry), and well-aerated (the pile is turned with a fork or has holes to let oxygen in).
• Food Preservation: To keep food edible, we construct abiotic barriers to stop decomposers:
  • Freezing/Refrigerating: Lowers the temperature to slow or stop enzyme reactions.
  • Salting/Drying: Removes moisture. Salt draws water out of microbial cells by osmosis, killing them.
  • Vacuum Packing/Canning: Excludes oxygen, preventing aerobic respiration. Canning also involves high-heat sterilization to kill existing bacteria.

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Calculating Rates of Decomposition

(Separate Biology only)

You must be able to calculate rate changes in the decay of biological material. The rate of decay is typically measured as the mass lost over a specific period of time.

$$Rate\; of\; decay=\frac{Change\; in\; mass}{Time}$$

If the initial mass and final mass are known, the percentage loss of mass can also be calculated:

$$Percentage\; loss\; of\; mass=\left(\frac{Initial\; mass-Final\; mass}{Initial\; mass}\right)\times 100$$

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