DECOMPOSITION is the breakdown of dead organic matter (dead organisms, their waste products and shed body parts) by MICROORGANISMS — primarily bacteria and fungi.
Decomposers are absolutely essential for life on Earth:
They RECYCLE NUTRIENTS — releasing minerals (nitrates, phosphates) back into the soil where plants can reabsorb them.
They complete the CARBON CYCLE — breaking down organic molecules and releasing CO₂ back to the atmosphere.
Without decomposers, nutrients would remain locked in dead organisms → soil would rapidly run out of minerals → plants could not grow → all life would eventually fail.
DECOMPOSERS vs DETRITIVORES:
DECOMPOSERS: bacteria and fungi — chemically break down organic matter using enzymes (EXTRACELLULAR DIGESTION — secreting enzymes outside the cell).
DETRITIVORES: earthworms, woodlice, millipedes — physically break down dead material into smaller pieces. This INCREASES SURFACE AREA for decomposers to work on, speeding up decomposition.
Both work together — detritivores break material up, decomposers chemically digest it.
Factors Affecting Decomposition Rate
Decomposers are living organisms — so decomposition rate is affected by the same factors that affect all enzyme-controlled reactions.
TEMPERATURE:
Higher temperature (up to ~40°C) → faster enzyme activity → faster decomposition.
Cold temperatures slow decomposition significantly — this is why refrigerators and freezers preserve food.
Above ~40°C, decomposer enzymes start to denature → rate falls.
MOISTURE (WATER AVAILABILITY):
Decomposers need water to survive and to transport dissolved nutrients.
Dry conditions → decomposition slows dramatically — this is why dried food lasts longer.
Waterlogged conditions can slow decomposition too — if the soil becomes anaerobic (no O₂).
OXYGEN AVAILABILITY:
Most decomposers are AEROBIC — they need oxygen for respiration.
AEROBIC conditions (well-aerated soil or compost) → faster decomposition.
ANAEROBIC conditions (waterlogged soil, sealed containers) → slower decomposition → some decomposers produce methane (CH₄) instead of CO₂.
pH:
Decomposers have an optimum pH. Very acidic or very alkaline conditions slow enzyme activity → slower decomposition.
Peat bogs are very acidic → slow decomposition → organic matter accumulates as peat (preserving bog bodies for thousands of years).
Decomposition in Human Contexts
Understanding decomposition has many practical applications:
COMPOSTING:
Mixing organic waste (vegetable peelings, garden waste) in a COMPOST HEAP.
Optimal conditions: moisture, oxygen (turn the heap regularly), warmth.
Decomposers break down the material → releases nutrients → creates COMPOST — a rich organic fertiliser for gardens.
FOOD PRESERVATION:
Methods work by removing conditions decomposers need:
REFRIGERATION — low temperature slows decomposition.
FREEZING — very low temperature stops decomposition.
DRYING — removes moisture.
PICKLING — low pH (vinegar) inhibits decomposers.
VACUUM PACKING — removes O₂.
SUGARING or SALTING — draws water out of food by osmosis, dehydrating decomposers.
SEWAGE TREATMENT:
Decomposers in sewage treatment works break down organic waste in human sewage.
Aerobic decomposers process the waste, removing pollutants.
Biogas (methane) captured from anaerobic decomposition used as a fuel.
⚠️ Common Mistake
DECOMPOSERS are bacteria and fungi — they chemically break down organic matter using enzymes. DETRITIVORES (earthworms, woodlice) physically shred material into smaller pieces — increasing surface area for decomposers. Detritivores do NOT chemically decompose material. Both are important but they are different things.
📌 Key Note
Decomposers = bacteria and fungi. Recycle nutrients, complete carbon cycle. Detritivores (earthworms etc.) = physically shred material, increase surface area. Decomposition faster with: higher temperature, more moisture, more O₂, neutral pH. Used in composting and food preservation.
Waterlogged, anaerobic soil — most decomposers need oxygen
Dry conditions — decomposers need water to function
Warm, moist, aerobic conditions — ideal for decomposer enzyme activity
Cold conditions — low temperature reduces enzyme activity
⭐ Higher Tier Only
Rate of decomposition depends on: temperature (enzyme activity), moisture (decomposers need water), O₂ availability (aerobic decomposers work faster), pH (optimum pH for decomposer enzymes). Compost heaps are managed to optimise these conditions. Biogas digesters use anaerobic bacteria to produce methane from organic waste — used as a renewable fuel. Students should be able to design an experiment to investigate the effect of one factor on decomposition rate.
🧪 Required Practical
🔬 RP7 — Investigate the effect of temperature on the rate of decay. Place a food sample (e.g. bread) in different conditions. Record mould growth over time. Or: measure rate of decomposition of organic material at different temperatures using respirometers.
Know the method, variables, equipment and how to analyse results.
🎯 Test Yourself
Question 1 of 2
1. What is the difference between decomposers and detritivores?
2. Why does food last longer in a refrigerator than at room temperature?
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