Microorganisms, decomposition and recycling carbon

Microbiology, Immunity and Forensics · Decay, DNA techniques and forensics · note 1 of 5

Spec 6.16

A Level only

A Level only: only in the full A Level, not the AS Level. AS Level students can skip it. What the labels mean

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Microorganisms, decomposition and recycling carbonSpec 6.16

In short

Saprotrophic bacteria and fungi decompose dead organisms, faeces and waste by secreting extracellular enzymes that digest organic matter and absorbing the soluble products. Their respiration releases carbon dioxide to the atmosphere, recycling carbon so it is available to producers for photosynthesis.

Decomposers are mostly saprotrophic bacteria and fungi. They feed by extracellular digestion: they secrete enzymes (proteases, lipases, carbohydrases such as cellulase) on to dead plants and animals, faeces and other waste. The enzymes hydrolyse large molecules into soluble ones (amino acids, fatty acids, glucose), which the microorganism absorbs. They use some of these for growth and respiration.

Producers take carbon dioxide from the atmosphere and fix it into organic compounds by photosynthesis.
Carbon passes to consumers when they feed.
Producers, consumers and decomposers all release carbon dioxide into the atmosphere by respiration.
Dead organisms, faeces and waste are digested by saprotrophs. Carbon in the organic molecules is converted to carbon dioxide when the decomposers respire, and the carbon is available again to producers.

Decomposition is fastest when conditions suit the microorganisms: warm temperatures (enzymes near their optimum), moisture, oxygen for aerobic respiration, and a suitable pH. Cold, dry, waterlogged, anaerobic or acidic conditions slow it, which is why peat and bog bodies persist. Without microorganisms, carbon would remain locked up in dead matter and producers would run short of carbon dioxide.

Carbon cycle: carbon dioxide taken up by producers in photosynthesis, passed to consumers by feeding, returned by respiration, dead organisms and waste digested by decomposers using extracellular enzymes, and decomposers respiring carbon dioxide back to the atmosphere. — The **carbon cycle**, showing the role of decomposers.

Exam tip:

Say that decomposers secrete extracellular enzymes, absorb the digested products and respire, releasing CO₂. 'They break down dead things' alone gains no marks.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

How do microorganisms recycle carbon?

Saprotrophic bacteria and fungi secrete extracellular enzymes that digest dead organisms and waste, then absorb the soluble products. They respire some of this, releasing carbon dioxide into the atmosphere. Producers can then take up this carbon dioxide in photosynthesis, so carbon returns to the living world.

How does PCR work?

PCR copies a section of DNA using repeated cycles. At about 95 °C the strands separate, at about 55 to 65 °C primers anneal, and at about 72 °C Taq polymerase builds new strands from free nucleotides. Each cycle doubles the amount of DNA, giving millions of copies after 30 cycles.

How does gel electrophoresis separate DNA?

DNA is negatively charged because of its phosphate groups. In an electric field it moves through an agarose gel towards the positive electrode. The gel is a mesh, so shorter fragments move faster and further than longer ones, separating the fragments by length into bands.

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