(5) Decomposition - factors affecting the rate of decay of biological organic material

As already described, waste materials from plants and animals must be recycled to maintain healthy ecosystems and avoid a build-up of too much waste, including dead plants and animals.

The decay processes ensure the constant recycling of the 'elements of life' to provide nutrients for new growth in plants and ultimately providing the minerals, protein and carbohydrate food for animals.

The organic material of waste products and dead animals or plants are broken down by decomposers - a variety of organisms.

Decomposers break down the smaller bits of dead material, in doing so they release waste carbon dioxide, water and energy (from their respiration), and, most importantly, nutrients that plants can use.

The three main types of decomposers are bacteria and fungi microorganisms and detritus feeders.

The latter include millipedes, springtails, woodlice, dung flies and other insects, maggots, slugs and worms - they obtain nutrients by digesting the dead plant and animal material - they are all well equipped with enzymes which they secrete to break the molecules down!

The smaller the pieces of dead material are, the greater the surface area exposed to enzyme action, so helping the decay chemistry to be optimised.

Decomposers decay waste in compost heaps (e.g. in garden) and in sewage works (as part of the purification process to make potable water).

Just as in GCSE chemistry, where you study the factors that control the rates of chemical reactions, we can look at what factors control the rate of decomposition of this organic material.

The rate of decomposition of organic material from dead organisms or their waste if living, is affected by various environmental factors.

Many of the factors you will come across in your chemistry, see ...

"Factors affecting the rates of chemical reactions"

The following discussion of the four factors affecting the rate of decay also apply to the rotting of food.

Generally speaking the quantity of mould (microscopic fungi) growing on a portion of food increases exponentially (curve upwards!), but the decay takes days, weeks or even months!

From the graph the average rate of mould growth over the 10 days:

= 17 / 10 = 1.7 arbitrary units of mould per day

For the initial 4 days the average rate is:

2.5 / 4 = 0.63 arbitrary units of mould per day.

See section on Methods of preserving food

 

(a) The availability of oxygen

Many decomposers require oxygen for aerobic respiration.

Therefore the rate of decomposition increases with increase in the ambient oxygen concentration.

This will produce more carbon dioxide, rather than methane gas (biogas - which you may wish to make).

This is the situation with a garden compost heap, where the decay is mainly via aerobic bacteria.

DO NOT confuse this situation with the production of biogas using anaerobic bacteria, where oxygen levels need to be minimised.

If the oxygen levels are low the rate of decomposition is reduced - you get this in the anaerobic conditions in water-logged soils and biogas digesters to make methane fuel.

However, some decomposer microorganisms can respire anaerobically (not needing oxygen) but this is transfers less energy (less exothermic) and these decomposers work more slowly.

Anaerobic decomposition-digestion produces more methane (biogas), rather than carbon dioxide.

See Respiration - aerobic and anaerobic in plants and animals.  gcse biology

 

(b) The temperature conditions

Most decomposers work most efficiently in warm conditions, but not at too high a temperature.

(Remember that enzymes are denatured at high temperatures and these enzymes control the digestion breakdown of organic material at the molecular level.)

All chemical reactions speed up with increase in temperature, but enzymes work best (fastest) within an optimum temperature range, often around 40^oC. The diagram on the right is for an enzyme with an optimum temperature around 35^oC.

At higher temperatures, reactant molecules have a greater average kinetic energy to overcome the activation energy barrier and so reactions can proceed faster.

The enzymes secreted the decomposer organisms that digest the dead or waste material, can therefore work faster, more efficiently, as the temperature is increased - but only so far!

However, at temperatures above ~50^oC, the protein structure of the enzymes begins to breakdown (denatured) and the decomposition rapidly decreases with further increase in temperature.

The disrupted structure of the protein enzymes means the substrate molecules can't 'dock in' to the active site where the chemical transformation takes place.

The resulting graph shows the result of the two competing effects - showing the optimum temperature and at high temperatures the enzyme controlled digestion reactions stop.

 

(c) The water content of the soil and rotting material

All decomposers (like all of life) require water to survive, no matter where their location.

The vast majority of chemical reactions in living organisms require the medium of water.

Decomposition increases in moist conditions compared to dry conditions.

The vast majority of chemical processes in living organisms need the medium of water.

Waste material to make biogas methane is mixed with water - all organisms need water to carry out their biological processes e.g. enzymes work better mixed with water - better contact with the organic waste being digested-decomposed.

However, there are situations where there is little oxygen and too much water!

A good example is water-logged soil, where it is difficult for air to permeate into the 'mud'.

As a consequence, the water contains little dissolved oxygen, but decomposers do need it to respire - but can do so anaerobically producing some methane - but this is a slower rate of respiration.

Consequently, in water-logged soils, the rate of decomposition is considerably slowed down.

 

(d) The concentration of the 'decay' organisms (decomposers)

The more digesting organisms (microorganisms/detritus feeders) in contact with a given amount of waste plant/animal material, the faster the rate of decomposition-digestion.

The bacteria and fungi that live on dead material secrete their digestive enzymes onto the food to digest it into soluble substances (smaller molecules) that can be absorbed.

This is called extracellular digestion because it happens outside the cells of the microorganisms.

The more microbes the better, because there are more enzymes in greater concentration!

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Key points about the importance of decomposition in nature's cycles

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

The Importance of Decomposers in Nutrient Cycles

Decomposers are organisms that break down dead plants, animals, and organic waste, returning vital nutrients to the environment.

They play a crucial role in the carbon cycle, nitrogen cycle, and other nutrient cycles.

Examples of decomposers include bacteria, fungi, and detritivores (such as earthworms and certain insects).

Roles of Decomposers:

• Nutrient Recycling: Decomposers break down organic material into simpler substances, making nutrients available for uptake by plants.

• Carbon Cycle: They break down dead organisms, releasing carbon dioxide (CO₂) back into the atmosphere through respiration.

• Nitrogen Cycle: Decomposers break down proteins and DNA from dead organisms, converting them into ammonia, which is then processed by nitrifying bacteria into nitrates for plants.

• Soil Fertility: The breakdown of organic matter enriches the soil, improving its texture and nutrient content.

• Waste Removal: They prevent the accumulation of dead organisms and organic waste, maintaining ecosystem balance.

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Factors Affecting the Rate of Decomposition (Speed of Decay)

The speed at which decomposition occurs depends on several environmental factors:

1. Temperature

   • Optimum Temperature: Decomposers (bacteria and fungi) work best within a moderate temperature range (20–40°C).

   • Too Cold: Enzyme activity in decomposers slows down, reducing decomposition rate.

   • Too Hot: High temperatures can kill decomposers or denature their enzymes, stopping decomposition.

2. Oxygen Availability

   • Aerobic Conditions: Most decomposers require oxygen for respiration, accelerating decay.

   • Anaerobic Conditions: In oxygen-poor environments (e.g., deep water, landfill sites), decomposition slows and may produce methane gas instead of CO₂.

3. Water Availability

   • Moist Conditions: Decomposers need water for metabolic processes, so decomposition occurs faster in damp environments.

   • Dry Conditions: Decomposition slows down as microorganisms struggle to survive and function.

4. pH Levels

   • Neutral to Slightly Acidic pH: Optimal for most decomposers.

   • Extreme pH Levels (High or Low): Can inhibit microbial activity, slowing decay.

5. Presence of Detritivores

   • Earthworms, beetles, and other detritivores help break down larger pieces of organic matter, increasing the surface area for microbial decomposition.

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Importance of Decomposition in Nature

• Maintains Ecosystem Balance: Without decomposition, dead organisms and waste would accumulate, disrupting ecosystems.

• Soil Health and Plant Growth: Nutrients are replenished, supporting plant life and food chains.

• Carbon and Nitrogen Cycling: Maintains atmospheric balance and ensures essential elements are available for life.

• Waste Management: Natural decomposition aids in composting and the breakdown of biodegradable materials.

Decomposers are essential for life on Earth.

Their role in nutrient cycling ensures the continuous renewal of organic material, supporting the growth and sustainability of ecosystems.

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Summary of learning objectives and key words or phrases

Be able to describe and explain the role of decomposers in natural cycles including the functions of bacteria, fungi, various microorganisms, detritus feeders and describe and discuss the factors affecting the rate of decomposition i.e. how decay is affected by oxygen, temperature, water and the concentration of decay organisms (decomposers).

• Know that many trees shed their leaves each year and most animals produce droppings at least once a day.

• All plants and animals eventually die and know that microorganisms play an important part in decomposing this material so that it can be used again by plants.

• Appreciate that the same material is recycled over and over again and can lead to stable communities.

  • You are expected to use your skills, knowledge and understanding to evaluate the necessity and effectiveness of schemes for recycling organic kitchen or garden waste.

  • Like using a compost bin!, to which you can add garden waste and kitchen waste. Its best if the compost material is shredded and the compost bin sides have holes/mesh to allow air in and circulate. You can add compost makers (decay accelerators) to speed up the process, but hopefully the bulk of the composting material warms up by heat released by the decomposition reactions to further speed up the decay.

• a) Appreciate that living things remove materials from the environment for growth and other processes.

  • Plants need carbon as carbon dioxide from air, hydrogen as water, oxygen from air/water and nitrogen from air/soil or as nitrates from the soil, plus other minerals via water through the roots.

  • From these elements and compounds, plants can make carbohydrates, fats and proteins as well as a source of absorbed minerals.

  • Therefore, when animals eat plants they digest the carbohydrates, fats, proteins and minerals, and then convert these materials into their own fats and proteins.

  • Know that these materials are returned to the environment either in waste materials or when living things die and decay.

    • These means all the essential elements of life are recycled through the soil and can re-enter the food chain again.

• b) Know and understand materials decay because they are broken down (digested) by microorganisms.

  • Know that microorganisms are more active and digest materials faster in (i) warm, (ii) moist and (iii) aerobic conditions.

    • (i) Warm conditions help speed up the chemical reactions of decay.

    • (ii) Water is an important medium for the reactions of living organisms.

    • (iii) Aerobic conditions require well oxygenated soil i.e. air able to circulate into the soil and litter.

• c) The decay process releases substances that plants need to grow.

• d) Know and understand that in a stable community, the processes that remove materials (plant/animal growth) are balanced by processes that return materials (microorganism decay) and the materials are constantly cycled (in a way 'recycled').

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