Decomposition

A saprobiont (saprotroph) is a decomposer — a bacterium or fungus — that feeds on dead organic matter.

It carries out extracellular digestion because it secretes digestive enzymes out of its cells onto the dead material, so the food is digested outside the organism, and the soluble products are then absorbed.

Carbon is recycled between the atmosphere (as carbon dioxide) and living organisms by several processes:

• Photosynthesis removes CO₂ from the atmosphere as producers fix it into organic molecules such as glucose;
• Respiration by all organisms (including decomposers) releases CO₂ back into the atmosphere;
• Combustion (burning of fossil fuels and biomass) and decomposition also release CO₂.

Carbon therefore moves out of the air into organic matter by photosynthesis and returns to the air through respiration, decomposition and combustion, keeping the cycle balanced (although burning fossil fuels is currently increasing atmospheric CO₂).

Earthworms are detritivores: they eat and physically break up the dead leaves into smaller pieces as they feed and burrow.

Breaking the leaves into smaller fragments increases the total surface area of the dead material that is exposed.

This gives the decomposer bacteria and fungi a larger surface area on which to secrete their enzymes, so more enzyme can act and the dead matter is digested and decomposed faster. (Earthworms also mix and aerate the soil, improving oxygen supply for aerobic decomposers.)

Atmospheric nitrogen gas (N₂) is inert and cannot be used directly by plants, so it must first be fixed.

Nitrogen fixation converts N₂ into ammonium / ammonia. This is carried out by nitrogen-fixing bacteria such as Rhizobium (in legume root nodules), Azotobacter (free-living in soil), and also by lightning (which fixes N₂ to form nitrate).

The ammonium produced is then converted by nitrifying bacteria into nitrate (NO₃⁻): Nitrosomonas oxidises ammonium to nitrite, and Nitrobacter oxidises nitrite to nitrate. Plants absorb this nitrate through their roots to make amino acids and proteins.

The mutualistic relationship. Rhizobium lives inside the root nodules of legumes (peas, beans, clover). The bacterium fixes nitrogen into ammonia, which the plant uses to make proteins; in return, the plant supplies the bacterium with carbohydrate (sugars from photosynthesis) and a suitable (low-oxygen) environment. Both partners benefit, so the relationship is mutualistic.

Nitrification is the oxidation of ammonium to nitrate, carried out in two steps by nitrifying bacteria: Nitrosomonas oxidises ammonium (NH₄⁺) → nitrite (NO₂⁻), and Nitrobacter oxidises nitrite (NO₂⁻) → nitrate (NO₃⁻). Because plants absorb nitrogen mainly as nitrate, nitrification increases the nitrate available for crops to take up and use in making amino acids and proteins. Nitrifying bacteria are aerobic, so nitrification is favoured by well-aerated, well-drained soil with plenty of oxygen.

Denitrification does the opposite: denitrifying bacteria (Pseudomonas) reduce nitrate (NO₃⁻) back to nitrogen gas (N₂), which is lost to the atmosphere. This removes nitrate from the soil, reducing the amount available to crops. Denitrifying bacteria are anaerobic, so denitrification is favoured by waterlogged, compacted, oxygen-poor soil.

Because the two processes need opposite conditions, farmers plough and drain their fields: adding oxygen promotes nitrification (more nitrate made) and suppresses denitrification (less nitrate lost), so the soil stays fertile.

The excess nitrate and phosphate enrich the water with mineral nutrients (eutrophication). These nutrients were previously limiting the growth of algae, so the algae now grow rapidly, forming an algal bloom across the surface of the river.

The dense algal bloom blocks light so that it cannot reach the plants and algae below the surface. Deprived of light, these submerged plants cannot photosynthesise, so they die (and the short-lived bloom algae soon die too).

The large mass of dead organic material is then broken down by aerobic (saprobiontic) decomposer bacteria. These bacteria multiply rapidly and respire aerobically, so they use up the dissolved oxygen in the water — the biochemical oxygen demand (BOD) rises sharply.

As a result, the dissolved oxygen concentration in the water falls to a very low level. With too little oxygen, the fish cannot respire (carry out aerobic respiration) and die.

Decomposers are saprobionts (saprobiontic bacteria and fungi) that feed on dead organisms and waste material. They obtain their nutrients by extracellular digestion.

In extracellular digestion the decomposers secrete digestive enzymes out of their cells onto the dead matter. These enzymes hydrolyse the large insoluble molecules in the dead organisms — for example proteins and carbohydrates — into small soluble products such as amino acids and sugars outside the organism, which the decomposers then absorb and respire.

As they break down the dead material, the decomposers release the mineral nutrients locked in it back into the soil. For example, the nitrogen in proteins is released as ammonium ions (ammonification), and other minerals such as phosphate are also liberated. These released ions become available in the soil once more.

Plants then absorb these mineral ions (nitrate, formed from the ammonium by nitrification, and phosphate) through their roots to build new proteins, nucleic acids and other compounds. In this way the decomposers prevent nutrients from being permanently locked in dead matter, recycling them so they can be reused and keeping the ecosystem's nutrient supply going.

(a) Describe (direction + quoted values). Downstream of the sewage discharge (X), the dissolved oxygen falls sharply from 11 mg dm⁻³ to a minimum of 2 mg dm⁻³ at 4 km. Beyond this point the dissolved oxygen rises again, recovering to 10 mg dm⁻³ by 20 km — close to its original level. There is therefore a characteristic oxygen sag with a minimum around 4 km.

(b) Explain. The sewage adds a large amount of organic matter to the river. Aerobic decomposer bacteria feed on this and multiply rapidly (their numbers peak near 4 km), respiring aerobically and so using up the dissolved oxygen — the biochemical oxygen demand (BOD) is high, which is why oxygen falls to its minimum where the bacteria are most numerous.

Further downstream the organic matter has been largely decomposed, so the bacteria decline, less oxygen is consumed, and oxygen is replenished by diffusion from the air and by photosynthesis of recovering plants/algae — so dissolved oxygen rises back towards normal.

The nitrate rises downstream because the decomposers carry out ammonification (releasing ammonium from the proteins/urea in the sewage), and then nitrifying bacteria (Nitrosomonas and Nitrobacter), which are aerobic, oxidise the ammonium to nitrate once the oxygen level recovers — so nitrate accumulates downstream of the oxygen minimum.

Decomposition is carried out by saprobiontic bacteria and fungi that secrete enzymes to digest dead matter (extracellular digestion) and then respire the products. Its rate therefore depends on the conditions affecting enzyme activity and microbial respiration, especially temperature and oxygen.

Temperature. Decomposer enzymes work fastest near their optimum temperature. In warm woodland soil, the higher temperature gives the enzyme and substrate molecules more kinetic energy, so there are more frequent, successful enzyme–substrate collisions and the rate of digestion (and decomposer respiration) is high — decomposition is fast. In a cold peat bog, the low temperature means low kinetic energy, few collisions and slow enzyme activity, so decomposition is very slow.

Oxygen availability. Most decomposers respire aerobically, so they need oxygen to release the energy for growth and reproduction. Well-drained woodland soil is aerated, supporting large, active aerobic decomposer populations. A waterlogged peat bog is anaerobic (oxygen is excluded by the water), so aerobic decomposers cannot thrive and decomposition almost stops. The bogs are also often acidic, taking the pH away from the enzymes' optimum, further slowing activity.

Link to fossilisation. Because decomposition is so slow in the cold, anaerobic, acidic peat bog, dead plant material is only partly broken down and accumulates rather than being fully recycled. Over thousands to millions of years, this partly decomposed carbon-rich material can be compressed and, given the right geological conditions, becomes a fossil fuel — this is fossilisation, which locks carbon away out of the carbon cycle until it is later released by combustion.

Conclusion. High temperature and good oxygen supply (with a suitable pH) maximise enzyme activity and decomposer respiration, giving fast decomposition and rapid nutrient recycling; cold, anaerobic, acidic conditions slow decomposition so much that carbon accumulates and is eventually fossilised rather than recycled.

Support for the conclusion. The data fit the known mechanism of eutrophication: more nitrate fertiliser → higher nitrate in the lake → an algal bloom that blocks light → submerged plants die → aerobic decomposer bacteria respire the dead matter and use up oxygen → dissolved oxygen falls → fish die (cannot respire). The observed higher nitrate, lower dissolved oxygen and fewer fish are all consistent with this cause-and-effect chain, so the conclusion is plausible.

Reasons for caution. However, the data show only a correlation, not proof of causation. Other factors changed at the same time could have lowered the oxygen or killed the fish — for example a rise in temperature (warmer water holds less oxygen), discharge of another pollutant or sewage, phosphate as well as nitrate, or overfishing/disease. The study has no control lake for comparison, and we are not told whether other variables (temperature, other run-off) were measured or kept constant. A single before/after comparison cannot rule these out.

Conclusion. The conclusion is reasonable and consistent with the eutrophication mechanism, but it is not proven by these data alone. To strengthen it, the scientist should control or measure other variables, use a comparable control lake with no fertiliser change, and repeat the measurements over time, because correlation does not by itself establish that the fertiliser caused the fish deaths.