Saprobiont decomposition

Some organisms obtain nutrients by breaking down dead organisms and waste outside their own bodies. These organisms are called saprobionts. Many bacteria and fungi act as saprobionts.

Extracellular digestion

Dead material and waste contain large, insoluble organic molecules that saprobionts cannot absorb directly. The saprobionts secrete enzymes onto the material. These enzymes break the large molecules into smaller, soluble products, which the saprobionts can then absorb.

Because digestion takes place outside the organism, it is called extracellular digestion.

Decomposition and nutrient return

As saprobionts digest dead organisms and waste, they decompose the organic matter. This process returns inorganic nutrients to the environment, where they can become available to plants and other organisms.

Mycorrhizae

A mycorrhiza is an association between a fungus and plant roots in which both partners benefit.

The fungus produces thin threads called hyphae, which extend from the roots into the soil. These threads increase the effective surface area available for absorption, helping the plant take up more water and inorganic ions, including phosphate.

In return, the plant supplies the fungus with organic compounds, such as sugars produced by the plant. The plant therefore gains help with water and ion uptake, while the fungus receives a source of organic compounds.

Nitrogen cycle

Nitrogen is recycled between the atmosphere, soil and organisms. Microorganisms change nitrogen between forms that can move through these stores. Plants take up nitrogen from the soil, and animals obtain it by feeding on other organisms.

Nitrogen fixation and uptake

Most plants cannot use atmospheric nitrogen gas, $$\ce{N2}$$, directly. Nitrogen-fixing bacteria convert it into ammonia, $$\ce{NH3}$$, or ammonium compounds. Some of these bacteria live freely in soil; others live in association with plant roots.

Plants absorb ammonium ions, $$\ce{NH4+}$$, or nitrate ions, $$\ce{NO3-}$$, and use the nitrogen to make organic nitrogen compounds, including proteins. These compounds pass to animals when they feed on plants or other animals.

Decomposition and ammonification

Dead organisms and waste contain organic nitrogen compounds. Saprobionts break down this material. During ammonification, microorganisms convert its organic nitrogen compounds into ammonia, which forms ammonium ions in the soil. This returns nitrogen to the soil in a form that plants can use or bacteria can change further.

Nitrification and denitrification

In nitrification, nitrifying bacteria oxidise ammonium ions in two stages: first to nitrite ions, $$\ce{NO2-}$$, and then to nitrate ions. Nitrification occurs under aerobic conditions, when oxygen is available. Plants can absorb the nitrate produced.

In denitrification, denitrifying bacteria convert nitrate into nitrogen gas, returning nitrogen to the atmosphere. This occurs particularly in anaerobic soil, where oxygen is scarce.

Phosphorus cycle

The phosphorus cycle recycles phosphorus between rocks, soil, water and organisms. Phosphorus moves through the cycle in different forms: as inorganic phosphate in the environment and as part of organic compounds in organisms.

From rocks to organisms

Weathering breaks down phosphate-containing rocks and releases phosphate ions into the environment. Plants absorb inorganic phosphate from soil or water and incorporate the phosphorus into organic compounds. When organisms feed on plants or other organisms, phosphorus passes along food chains.

Return and long-term storage of phosphate

Waste and dead organisms return organic phosphorus to the environment. Decomposers, including microorganisms, break down this material and release inorganic phosphate. Plants can absorb the phosphate again.

Some phosphate enters bodies of water and becomes part of sediments. Over long periods, these sediments can form phosphate-containing rocks. Geological processes can expose the rocks to weathering, releasing phosphate and returning it to the cycle.

Fertilisers and nutrient replacement

Plants need mineral nutrients from the soil to grow, including nitrate as a source of nitrogen and phosphate as a source of phosphorus. These nutrients become part of the plants’ tissues.

When crops are harvested, nitrogen- and phosphorus-containing material is removed from the ecosystem. Removing livestock also takes away material containing these elements. Less material is therefore left to decompose and return nutrients to the soil. Fertilisers replace nutrients removed by agricultural production, helping to maintain the supply available for plant growth.

Natural and artificial fertilisers

Natural fertilisers, such as manure, contain organic material from living organisms. Decomposers break down this material and release inorganic nutrients into the soil. Plants can then absorb these nutrients through their roots.

Artificial fertilisers are manufactured products that supply mineral nutrients in inorganic forms. They can provide sources of nitrate and phosphate directly to the soil. Both natural and artificial fertilisers can replenish nutrients removed by farming.

Fertiliser leaching and eutrophication

Fertiliser supplies plants with nutrients, but some fertiliser nutrients dissolve in water and are washed out of the soil. This is called leaching. The water can carry the dissolved nutrients into rivers, lakes and other water bodies.

Fertiliser nutrients and algal blooms

High concentrations of nutrients such as nitrate and phosphate can cause algae to grow rapidly, forming an algal bloom. The bloom blocks light from penetrating the water. Submerged plants and algae then receive too little light to photosynthesise sufficiently and may die.

Oxygen depletion during eutrophication

Saprobionts are organisms that feed on dead material. They decompose the dead plants and algae and use dissolved oxygen for aerobic respiration. As decomposition continues, the concentration of dissolved oxygen can fall so low that fish and other aquatic organisms die.

This enrichment of water with nutrients, together with its effects on the aquatic ecosystem, is called eutrophication.