Cycling matter through ecosystems

Why is it important that materials are recycled? We have studied the flow of energy through ecosystems and seen how energy is continually lost as heat from the ecosystem and has to be replaced as light. Nutrients, however, are recycled.
There is a finite amount of each nutrient in an ecosystem and so the same atoms must constantly be re-used, over and over again. This happens at the ecosystem level and also globally. Ecosystems look unchanging, but they are in fact always changing. Materials are always being ‘moved around’ within an ecosystem. Nutrients are always being taken in by organisms and materials are lost when they breathe and excrete. What is a waste product to one organism becomes a vital nutrient to another. All the organisms in the ecosystem are interdependent and interact with their physical environment.

Materials are moved around an ecosystem when
organisms:
• feed
• excrete
• respire and breathe
• die and are decomposed
The molecules that are moved around, particularly the organic molecules, store large amounts of energy in the bonds holding the atoms together. So, as materials are moved, energy is transferred also. But, as noted earlier, energy is eventually lost from the ecosystem as heat and must be replaced as light. The nutrients just keep on being recycled …
As you can see from Figure below, de-composers (bacteria and fungi) are key in returning nutrients to the ecosystem. Important mineral elements such as nitrogen and phosphorus are returned to plants as a result of the action of de-composers.

De-composers feed by a method known as saprobiotic nutrition. They feed on dead matter – and so do you. You digest the parts of dead animals and plants that you eat and so must the de-composers. To do this, they secrete enzymes onto the dead matter. The enzymes digest the complex organic molecules into simpler, smaller ones and the micro-organisms absorb these products of digestion (just like you). But, unlike you, their extracellular digestion does not take place in a gut, it takes place in the soil, or wherever the dead matter
happens to be.

However, besides hydrolytic enzymes that break down complex organic molecules many micro-organisms have enzymes for other purposes. For example, many of the de-composers have an enzyme that releases the amino group from amino acids and converts it to ammonia. This is known as ammonifiation and is important in the nitrogen cycle. But that is not why it takes place. Ammonifiation
is carried out by a range of bacteria and fungi as a way of obtaining energy from organic, nitrogen-containing compounds. The ammonia, vital to the nitrogen cycle, is just a useless by-product to these micro-organisms. This is typical of many of the chemical reactions that take place in all the nutrient cycles. The reactions are primarily energy-releasing reactions for a particular type of microorganism. It is a ‘happy chance’ that the reactions produce a byproduct that can be processed in the next stage of the nutrient cycle.

Interesting Pieces

Decomposers release many mineral elements, nitrogen, phosphorus and others from organic compounds in dead bodies. These are then absorbed from the soil by plants. However, carbon is released into the air as carbon dioxide when the decomposers respire – just as with all other organisms.

However, besides hydrolytic enzymes that break down complex organic molecules many micro-organisms have enzymes for other purposes. For example, many of the decomposers have an enzyme that releases the amino group from amino acids and converts it to ammonia. This is known as ammonifiation and is important in the nitrogen cycle. But that is not why it takes place. Ammonifiation is carried out by a range of bacteria and fungi as a way of obtaining energy from organic, nitrogen-containing compounds. Th ammonia, vital to the nitrogen cycle, is just a useless by-product to these micro-organisms. This is typical of many of the chemical reactions that take place in all the nutrient cycles. Th reactions are primarily energy-releasing reactions for a particular type of microorganism. It is a ‘happy chance’ that the reactions produce a byproduct that can be processed in the next stage of the nutrient cycle.




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