Examining the carbon cycle
Monacia Williams, Contributor

Did you manage to construct the aquatic food chains? It was easy, wasn't it? I am sure you went through the following steps to do so successfully! You would have:

• identified the producers, the first organisms in the food chain
• identified the herbivores, the primary consumers that would follow the producers
• identified the carnivores, the secondary and tertiary consumers.

Having done this, it now becomes very easy to construct the food chain and the food web. Have you ever thought of what happens to all these organisms when they die? Does the energy that they contain remain in their bodies? No, it does not! These organisms now become food for another set of organisms that are not included in the food web, but are extremely important in maintaining life. These are the decomposers.

Decomposers are bacteria and fungi. While bacteria are microscopic organisms and, therefore, not visible to the naked eye, fungi are all around for us to see! Do you want to see some? Try this. Get a slice of bread, sprinkle a little water on it and tie it up in a plastic bag. Watch your bread for a period of about a week and notice the changes; see the colour of your bread change. That's fungus at work. Your bread is decomposing!

Decomposers produce enzymes which dissolve dead tissues into liquid which they then absorb. These organisms are of great importance because they:

• Remove dead animals and plants by decomposing them. When they do this, they release nutrients back into the soil.
• Keep these minerals, essential for the proper growth of plants, in circulation in the biogeochemical cycles. If plants do not thrive, then food chains would be affected because animals would lack food and eventually die off.
• Recycle minerals taken from the soil by the plants.
  Two of the important minerals that are recycled are carbon and nitrogen. Carbon is present in all living organisms. Its ultimate source is the carbon dioxide in the air.

Summary of carbon cycle

• Plants and animals give out carbon dioxide when they respire.
• Bacteria and fungi respire when they are decomposing the bodies of dead animals and plants.
• Wood, coal, gas and gasolene contain carbon. When they are burned, the carbon combines with the oxygen of the air to form carbon dioxide, which is then returned to the air.
• Plants remove carbon dioxide from the air for use in photosynthesis.

All of these processes serve to keep the percentage of carbon dioxide in the air constant at 0.03 per cent! Remember that carbon dioxide is constantly being removed by the plants for photosynthesis, so if it is not replaced, the supply would become depleted.

Another important cycle is the nitrogen cycle. Nitrogen is needed by living organisms to make proteins, but even though nitrogen is very abundant in the air, at 79 per cent, living organisms cannot utilise it in this form. It has to be converted to ammonium or nitrate before it can be used by plants. Consumption of these plants by herbivores then makes it available to animals.

The processes of nitrogen cycle

Nitrogen fixation:

• By lightning which acts as a catalyst for the reaction between nitrogen and oxygen. Rain washes the end products into the soil where they form nitrates. Only a small portion of nitrates is fixed in this manner.
• By bacteria, known as nitrogen-fixing bacteria, which live freely in the soil and in nodules on the root of leguminous plants (peas and beans). These nodules are small bumps on the roots of these plants. Ploughing the remains of these plants back into the soil increases the number of nitrogen-fixing bacteria into the soil and, hence, the soil's fertility.
• Nitrification by nitrifying bacteria which make nitrates from ammonium compounds in urine and faeces and also from the decomposition of dead animals and plants.
• Denitrification by denitrifying bacteria which are found in waterlogged or poorly aerated soil. They convert nitrates back to nitrogen which goes back into the atmosphere. Note that this is not beneficial to plants because it reduces the amount of nitrates available.

Using your class text, see if you can fit these summaries into the diagrammatic representations of the carbon and nitrogen cycles. Next week, we will look more closely at the relationship which exists between the root nodules of the leguminous plants and the bacteria, as well as other special relationships that exist between living organisms.

This Excelsior High School student pays close attention to what will be a masterpiece during art class, recently. - Michael Lee photo

Monacia Williams teaches at Glenmuir High School.