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The total mass of all the organisms that have ever lived is many times greater than the total mass of the carbon and nitrogen atoms on Earth. Therefore, since the amount of carbon, nitrogen and other atoms on Earth is finite, and since no important additional quantity reaches it from space, how does life survive?
The answer that there is a constant exchange and circulation of elements in the structures of living things. Nothing is wasted, therefore. The compounds in plant and animal corpses and dead organisms do not go to waste, but are reused repeatedly, thanks to the flawless recycling systems in nature. These cycles are to a large extent performed by living things we humans never see and have never even heard of.
One of these recycling processes involves the element of carbon. As we know, plants absorb carbon dioxide—consisting of one carbon and two oxygen atoms, CO2—to perform photosynthesis. However, this is not sufficient to maintain the carbon balance, because a large amount of carbon remains gathered in dead plants and animals. At this point, bacteria and fungi enter the equation and release the carbon in dead bodies back into the soil and atmosphere.
The nitrogen cycle is also of great importance to the continuation of life. Plants need nitrogen to synthesize amino acids and proteins. Yet they cannot make direct use of the gaseous nitrogen in the atmosphere, but absorb it in the form of nitrates from the soil. This conversion is the work of various micro-organisms. Nitrite bacteria convert nitrogen into a form that plants can use. Human beings and animals obtain the nitrogen they need from plants, but single-celled organisms are of essential importance in converting nitrogen into forms that can be used by other living things.
If there were insufficient nitrogen in the soil, then plants, and thus human beings and animals that depend on them, could not exist. And in nitrogen levels were any higher than they are, then the poisonous gas nitric oxide NO—which causes air pollution and acid rain and which damages the ozone layer and the ecology—would accumulate in the atmosphere. Drinking water would become polluted, and lakes, rivers and other freshwater ecosystems would be damaged.134
Forests perform an important function in the circulation of water on Earth.135 Rain or melted snow absorbed by the soil returns to the atmosphere in the form of water vapor as the result of the transpiratoin of plants and trees. An enormous amount of water vaporizes through the leaves of trees: Plants work like living pumps that release water into the atmosphere by passing up through their stems and branches. In this way, water is constantly being recycled without disappearing into the ground.
In addition, living things also play important roles in the global recycling of such elements as phosphorus, sulfur and others. One point requiring emphasizing is that these cycles operate with perfect efficiency. Despite all the advanced technological means of the age we live in, only about 10% of our waste is ever recycled.136 Yet the recycling efforts carried out by living things for millions of years is nearly 100%. No doubt, this is one of the countless marvels of creation in the system of interconnected living things.