THE CARBON CYCLE	THE NITROGEN CYCLE
THE SULFUR CYCLE	THE PHOSPHORUS CYCLE

Introduction to Biogeochemical Cycles

Some Important Implications and Observations

1. Balance.
A balance must be achieved if the nature of the planet, and of life as we now it, are to continue. An imbalance in a biogeochemical cycle would have dramatic consequences on a global scale. For instance, the advent of cyanobacteria made the atmosphere oxygenic and greatly affected the biology of this planet. This was a truly historical event.

2. Role of Microorganisms.
Microorganisms are intimately involved in the recycling of biological matter. Some materials that higher organisms depend on would not exist without microorganisms. For instance, molecular nitrogen is converted to ammonia only by certain species of bacteria. As a result, all the nitrogen atoms found in all living things started out in bacteria. Equally important, without microorganisms to recycle their wastes, "higher" organisms would be killed.

3. Reservoirs.
Reservoirs are sources of atoms. Not all reservoirs can be tapped by biological organisms - some forms of atoms cannot be utilized by incorporation into organic molecules. In addition, materials that can be recycled do not necessarily make up the largest reservoir available. For instance, although the largest carbon reservoir is within rocks in the Earth's crust, this source is biologically useless since organisms do not perform reactions capable of utilizing it.

4. Types of Reactions.
The reactions that organisms perform to interconvert inorganic and organic molecules invariably involve oxidation-reduction, i.e. "redox", chemistry:

a. Synthesis of more complicated molecules usually involves fixation and/or reduction. This requires an electron donor and an energy source.

b. Degradation of molecules usually involves their oxidation (equivalent to burning) to obtain the energy that they contain. This requires an electron acceptor, the most efficient of which is molecular oxygen.

5. Sources and Sinks of Electrons.
Organisms adapt to use the best electron sources (donors) or sinks (acceptors) available. When the "favorite" is used up, gene expression is altered to permit the use of the next best source or sink. This situation is analogous to carbon utilization wherein glucose is favored but other sugars can be utilized when glucose is lacking.

Be sure to look over the UNITS FOR GLOBAL QUANTITIES.