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.
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. Types of Reactions. This requires an electron donor and an energy source.