The Organic Carbon Cycle
The organic carbon cycle is the set of natural processes by which inorganic carbon is converted to organic carbon, is then cycled through different forms of organic carbon, and finally is converted back to inorganic carbon. Before proceeding further, a definition of organic matter would be in order. A commonly used definition is that organic matter is any material that is part of a living organism or came from a living or once living organism. Although there are many elements present in small amounts, organic matter is made up predominantly of carbon, hydrogen, and oxygen. Organic carbon is a term used for the carbon that is in organic matter, while inorganic carbon is carbon in other compounds, such as carbon dioxide, carbonate and bicarbonate compounds, diamonds, or graphite.
The two major reactions that take place in the organic carbon cycle are photosynthesis and biological oxidation of organic matter. Photosynthesis is the process by which green plants convert carbon dioxide and water into organic matter in the presence of sunlight. Biological oxidation is the process by which aerobic bacteria in the presence of oxygen break down organic matter to carbon dioxide and water as end products. These two reactions can be represented by the following two equations:
Photosynthesis: CO2 + H2O + sunlight → organic plant matter + oxygen (catalyzed by the chlorophyll in green plants)
Biological Oxidation: waste organic matter (primarily C, H & O) + O2 → CO2 + H2O + energy (This reaction is carried out by aerobic microorganisms; it is the 'death and decay' shown in the diagram.) This reaction causes biochemical oxygen demand (BOD) when organic pollutants are present in water and biological oxidation uses dissolved oxygen from the water.
You may notice that these two reactions are each just the opposite of the other. Together they make up a major part of the organic carbon cycle, as shown in the diagrams. The organic plant matter may be eaten by an animal and converted to organic matter in the animal, or the plant may die resulting in decay and biological oxidation of the plant matter. The animals will give off waste organic material and will die at some point. The waste and dead organic matter will decay and be converted to CO2 and water by biological oxidation. If organic pollutants are present in water, their effect as pollution of water is to cause a reduction in dissolved oxygen level in the water.