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Chemistry LibreTexts

Biochemical Cycles

Plants such as trees and algae undergo the photosynthesis reaction where carbon dioxide and water in the presence of sunlight are converted to organic materials and oxygen. An important reverse reaction occurs in the water: Fish use metabooilism where oxygen and organic materials - other small fish or algae - as food is converted to carbon dioxide, water, and energy. Bacteria in water, as well as land, also undergo metabolism and use oxygen and decompose organic wastes as food to convert to carbon dioxide, water, and energy. By products in the decomposition of organic waste are nitrates and phosphates. The major natural biochemical cycles include the carbon, nitrogen, and phosphate cycles. They are presented in brief in this graphic.



The overall health of a body of water depends upon whether these factors are in balance. Municipal sewage systems are now doing a better job of removing most of the organic waste products in the discharge water, but some organic waste still enters the streams and lakes. If an excess amount of organic waste is present in the water, the bacteria use all of the available oxygen in the water in an attempt to decompose the organic waste.

The amount of organic waste in water is represent by a chemical test called BOD - Biological Oxygen Demand. The concentration of oxygen is measured in a water sample at the beginning of the test and again after five days. The difference between the oxygen concentrations represents the amount of oxygen consumed by the bacteria in the metabolism of the waste organics present.


In situations where eutrophication occurs, the natural cycles are overwhelmed by an excess of one or more of the following: nutrients such as nitrate or phosphate, or organic waste.

In the first case under aerobic conditions (presence of oxygen), the natural cycles may be more or less in balance until an excess of nitrate and/or phosphate enters the water. At this time the water plants and algae begin to grow more rapidly than normal. As this happens there is also an excess die off of the plants and algae as sunlight is blocked at lower levels. Bacteria try to decompose the organic waste, consuming the oxygen, and releasing more phosphate and nitrate to begin the cycle anew. Some of the phosphate may be precipitated as iron phosphate to remove the soluble form from the water solution.

In the second case under anaerobic conditions (absence of oxygen), as conditions worsen as more phosphates and nitrates may be added to the water, all of the oxygen may be used up by bacteria in trying to decompose all of the waste. Different bacteria continue to carry on decomposition reactions, however the products are drastically different. The carbon is converted to methane gas instead of carbon dioxide, sulfur is converted to hydrogen sulfide gas. Some of the sulfide may be precipitated as iron sulfide. Under anaerobic conditions the iron phosphate in the sediments may be solubilized into solution to make it available as a nutrient for the algae which would start the growth and decay cycle over again. The pond may gradually fill with undecayed plant materials to make a swamp.