There is almost no known type of metabolic activity that cannot be associated with some bacterial group. Many kinds of metabolic reactions are brought about uniquely by special groups of bacteria.
Some bacteria exhibit a high degree of nutritional versatility. It has been shown, for example, that pseudomonas species may be able to satisfy their energy and carbon requirements by oxidizing any one of approximately one hundred simple organic compounds. However, other bacteria may show a high degree of nutritional specialization. For example, the nitrifying bacteria, nitrosomonas, can obtain its energy only by the oxidation of ammonia to nitrate.
With respect to oxygen, every conceivable mode of response may be found in bacteria. Some are strict aerobes, some are strict anaerobes, some may grow best in the presence of low concentrations of oxygen and some can develop well in either the presence or absence of oxygen.
The same diversity is shown with respect to pH, the hydrogen ion concentration. Although the majority of bacteria grow best under neutral or slightly alkaline conditions, certain sulfur oxidizing forms flourish in environments with a pH close to 0. Meanwhile the urea decomposing bacteria can grow well at a pH of 11.
The temperature range for bacteria is equally wide. Certain bacteria are able to grow at temperatures slightly below the freezing point of pure water, while thermophilic forms can be found in hot springs at temperatures of 80°C.
In view of these facts, it is not surprising that the bacteria are wide-spread in nature. Indeed, there is probably no natural environment capable of supporting the development of living organisms in which bacteria cannot be found.
Clearly therefore, the kind of ecological generalization that can be made concerning one of the higher groups of plants or animals would be meaningless here. Nevertheless, it may be contended that the ecology of the bacteria can be studied with a greater precision and elegance than that of any other living group.
It is, however, a microecology since the significant environment for a given type of bacterium may be contained in a volume of a few cubic microns. A single cellulose fiber undergoing decomposition in mud or soil will support a characteristic and highly specialized microflora, and in closely adjacent regions, wholly different microfloras may predominate.
Thousands of such microenvironments may lie concealed from the gross ecological eye in a few grams of soil or mud. Thanks to their physiological versatility, the bacteria play cardinal roles at a number of different points in the cycle of matter in nature.
Bacteria are principle agents in the mineralization process. They are found in almost every situation in which organic matter is either formed by the metabolic activities or by death of other living organisms.
The decomposition of this organic matter produces the eventual liberation of carbon as carbon dioxide, nitrogen as ammonia, phosphorus as inorganic phosphate, and sulfur as hydrogen sulfide. The further oxidation of if ammonia to nitrates and of hydrogen sulfide to sulfates are also brought about by special groups of bacteria.
Other bacteria may reduce these highly oxidized forms of carbon, nitrogen and sulfur, thereby converting carbonates to methane, nitrates to nitrites, nitrous oxides or elementary nitrogen and sulfates to hydrogen sulfide.
The fixation of atmospheric nitrogen, an essential reaction in the maintenance of the Earth’s fixed nitrogen supply, is very largely a bacterial activity although blue green algae also participate.
In any natural environment such as a body of water, all these bacterial transformations of matter proceed simultaneously. Although at any given time and place, one particular process may predominate, thus leading to a temporary mass development of the responsible microbial agents. When several different kinds of bacteria are capable of performing the same chemical transformation, the exact nature of the microflora will be determined by the physical conditions such as the presence or absence of oxygen and light or the pH.
This point can be illustrated by considering a very simple example. The mineralization of acetate is a common product of the breakdown of carbohydrates, fats and proteins. When acetate is in a dark, anaerobic environment, it can be oxidized by representatives of three special groups.
The methane bacteria which couple the oxidation with the reduction of carbon dioxide to methane; the sulfate reducing bacteria which couple the oxidation with a reduction of sulfates to hydrogen sulfide; and the nitrate reducing bacteria which couple the oxidation with a reduction of nitrates principally to N2.
Which group actually predominates will be determined by the relative availability of carbon dioxide, sulfate and nitrate. Under anaerobic conditions where light is available, the bacteria that employ acetate as an oxidant for the photosynthetic reduction of carbon dioxide will come to the forefront. If oxygen is present, other bacteria will predominate.
Notably, these will be the pseudomonas types. Most of which are strict aerobes and oxidize acetate with molecular oxygen. When the supply of combined nitrogen is limited in an oxygen rich environment, the pseudomonas types will be supplanted as acetate oxidizers by nitrogen fixing bacteria of the azotobacter group.