Bacteria are organisms of special significance to the study of clean and polluted waters because they break down organic matter. While most of them are not harmful to humans, some bacteria (e.g. Clostridium) are pathogenic. Most bacteria are retained on a filter of pore size 0.45 μm and all bacteria are trapped on a filter of 0.22 μm. They are important in sewage treatment, and in solid waste disposal. They are extremely abundant in almost all parts of the aquatic environment. They occur suspended in the water, on the surface of submerged objects, in river bed debris, and on and in other organisms in the environment. A bacterial cell can be seen with the aid of a microscope; when enough of them are grouped together, they can be seen with the naked eye as a 'colony'.
Most species of bacteria feed on the biodegradable natural wastes produced by other living organisms, as well as on the remains of the dead ones, as these are energy-rich resources. They can also utilise any biodegradable compounds released into the river by soil run-off or waste flows. Many of these bacteria are classified as aerobes: they require dissolved oxygen for aerobic respiration in order to oxidize (burn up) the nutrient compounds so that they can gain energy for their other life processes, such as building new cell material, and for functions such as movement, reproduction and growth. A generalised equation for aerobic bacterial degradation would be:
If dissolved oxygen is not available in the water, aerobic bacteria or aerobes cannot thrive; instead, another class of bacteria, anaerobes, make use of the bound oxygen in inorganic salts such as nitrates, sulphates and phosphates. The toxic gas hydrogen sulphide is often produced. In addition, anaerobes convert organic material to methane and carbon dioxide. A generalised equation for anaerobic bacterial activity is:
Certain bacteria use dissolved oxygen if it is available but convert to anaerobic processes when dissolved oxygen is absent. Such bacteria are called facultative anaerobes.
Besides organic compounds, and oxygen in the case of aerobic decomposition, bacteria require a supply of various elements such as iron, magnesium, potassium, calcium and sodium. If any essential nutrient is below the minimum concentration necessary for certain processes, bacterial growth will be prevented. Temperature also has a direct influence, not only on the rate of decomposition, but also on the particular species that accomplish it. Between 10°C and 20°C is the optimum range for most river bacteria (mesophiles); while others may flourish at higher temperatures (thermophiles), again within an optimum temperature range. There are also psychrophiles which can live at around 0°C. An increase in temperature within the appropriate range will tend to speed up the general rate of metabolism of bacteria, thereby increasing the rate of decomposition of organic compounds.