Although some bacteria, the pathogens, are injurious to human life and welfare, the majority of the saprophytes, when naturally occurring are beneficial. Certainly, as the foundation of the food chain, bacteria are essential to life. Bacteria act as anti-pollution agents and the dissolution of organic matter is the prime role of bacteria in water self-purification. Because of this function, certain types of bacteria are the work horses of biological sewage treatment plants.
Bacteria are grouped into aerobic and anaerobic classes. Aerobic bacteria thrive in the presence of O2 (free oxygen), anaerobic bacteria thrive in the absence of free oxygen.
Pollution can produce a superabundance of the saprophytic population. Nutrients from sources such as organic matter and other oxygen demanding wastes, toxic chemicals, nitrogen and phosphorus can promote nuisance aquatic growths. The decomposition of these growths then can produce undesirable conditions for aquatic life and recreational or potable uses of the water.
This decomposition of aquatic growths can consume the free oxygen and create conditions that are favorable for the growth of anaerobic bacteria. These anaerobic bacteria are a major factor in low oxygen conditions in lakes that produce fish kills.
Some sheath-forming bacteria are the primary nuisance type growths in rivers, lakes and ponds. A notable problem associated with this group occurs in areas subjected to organic enrichment.
The most common offenders belong to the genus sphaerotilus. These bacteria are prevalent in areas receiving raw domestic sewage, improperly stabilized paper pulp effluents or effluents containing simple sugars. For example, the growths they produce interfere with fishing by fouling lines, clogging nets and generally creating unsightly conditions in the infested areas.
The metabolic demands of these bacteria while they are living and their decomposition after death impose a high BOD load on the stream and can severely deplete the dissolved oxygen. It has been suggested that large populations of sphaerotilus render the habitat noxious to animals and hence its presence may actively exclude desirable fish and invertebrates.
Clear lakes and streams contain little dissolved organic matter and have relatively high oxygen content. The total bacterial population of these systems is low, generally on the order of 1 to 1,000 organisms per milliliter, except for those areas in close contact with surfaces.
In lake sediment studies, no significant reduction in bacterial numbers with depth was found in-the top 5 cm. Below this, however, a gradual reduction occurs. It has been noted the turnover of phosphate under natural conditions appears to be caused by aquatic bacteria and that bacteria may compete with algae for available inorganic phosphate.
Some findings suggest that the aquatic bacteria take up dissolved, inorganic phosphorus and release it again in an organic form, not immediately available to the algae.
Occasionally, bacteria that are ordinarily considered saprophytic have infected fish and caused disease. Oxygen utilized by bacteria during decomposition may significantly deplete dissolved oxygen and result in fish kills, especially in winter with an ice cover.