Fungi are sometimes considered an all-inclusive category for the nonphotosynthetic organisms of the plant kingdom. There are approximately 80,000 known species of fungi, a group far more numerous than the 19,000 to 25,000 species of algae.

The plant bodies of fungi are either unicellular, colonial or filamentous. Their growth phase is always simple, although certain fruiting structures as mushrooms and those of shelf and cup fungi may be intricate. Nutritionally, fungi lack chlorophyll and therefore are nonphotosynthetic, resulting generally in a dependence on metabolites provided by other organisms. This is particularly significant in cooling water systems, since approximately ten percent of the fungal groups are capable of utilizing wood as a source of organic nutrient, and thus are capable of destroying lumber in cooling towers.

The mold type usually reproduces by forming spores, a secondary life form usually set apart from the normal filamentous reproductive structure (hypha) of the organism as a resting stage. 

Spores emitted by the organism may remain dormant for a lengthy period, especially under conditions unsuitable for growth. When conditions become suitable, the spore will develop into an active organism.

Spores are generally resistant to most microbiocides and can present very difficult control situations. It is important to note, however, that the organisms' dormant state at this stage makes them relatively harmless. Certain filamentous fungi may also reproduce by the physical breaking off of part of the filament. This is a less common reproduction mechanism and is generally associated with bacteria.

Yeastlike fungi are unicellular, nucleated microorganisms, spherical, ovoid or elongated. They reproduce primarily by budding, although some cells undergo fission. Fission reproduction is achieved by one cell splitting in half to form two new cells. Yeastlike fungi are responsible for water and wood discoloration.

Table summarizes some of the important parameters of fungal life. Again, it can be seen that the pH and temperature requirements for growth are generally satisfied in most cooling water systems.