Fungi are found as vegetative colonies or as spores nearly everywhere in the water. They grow weakly or luxuriantly or await favorable conditions for growth.
Fungi lack chlorophyll and consequently, most are dependent on other organisms. They secrete extracellular enzymes and reduce complex organic material to simple compounds which they can absorb directly through the cell wall.
As agents of decomposition along with bacteria, they serve a useful purpose in the breakdown of dead organisms either fully into inorganic constituents or partially into minute fragments suitable as food for the many small scavenger animals.
However, fungi generally require some free oxygen. Fungi are said to be more effective than bacteria in breaking down the hard parts of the bodies of insects crustaceans and in the lignin of wood.
Aquatic fungi, or phycomycetes, exists on a wide variety of substrata. They are found as parasites or saprophytes on algae composing the phytoplankton, on nonplanktonic algae, other aquatic fungi, spores of higher plants, vegetable and animal debris, eggs, embryos and the adults of microscopic animals and empty integuments of aquatic insects.
Others are wound parasites of larger aquatic animals such as fish, amphibia, etc. on which they continue to live as saprophytes after the death of the host.
Many are found on twigs and fruits that have fallen into the water or have been washed in from surface flow.
The phycomycetes as a group are characterized by having an indefinite number of spores born in a sporangium and by a vegetative system or mycelium which is composed of elements or hyphae lacking cross walls, except where the reproductive structures are delimited.
Although this vegetative system in certain higher forms may be visible to the naked eye where for example it may form a cottony halo around a dead fish or insect in the water. In a very large number of species, it may be completely within the substratum and, further, may consist of only a few delicate, strongly tapering, very minute rhizoids.
The fungus saprolegnia called “water mold” is a typical example of an aquatic phycomycete. It commonly causes infection of small fish and appears as a whitish fuzzy mass on the animal. Other molds very similar in appearance attack fish eggs during development and in the natural habitat or in hatcheries may be of serious consequence.
Other aquatic fungi such as leptomitus and achlya are important participants in the decomposition of sewage and heavy organic material.
A small number of sack fungi, ascomycetes, occur in natural waters. These usually resemble miniature mushrooms (which they are not) on decaying plants. Many sack fungi cause diseases and others such as the famous penicillin produce antibiotic substances. The importance of these antibiotic materials in the natural environment is not known well.