Fungi (e.g. species such as Penicillium which are used for manufacture of antibiotics, and yeast) are generally unicellular non-photosynthetic organisms which can tolerate acid conditions. They are capable of degrading highly complex organic compounds. They utilise much the same food sources as bacteria but they require less nitrogen since their protein content is lower. Fungi play an important role in sewage treatment.
In polluted water, particularly near to a sewage works outfall, a material known as 'sewage fungus' is often found. This is not a single organism but a mixture of different species of bacteria, fungi, algae and protozoa. They form a slimy, furry growth (not unlike plumes of cotton wool) (Figure 15) on the river bed and river bank. The organisms most often seen in sewage fungus are the bacterial species Sphaerotilus natans and Zoogloea ramigera. Other important species are the bacteria Beggiatoa alba and the Flavobacterium spp., the fungi Geotrichum candidum, Leptomitis lacteus, Fusarium aquaductum, the alga Stigeoclonium tenue and the protozoan Carchesium polypinum.