Fungi usually are saprophytic organisms and are classified by their mode of reproduction. As saprophytes they obtain their nourishment from the degradation of dead organic matter. Most fungi are free-living and include yeast, molds, and mushrooms.
Most fungi are strict aerobes and can tolerate a low pH and a low nitrogen environment. Although fungi grow over a wide range of pH values (2-9), the optimum pH for most species of fungi is 5.6, and their nitrogen nutrient requirement for growth is approximately one-half as much as that for bacteria.
In the activated sludge process filamentous fungi (Figure 1.2) may proliferate and contribute to settleability problems in secondary clarifiers. The proliferation of filamentous fungi is associated with low pH (<6.5) and low nutrients. Although filamentous fungi contribute to settleability problems in the activated sludge process, the presence of a large and diverse population of fungi is desired for the treatment of some industrial wastewaters and composting of organic wastes. Fungi have the ability to degrade cellulose, tolerate low nutrient levels, and grow in the presence of low moisture and low pH conditions.
An example of a unicellular fungus is the yeast (Saccharomyces).They reproduce by budding. Budding results in the production of numerous daughter cells (offspring) from one parent cell. Yeast can degrade organic compounds to carbon dioxide and water with the use of free molecular oxygen (O2), or as facultative anaerobes they can degrade organic compounds such as sugars to ethanol (CH3CH2OH) in the absence of free molecular oxygen.
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