Molds, or mould, are various fungi that cover surfaces as fluffy mycelia and usually produce masses of asexual, or sometimes sexual, spores. Mold is a growth of minute fungi forming on vegetable or animal matter, commonly as a downy or furry coating and associated with decay or dampness. They are genetically similar to yeasts. There are thousands of known varieties of molds. Their primary energy source is organic matter which is broken down by enzymes released from the mycelia (the mass of hyphae) into simpler compounds. By decomposing organic matter, molds play a big part in material biodegradation, enabling decay and rot necessary in all ecosystems. The enzymes and mycotoxins can also inhibit the growth of other molds and microorganisms. Some mycotoxins are considered to be harmful to health. Adequate humidity and temperature are needed for optimal growth. Molds do not use photosynthesis to receive energy.