Moulds, together with mushrooms and yeast, belong to the fungi organisms. In between plants and animals, fungi are genetically more closely related to animals than to plants. Their cell walls are made of chitin, a primary component also found in the exoskeletons of insects and crustaceans. Mycology, the study of fungi, is now considered a subset of biology not botany. There are thousands of known species of moulds and all require moisture for growth. Aside from the transmission of large number of tiny spores variously adapted for wind or water dispersal, moulds move by growth. They can grow on dead organic matter everywhere in nature, visible to the unaided eye only when they form large colonies. Moulds are the undertakers of ecosystems playing a major role in the decomposition of organic material to enable the recycling of nutrients. Moulds, like all fungi, do not photosynthesise but derive energy from organic matter by secreting enzymes to degrade complex matter into simpler substances.
The largest living organism on the planet is a fungus that is around 1000 hectares in size in the earth in the Blue Mountains in Oregon, USA. From estimates made of the rate of its growth it is thought to be at least 2800 years old but maybe up to 8000, which would also make it the oldest living creature on earth. Fungi perform a multitude of roles in ecosystems in addition to their role as recycler. In forests, fungi feeding off trees (it is estimated some fungi take up a third of a tree's total food production) penetrate tree roots and surrounding soils to form webs of living material that connects surrounding trees. Tree cannot thrive in isolation and the fungi contribute to the survival of the group by redistributing sugars in abundance in thriving trees to malnourished trees. Where trees lack nitrogen, fungi can release a toxin into soils that cause minute organisms to die to become fertiliser and transmit signals from one tree to the next to give trees not yet infested with an insect predator sufficient time to disseminate a repellent through their foliage. A Casselman, "Strange but True: The Largest Organism on Earth is a Fungus," Scientific American, 4 October 2007 and J Fraser, "Root Fungi Can Turn Pine Trees into Carnivores - Or at Least Accomplices," Scientific American, 12 May 2015