All fungi lack chlorophyll and are heterotrophs (obtaining carbon and energy from organic matter). They obtain their food by direct absorption from the immediate environment and are thus absorptive heterotrophs. Most fungi are saprotrophs (or Saprobes), decomposers that obtain their food (energy, carbon and nitrogen) directly from dead organic matter.
They secrete out digestive enzymes which digest dead organic matter, and the organic molecules thus produced are absorbed back into the fungus. Saprobic fungi anchor to the substrate by modified hyphae, the rhizoids. Fungi are the principal decomposers of cellulose and lignin, the main components of plant cell walls (most bacteria cannot break them). Extensive system of fast growing hyphae provides. Enormous surface for absorptive mode of nutrition. Saprobic fungi, alongwith bacteria, are the major decomposers of the biosphere, contributing to the recycling of the elements (C, N, P, O, H etc) used by living things.
Some fungi are parasites, some are even predators, and still others are mutualists. Parasitie fungi absorb nutrients directly from the living host cytoplasm with the help of special hyphal tips called haustoria. They amy be obligate or facultative. Obligate parasites can grow only on their living host and cannot be grown on available defined growth culture medium. Various mildews and most rust species are obligate parasites. Facultative parasites can grow parasitically on their host as well as by themselves on artificial growth media.
Some fungi are active predators. The oyster mushroom (pleurotus ostreatus) is a carnivorous (predatory) fungus. It paralyses the nematodes (that feed on this fungus), penetrate them, and absorb their nutritional contents, primarily to fulfil its nitrogen requirements. It fulfills its glucose requirements by breaking the wood. Some species of arthrobotrys trap soil nematodes by forming constricting ring, their hyphae invading and digesting the unlucky victim. Other predators have other adaptations, such as secretion of sticky substances.
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Fig 8.2:Carnivorous fungi (a) the oyster mushroom decomposes wood, and also uses nematodes as a source of nitrogen (b) A nematode is trapped in constricting ring of a soil- dwelling carnivorous fungus (arthrobotrys sp).
Fungi from two key mutualistic symbiotic between certain fungi (mostly Ascomycetes and imperfect fungi, and few basidiomycetes – about 20 out of 15000 species of lichens) and certain photoautotrophs – either green algae or a cyanobacterium, or some times both. Most of the visible part of lichen consists of fungus, and algal components are present within the hyphae (fig 8.3). fungus protects the algal partner from strong light and desiccation and itself gets food through the courtesy of alga. Lichens can at such places where neither f the components alone can, even at harsh places such as bare rocks etc. lichens vary in colour, shape, overall appearance, growth form (fig. 8.3). they are ecologically very important as bioindicators of air pollution.
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Fig. 8.3Lichens (a) Cross section of a typical lichen showing different layers. (b) Different types of lichens varying in size, colour and appearance. Three growth forms – crustose grow tightly attached to rocks, tree trunks etc; foliose are leaf – like, fruticose are branching.
Mycorrhizae are mutualistic association between certain fungi and roots of vascular plants (about 95% of all kinds of vascular plants). The fungal hyphae dramatically increase the amount of soil contanct and total surface area for absorption and help in the direct absorption of phosphorus, zinc, copper and other nutrients from the soil into the roots. Such plants show better growth than those without this association. The plant, on the other hand, supplies organic carbon to fungal hyphae.
There are two main types of mycorrhizae (Fig. 8.4): endomycorrhizae, in which the fungal hyphae penetrate the outer cells of the plant root, forming coils, swellings, and minute branches, and also extend out into surrounding soil; and ectomycorrhizae, in which the hyphae surround and extend between the cells but do not penetrate the cell walls of the roots. These are mostly formed with pines, first etc. however, the mycelium extends far out into the soil in both kinds of mycorrhizae.
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Fig. 8.4Endomycorrhizae and ectomycorrhizae. (a) in endomycrrhiza (left side of figure), fungal hyphae penetrate and branch out in a root cells. In ectomycorrhiza (right side of figure), fungal hyphae simply grow around but do not penetrate the root cell (b) ectomycorrhizae on roots of pines.
Fungi grow best in moist habitats, but are found wherever organic matter is present. They survive dry conditions in some resting stage or by producting resistant spores. They can also tolerate a wide range of pH from 2 – 9, a wide temperature range, and high osmotic pressure such as in concentrated salt/sugar solutions as in jelly, jam etc. these features also help them in their survival on land. Fungi store surplus food usually as lipid droplets or glycogen in the mycelium.