All fungi lack chlorophyll. So fungi are heterotrophs. They obtain their energy from organic matter. They absorb their food directly from the environment. So fungi are called absorptive heterotrophs. Fungi may be saprotrophs, parasites, predators or mutualists.
Most fungi are saprotrophs (saprobes) or decomposers. They obtain their food (energy, carbon and nitrogen) from dead organic matter. They secrete digestive enzymes. These enzymes digest organic matter. These digested molecules are absorbed back by the fungi. Saprotrophs fungi attach with the substrate by a modified hyphae, rhizoids.
Fungi decompose the cellulose and lignin present in the cell wall of plant. Most bacteria cannot decompose these compounds. The fast growing hyphae form extensive system of hyphae. It increases the surface area for absorption Saprotrophs fungi and bacteria are major decomposer of the biosphere.
- Obligate parasites: These fungi can grow only on their specific hosts and cannot grow on available defined growth culture medium various mildews and rust species are obligate parasites.
- Facultative parasites: These fungi grow on their host and can also grow on artificial growth media.
Some fungi are active predators. The oyster mushroom (Pleurotus ostreatus) is a carnivorous fungus. It paralyses the nematodes (that feed on this fungus). It penetrates into nematods and absorbs their nutritional contents and fulfills its nitrogenous requirements. While it fulfills its glucose requirement by breaking the wood.
Some species of Arthrobotrys capture soil nematods by forming constricting ring. Their hyphae digest the nematods. Other predators have different adaptations for capturing their prey. Some secretes sticky substances.
4. Mutualistic Fungi
Mutualistic symbiotic association is an association between two organisms in which both organisms get benefits. There are two such associations: Lichens and Mycorrhizae.
The photoautotrophs of lichens are green algae or cyanobacterium or sometimes both. There are 15000 species of lichens. Most of the visible part consists of fungi. The algal components are present within the hyphae. The fungi protect the algae from strong light and desiccation and algae provide food to fungi.
Lichens grow at harsh places like bare rock. Algae or fungi cannot grow alone in such places. Lichens vary in colour, shape, overall appearance and growth form. They are ecologically very important and act as bio indicators for air pollution. There are three growth forms of lichens:
- Crustose Lichens: These grow tightly attached to the rock, tree trunks etc.
- Foliose Lichens: These are leaf like.
- Fruticose Lichens: These form branching system.
Mycorrhizae are mutualistic association between certain fungi and roots of vascular plants. About 95% of all vascular plants form mycorrhizae. The fungi increase the total amount of soil contact and area of absorption. It also helps the root in the direct absorption of phosphorus, zinc, copper and other nutrients form soil.
Such plants show better growth than the plants without this association. On the other hand, the plant supplies organic hyphae to fungal hyphae. There are two main types of mycorrhizae.
- Endomycorrhizae: In this case, the fungal hyphae penetrate the outer cells of the plant root. It forms coils, swellings and minute branches. These branches extend into surrounding soil and absorb water.
- Ectomycorrhizae: In this case, the hyphae surround and extend between the cells but do not penetrate the cell wall the root cells. However, the mycelium extends outside in the soil. These mycorrhizae are formed with pines, firs etc.
Adaptations for Nutrition in Fungi
- Fungi grow best in moist habitats. They are found wherever organic matter is present.
- They pass dry conditions by forming some resting stage or by producing resistant spores.
- They can tolerate a wide range of pH form 2 — 9.
- They can tolerate a wide range of temperatures.
- They can live ¡n high osmotic pressure, for example, concentrated salt of sugar solution as in jelly, jam etc.
- Fungi store surplus food in the form of lipid droplets or glycogen.
Reproduction in Fungi
Most fungi can reproduce asexually and sexually, expect imperfect fungi. Sexual reproduction is absent in imperfect fungi.
Asexual reproduction takes place by spores, conidia, fragmentation and budding.
Spores are common means of reproduction in fungi. Spores are produced inside the reproductive structures called sporangia. The sporangia are cut off from the main hyphae by complete septa.
Spores may be produced by asexual or sexual process. Spores are small, haploid and non-motile. They are produced in large number. They do not need water for their dispersal. Wind disperses the spores to great distances.
It causes wide distribution of many kinds of fungi, including many plant pathogens. The spores settle in a suitable place and germinate to form new fungal hyphae. Spores may also be dispersed by insects and other animals and by rain splash.
Conidia are non-motile asexual spores which are cut off at end of modified hyphae called conidiophores. They are not produced inside the sporangia. Conidia are present in chains or clusters. They are produced in large number. They can survive for weeks and rapidly settle on new food.
In this case, some hyphae of mycelium are broken. Each broken fragment gives rise to a new mycelium.
Unicellular yeasts reproduce by budding. A tiny outgrowth or bud is produced in budding. This bud separates and grows by simple relatively equal cell division and forms new hyphae.
Sexual reproduction takes place by the fusion of haploid nuclei and meiosis in all fungi. But different fungi have different detail of sexual reproduction. Hyphae of two genetically different but compatible mating types come together. Their cytoplasm first fuse and then nuclei fuse. The fusion of cytoplasm is called plasmogamy and fusion of nuclei is called karyogamy.
In two groups of fungi, Basidiomycetes and Ascomycetes, fusion of nuclei does not take place immediately after the fusion of cytoplasm. So the two genetic types of haploid nuclei of two different individuals are present in the same hyphae. These nuclei divide in same hyphae for most of the life of fungus. The fungal hyphae or cells with two nucleic of different genetic types are called dikaryotic (or heterokaryotic) hypha.
Different groups of fungi produce different types of haploid sexual spores like basidiospores and ascospores. These sexual spores are produced by meiosis in zygote. These spores may be produced in their characteristic fruiting bodies like basidia or basidiocarps and asci or ascocarps.