Nutrition in Fungi: 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 to the substrate by modified hyphae, and rhizoids.
Fungi decompose the cellulose and lignin present in the cell wall of the plant. Most bacteria cannot decompose these compounds. The fast-growing hyphae form an extensive system of hyphae. It increases the surface area for absorption Saprotrophs fungi and bacteria are major decomposers of the biosphere.
Parasitic fungi absorb nutrients directly from the living host cytoplasm with the help of special hyphal tips called haustoria. They may be obligate or facultative parasites.
- Obligate parasites: These fungi can grow only on their specific hosts and cannot grow on available defined growth culture mediums 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 paralyzes the nematodes (that feed on this fungus). It penetrates into nematodes 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 nematodes by forming a constricting ring. Their hyphae digest the nematodes. Other predators have different adaptations for capturing their prey. Some secrete sticky substances.
4. Mutualistic Fungi
The mutualistic symbiotic association is an association between two organisms in which both organisms get benefits. There are two such associations: Lichens and Mycorrhizae.
Lichen is a mutualistic association between fungi and certain photoautotrophs like algae or blue-green algae. The fungi are certain Ascomycetes, imperfect fungi, and a few Basidiomycetes (20 species).
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 in harsh places like bare rocks. Algae or fungi cannot grow alone in such places. Lichens vary in color, shape, overall appearance, and growth form. They are ecologically very important and act as bioindicators of 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 a branching system.
Mycorrhizae are a 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 the area of absorption. It also helps the root in the direct absorption of phosphorus, zinc, copper, and other nutrients from the soil.
Such plants show better growth than plants without this association. On the other hand, the plant supplies organic hyphae to fungal hyphae. There are two main types of mycorrhizae.
- Endo mycorrhizae: 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 the surrounding soil and absorb water.
- Ectomycorrhizae: In this case, the hyphae surround and extend between the cells but do not penetrate the cell wall of 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 from 2 — 9.
- They can tolerate a wide range of temperatures.
- They can live ¡n high osmotic pressure, for example, in concentrated salt or 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, except imperfect fungi. Sexual reproduction is absent in imperfect fungi.
Asexual reproduction takes place through spores, conidia, fragmentation, and budding.
Spores are a 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 the asexual or sexual process. Spores are small, haploid, and non-motile. They are produced in large numbers. They do not need water for their dispersal. Wind disperses the spores to great distances.
It causes the 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 splashes.
Conidia are non-motile asexual spores that 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 numbers. They can survive for weeks and rapidly settle on the 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 through 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 the fusion of nuclei is called karyogamy.
In two groups of fungi, Basidiomycetes, and Ascomycetes, the 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 the same hyphae for most of the life of the fungus. The fungal hyphae or cells with two nuclei 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 the zygote. These spores may be produced in their characteristic fruiting bodies like basidia or basidiocarps and asci or ascocarps.