In this article we will discuss the nutritional requirements, essential elements, sources of macro elements, modes and mechanism of nutrition in fungi.
The fungi utilise both organic compounds and inorganic materials as the source of their nutrient supply. In other words, organic and inorganic compounds constitute their food. No fungus is able to make any increase in its dry weight in the absence of organic food materials, why?
Lacking chlorophyll the fungi are unable to photosynthesize or use carbon dioxide to build up organic food materials. They are, thus heterotrophic for carbon (organic) food compounds which they in their natural habitats obtain by living as saprophytes or parasites from dead or living plants, animals or micro-organisms or their wastes.
The constituent elements of the organic and inorganic substances which fungi make use of are C, O, H, N, P, K, Mg, S, P, Mn, Cu, Mo, Fe and Zn. Calcium is required by some fungi but not all. These elements which fungi require as food are termed the essential elements. Some of these elements, the fungi need in extremely small trace amounts and the others in comparatively larger amounts.
The former are called the trace or micro elements and the latter macro elements. The fungal growth is adversely affected or the fungus fails to grow if any one of the essential elements is lacking in the culture medium. Examples of the macro elements are C, N, O, H, S, P, K and Mg. The macro elements are body builders and provide energy for metabolic processes.
Sources of Macro Elements:
The organic substances usually utilized by fungi are very varied in nature. Yeasts, for example, can use acetates as sources of carbon but for most fungi the chief sources of carbon are the carbohydrates. The carbohydrates are needed for building up the body and also as a source of energy. In a typical fungus, 50% of the dry weight is carbon of the carbohydrate source of carbon, most fungi use simple sugars.
Glucose, for instance, is suitable for almost all fungi. Next in preference are the fructose. Less commonly used are the hexose sugars and some pentoses. Xylose among the pentoses has been reported to be superior to glucose for some fungi. Mannitol is equivalent to glucose for many fungi. Maltose which occurs in nature as a byproduct of starch hydrolysis is utilized by many fungi. Sucrose is also a good source of carbon for some.
From among the polysaccharides, starch and cellulose are utilised by a fewer fungi which can synthesize the appropriate hydrolytic enzymes. Some fungi are able to make good growth on fats as the only source of carbon.
Organic acids are generally poorer sources of carbon for most fungi. Basidiomycetes include most of the lignin-utilizing fungi. Proteins, lipids some organic acids and higher alcohols are utilised by some fungi as a sole energy source’ Growth, however, is always better on a substance containing a suitable carbohydrate.
Besides carbon, fungi require nitrogen. To obtain nitrogen, they utilise both organic and inorganic materials as the source. The chief organic sources of nitrogen are protein, peptide or an amino acid certain groups of fungi show specializations in respect of certain nitrogen sources For example, the Saprolegniaceae and Blastocladiales include a number of species which grow only with organic nitrogen such as amino acid.
In nature, fungi decompose proteins and other materials to obtain their supply of nitrogen. In pure cultures amino acids, peptides, or peptones gelatin, casein and egg albumin can serve as sources of organic nitrogen for building up protoplasm. Urea is also considered as a utilisable nitrogen source for some fungi.
Many fungi, however, obtain nitrogen from inorganic sources. A number of fungi are known which use both nitrate and ammonium salts. Robbin (1939) and Lindberg (1944) reported that Absidia sp., Mucor hiemalis, Lenzites trabea and Marasmius sp. use ammonia but are incapable of utilizing nitrate salts. Fewer fungi are able to utilize nitrate salts.
Nitrites can be toxic. Organic sources of nitrogen can also serve as sources of carbon. There is not much evidence to support nitrogen fixation in saprophytic fungi. Metcalfe and Chayen (1954) reported that soil inhabiting Rhodotorula and yeast-like Pullularia pullans fix atmospheric nitrogen. It is certain; however that nitrogen fixation is not a widespread ability in fungi.
Hydrogen and oxygen are supplied in the form of water which is the major constituent of fungus mycelium forming about 85-90% of the entire weight.
The chief among the inorganic nutrients which the fungi require in fairly large amounts for their mineral nutrition are sulphur, phosphorus, potassium and macronutrients the fungi obtain from simple inorganic salts or sources such as suIphates for sulphur, and phosphates for phosphorus.
These must be supplied in any culture medium. Calcium is not known to be needed by the fungi in general. Some, however, require it as a micronutrient. Some fungi are reported to require only minute traces of iron, zinc, copper, manganese and cobalt and molybdenum.
These trace elements or micronutrients are considered essential of growth. The form in which the major and the minor metallic element requirements are utilised is the anion. Fungi store excess food in the form of glycogen or lipids.
The fungi like all other organisms require minute amounts of specific, relatively complex organic compounds for growth. These are the vitamins or growth factors. Many fungi synthesize their own supply of appropriate growth factors from a simple nutrient solution of defined composition. Such fungi are thus autotrophic for vitamins and are called need no exogenous supply.
There are others which depend in whole or in part on an external source because they are unable to synthesize one or more of the essential growth substances. The fungi heterotophic for their needs of growth factors are termed auxo-trophic. There are marked difference between the vitamin demands of the different species of a genus or even the strains of a single species.
The important fungal vitamins, which may function in enzyme systems include thiamine (B1), biotin, pyredoxine (B6) and riboflavin (B2). A few fungi also need nicotinic acid and pantothenic acid. The vast majority, however, require thiamine (B1). The growth factors are catalytic in their actions.
To sum up, the basic nutritional needs of fungi are:
(i) A suitable organic compound as a source of carbon and energy.
(ii) A suitable source of nitrogen.
(iii) Inorganic ions of sulphur, phosphorus, potassium and magnesium in significant amounts.
(iv) Inorganic ions of iron, zinc, copper, manganese and molybdenum only in minute traces,
(v) Certain vitamins or organic growth factors in trace amounts.
Besides the nutritional requirements listed above the growth of fungi is habitat factors such as temperature, oxygen supply, moisture, pH value and by-products of metabolism.
Modes of Nutrition:
The fungi lack chlorophyll. They are, therefore, unable to synthesize carbohydrate food from inorganic materials and get it readymade from themselves. These heteromorphs according to their method of obtaining food are divided into two categories, namely, the saprophytes or saprobes and parasites.
The organic nutrients directly through the cell membrane from the substratum which abounds in dead organic matter of both animal and plant origin. The saprophytes cannot ingest solid food. Yeast and molds are the common examples of saprophytic fungi.
The parasite lives in or the Living body of a plant or animal and absorbs organic molecules as nutrients through the cell walls from the tissues of the host. Rusts and smuts are the common parasites.
Mechanism of Nutrition (Fig. 1.15):
The whole mycelium may have the power to absorb these nutrients or this task may be assigned to special portions of the mycelium. In saprophytic fungi the hyphae (Mucor mucedo) or rhizodial hyphae (Rhizopus stolonifer) come in intimate contact with nutrients in the substratum (A) and absorb soluble smaller molecules such sugars and amino acids.
Insoluble complex substances such as proteins, lipids and Poly are first broken into soluble monomeres (digested) by secreting extra-cellular enzymes and then absorbed.
The fungal hyphae secrete enzymes which convert insoluble complex food materials in the substratum to soluble ones. The latter are then absorbed by direct diffusion either through the hyphal walls of the hyphae that penetrate the substratum or by the rhizoidal hyphae.
The mycelium of the parasites is rarely ectophytic but frequently it grows inside the host. The hyphae either ramify in the intercellular space between the host cells (D) or penetrate into the host cells (G). The former are called intercellular hyphae and the latter intracellular hyphae.
The intercellular hyphae obtain nutrition through the cell walls or membranes of the host cells. This they do by secreting an enzyme upon the plasma membrane of the host cell.
It makes the membrane more permeable to the contained solutes. The latter diffuse out and are absorbed by the hyphal walls. The hyphal walls of the intracellular hyphae come in direct contact with the host protoplasm (G) and obtain food by direct diffusion.
The intercellular hyphae of some highly specialised (obligate) plant parasites give out slender lateral outgrowths. The hyphal outgrowth punctures the host cell wall making a minute pore through which it enters the host cell. Within the host cell, it enlarges to form a globose (D), lobed (B), or branched (F) absorptive organ.
This type of feeding organ of the parasitic fungi is called a haustorium. It is markedly specialised in structure to absorb nutrition from the host tissues. The haustonum is intracellular and thus robs the host of its food without killing it. Haustoria are characteristic of obligate parasites.
They vary in shape and size in different fungi. In Albugo the haustorium is a button-like (D) or spherical structure. Peronospora parasitica has sac-like haustoria (E) in the leaf cells of Capsella. Peronospora calotheca produces branched filamentous haustorium in the stem cells of Galium (F). Erysiphe graminis forms an elongated branched haustorium inside the host cell (B).
Each haustorium (Fig. 1.16) usually consists of two parts, a constricted region which is in the form of a narrow penetration tube and the expanded or branched region on the host cell. The penetration tube is usually clasped by a ‘collar’ of host wall material. The enlarged or expanded region of the haustorium causes Invagination of the cytoplasmic membrane of the host cell.
The latter remains closely appressed to the wall of the haustorium. There is a zone of apposition enclosing the haustorium between the fungal wall and the unbroken cytoplasmic membrane of the host. Its origin is in dispute.
The secretion from the haustorium upon the plasma membrane of the host makes it permeable to solutes contained in the sap cavity. They diffuse out and are then absorbed by the haustorium parastic fungi do not produce haustoria in artificial cultures. The haustona are also not produced by fungi which live as parasites on animals.
The fungi, as mentioned above, are unable to synthesize sugars from carbon dioxide and water. They, however, can synthesize from soluble sugars the more complex carbohydrates which I the chief components of their cell walls.
They are also able to synthesize proteins and eventually protoplasm if supplied with carbohydrates and simple nitrogen compounds such as ammonium salts. Besides ammonium salts, they can absorb and utilize many complex but soluble organic nitrogenous compounds.
Many fungi obtain nutrition by living in mutually beneficial associations with other p ants. The Association is not causal but permanent and is established during long process of evolution. The two best known examples of mutualisitc associations of fungi with other plants are Symbiosis and Mycorrhiza.
The common example of symbiosis is an association of a fungus and an alga in a lichen thallus. The two organisms in this association are so intertwined as to form a single composite thallus plant which different from either of the partners in form and habit.
The duty of alga in this partnership is to synthesize food with the help of green chloroplasts and share it with its fungal partner. The fungus absorbs minerals in solution and water from the substratum and press on to the alga. The fungal hyphae which form the bulk of the lichen thallus provide shelter to the alga, in addition.
(b) Mycorrhiza. (pl. Mycorrhizae or mycorrhizas):
It is defined as the symbiotic association between the hypha of certain fungi and roots of plants.
It is of three types:
(ii) Endomycorrhiza, and
The fungal hyphae in this case form a complete envelope around the root tip and also penetrate and extend into the first few cortical layers to form an intercellular network of hyphae known as the Hartignet.
The hyphal strands that extend into the substrate from the envelope absorb water and nutrients from the soil and pass them on to the roots of the plant through the Hartig’s net. The presence of the fungus thus increases root absorption. In return the fungus receives food and shelter.
The fungal hyphae, in this case, penetrate root hairs, epidermis and reach the cortex where they grow intracellularly forming fungal knots in the cortical cells. A portion of the mycelium lives in the soil but it forms no dense hyphal growth (envelope on the surface of the root).
It is a combination of the two. The fungal hyphae form a sheath at the surface of the root. Within the root, they grow intercellularly and intracellulary.