Excerpt from

GENETICS AND BREEDING OF EDIBLE MUSHROOMS

Shu-ting Chand

John A. Buswell

Philip G. Miles

Copyright 1993

4.2. Nutritional Requirements

The nutritional requirements for mycelial
growth are relatively simple. Since the fungi are heterotrophic organisms, they
must be supplied with a source of carbon. While many carbon sources may
be used, individual species commonly
have a preference, but among the simple sugars glucose is most frequently preferred,
and in amounts of approximately 2%. In addition to the simple sugars, polysaccharides
may provide carbon for the fungi. Such polysaccharides are the usual source
of carbon for fungi in nature, and the insoluble polysaccharides, such as cellulose,
are broken down by extracellular enzymes to simpler, soluble units which are
then taken into the fungal hyphae by absorptive (osmotrophic) nutrition.
Under certain conditions a number of organic compounds (alcohols, organic acids,
polycyclic compounds, and ammo acids) may also provide carbon for mycelial growth.
It is worth mentioning at this point that a mixture of sugars may give greater
growth than simply the summation of growth to be obtained by each separately
(Horr, 1936). On the other hand, another frequent observation in studies of
carbon nutrition is that when a fungus is supplied with a mixture of carbon
sources, it may use one preferentially over the others. The matter of concentration
of carbon source is also important in determination of the effectiveness of
promotion of growth. This has been shown in Coprinus lagopus (= C.
cinereus) by Moore (1969) with the demonstration that growth on sucrose
is negligible at low concentrations but occurs at higher sugar concentrations.

Obviously, all organic compounds (carbohydrates, amino acids, lipids, nucleic acids) require carbon in their skeletal framework but it should not be overlooked that the carbon compounds supplied to fungi also provide the energy required for the organism's metabolic activities.

Nitrogen is a required element
in media used for the growth of fungi. It is essential for the synthesis of
fungal proteins, purines, pyrimidines, and is also necessary for the production
of chitin, a common lunyal cell wall polysaccharide that is composed of units
of N-acetylglucosamine. While there are a few fungi that have been reported
in the past to fix atmospheric nitrogen, there is no confirmation, using modern
techniques, that this is true and there certainly are no filamentous fungi that
do. To date, nitrogen fixation is known to occur only in prokaryotic organisms.
Thus, the common sources of nitrogen in fungal media are salts of nitrate and
ammonium, and organic nitrogen compounds. A generalization can be made to the
effect that the nitrogen requirements of all fungi can be met by organic nitrogen
(e.g., peptone or amino acids), some may utilize the ammonium ion, and some
may use nitrates. Those that utilize nitrate are also able to use the ammonium
ion. In the ceil the ammonium ion is combined with a-ketoglutaric acid in the
presence of glutamic dehydrogenase to form glutamic acid, and other amino
acids may be formed by transaminase reactions.
Thus, there is a relationship between ammonia and TCA cycle intermediates which
leads to the formation of amino acids.

A medium for the growth of fungi must contain minerals. The mineral requirements are similar to those for plants. While some fungi require a reduced form of sulfur, most species utilize sulfur as sulfate (e.g., magnesium sulfate) in a range of 0.0001 to 0.0006 M. The role of sulfur is for sulfur-containing amino acids (e.g., cysteine and methionine), for vitamins such as thiamine and biotin, and in some cases for products of secondary metabolism (e.g., penicillin). Phosphorus is present in ATP, nucleic acids, and the phospholipids of membranes. It is commonly included in growth media as potassium phosphate at a concentration of about 0.004 M. Potassium has the role of a cofactor in many enzyme systems and its requirement is fulfilled at a concentration of 0.001 to 0.004 M.

Many enzymes are activated by magnesium, essential to all fungi, and magnesium sulfate, when supplied at a concentration of 0.001M, satisfies this requirement.

Equally important mineral elements, although required in lower concentrations, are the trace elements: iron, zinc, manganese, copper, and molybdenum. These are constituent elements in enzymes and are not all universally required by fungi.

Vitamins are organic molecules
required in small amounts and not used as a source of energy or structural material
of protoplasm. The vitamin has a catalytic action and imparts specificity in
its function as a coenzyme. The vitamin requirement is influenced by temperature
and pH since it is concerned with enzyme activity. Most fungi are able to make
their own vitamins, but sometimes in amounts too low to givp optimal growth.
Thiamine (vitamin B) is a natural deficiency of a number of basidiomycetes,
including the wood-rotting edible mushrooms Lentinus edodes and Flammulina
velutipes. Biotin (vitamin B₇ or vitamin H) is a natural deficiency for
some fungi such as the ascomycetes Neurospora and Sordaria.

A chemically defined medium that supports the growth of many edible basidiomycetes is as follows:

Dextrose 20.0 g

Asparagine 2.0 g

KH₂PO₄ 0.46 g

K₂HPO4 1.0 g

MgSO₄.7H₂O 0.5 g

Thiamine-HCl 0.12 mg

Distilled H₂O 1000 ml