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FUNGAL NUTRITION 111

• Some fungi cannot utilize nitrate or ammonium
  as their sole nitrogen source; instead they require
  a source of organic nitrogen. In these cases the
  requirement usually can be met by supplying a
  single amino acid such as asparagine. Several
  Basidiomycota fall in this category.


• Some fungi have more specific individual require-
  ments, which will be found in their natural habitats.
  For example, some Oomycota (e.g. Phytophthora
  infestans) need to be supplied with sterols for
  growth; some other Oomycota (e.g. Leptomitis
  lacteus) need sulfur-containing amino acids such as
  cysteine, because they cannot use inorganic sulfur.
  Some of the fungi from animal dung need to be sup-
  plied with the iron-containing haem group, a com-
  ponent of cytochromes in the respiratory pathway.

It should be emphasized that all these comments refer
to the minimumrequirements for growth, not to
the optimal requirements. Also, they relate to somatic
(vegetative) growth, and additional nutrients may
be required to complete the life cycle. For example,
several Pythiumand Phytophthoraspp. do not require
sterols for hyphal growth but they need them for both
sexual and asexual reproduction. Many fungi have
extra nutrient requirements for growth in suboptimal
environmental conditions. For example, Saccharomyces
cerevisiae has relatively simple requirements for
growth in aerobic culture but needs to be supplied
with a wide range of vitamins and other growth
factors in anaerobic conditions, because some of the
basic metabolic pathways that normally supply these
requirements do not operate in anaerobic conditions
(Chapter 7).

The carbon and energy sources of fungi

An enormous range of organic compounds can be

utilized by one fungus or another. This is illustrated

diagrammatically in Fig. 6.1, where nutrients are

arranged in approximate order of structural com-

plexityfrom left to right, and in approximate order

of their degree of utilization(vertical axis).

At one extreme, the simplest organic compound,

methane(CH 4 ), can be used by a few yeasts. They con-

vert methane to methanol by the enzyme methane

monooxygenase. Then this is converted to formalde-

hyde by the enzyme methanol dehydrogenase. Then

formaldehyde combines with ribulose-5-phosphate,

and two further reaction steps generate fructose-6-

phosphate.

CH 4 →H 3 C—OH →H—C—H→→→fructose-6-phosphate

O

methane methanol formaldehyde

Several more yeasts (e.g. Candida spp.) and a few

mycelial fungi can grow on the longer-chain hydro-

carbons (C 9 or larger) in petroleum products. The

main limitation with these compounds, as with

methane, is that they are not miscible with water

and so growth is restricted to the water–hydrocarbon

interface. Two mycelial fungi are notorious for their

growth on the long-chain hydrocarbons in aviation

kerosene: Amorphotheca resinae (Ascomycota) and

Paecilomyces varioti(a mitosporic fungus). A. resinaehas

the dubious distinction of having caused at least one

major aircrash, by blocking filters and corroding the

walls of fuel storage tanks, but this problem is easily

avoided (Chapter 17).

Moving further along the spectrum in Fig. 6.1, a larger

number of fungi can utilize the common alcohols

such as methanol and ethanol. In fact, ethanol is an

excellent carbon source for Candida utilis, Emericella

nidulans, and Armillaria mellea, and it can even be

their preferred carbon substrate. Glycerol and fatty

acids will support the growth of several fungi, and

can be the preferred substrates for a few fungi such

as the common “sewage fungus,” Leptomitus lacteus

(Oomycota). Amino acids also can be utilized, but

they contain excess nitrogen in relation to their car-

bon content, so ammonium is released during their

metabolism and this can often lower the pH to

growth-inhibitory levels unless the culture medium is

strongly buffered.

The vast majority of fungi can utilize glucose and

other monosaccharides or disaccharides. Some also

can use sugar derivatives such as aminosugars (e.g.

glucosamine), sugar acids (e.g. galacturonic acid), or

sugar alcohols (e.g. mannitol). However, the utilization

Table 6.1A chemically defined liquid culture medium for
fungi.

Chemical Quantity

NaNO 3 or NH 4 NO 3 or 2 g
L-asparagine at equivalent
nitrogen content
KH 2 PO 4 (alone or in a buffered 1 g
mixture with K 2 HPO 4 )
MgSO 4 0.5 g
KCl 0.5 g
CaCl 2 0.5 g
FeSO 4 , ZnSO 4 , CuSO 4 0.005–0.01 g each
Sucrose or glucose 20 g
Distilled water 1 liter

Note: Common supplements required by fungi include the vit-
amins biotin(10mg) or thiamine(100mg).

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