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120 CHAPTER 6

less concerned with this, although it has important
implications for environmental processes. The two
most commonly used criteria of the efficiency of sub-
strate conversion are the economic coefficientand the
Rubner coefficient:

Economic coefficient (expressed as a percentage) =

Rubner coefficient (expressed as a proportion) =

Typical values for the economic coefficient range
from 20 to 35 if a fungus is grown in a dilute medium,
but the values fall markedly if the medium is made
progressively richer – fungi seem to be “wasteful” if
supplied with excess substrate. Values for the Rubner
coefficient typically are higher than for the eco-
nomic coefficient. For example Aspergillus nigerhad a
Rubner coefficient of 0.55 – 0.61 (equivalent to 55 – 61%)
over a range of cultural conditions, compared with
an economic coefficient of 35–46. The difference is
explained mainly by the synthesis of lipid storage
reserves during growth, because lipids have higher
calorific values than the carbohydrates used as substr-
ates in culture media. However, the important point
revealed by all these values is that a considerable
proportion of the substrate supplied to a fungus is con-
sumed in energy production rather than being converted
into biomass.
Substrate conversion efficiencies are difficult to obtain
in natural systems, but Adams & Ayers (1985) did this
in laboratory conditions by collecting all the spores pro-
duced by a mycoparasite (Sporidesmium sclerotivorum)
when it was grown on sclerotia of its main fungal host,
Sclerotinia minor. This experimental system mimics the
conditions in nature, because sclerotia are produced
as dormant survival structures on infected host plants
and then overwinter in the soil, where they can be
attacked by the mycoparasite. The reported substrate
conversion efficiency was exceptionally high: an eco-
nomic coefficient of 51– 60 and a Rubner coefficient of
0.65 – 0.75. This mycoparasite has a remarkable way
of parasitizing the host sclerotia: it penetrates some of
the sclerotial cells initially but then grows predomin-
antly betweenthe sclerotial cells, scavenging small
amounts of soluble nutrients that leak from them, and
thereby creating nutrient stress. The host cells respond
by converting energy storage reserves (principally
glycogen) to sugars, which leak from the cells to sup-
port further growth of the mycoparasite. This essentially
noninvasive mode of parasitism is also employed by
endophytic fungiin plants (Chapter 14). They grow

Heat of combustion of biomass produced

Heat of combustion of substrate consumed

Dry weight of biomass produced

Dry weight of substrate consumed

slowly and sparsely, between or within the plant cell

walls, exploiting nutrients that leak from the host cells.

In terms of substrate efficiency we should also note

that fungi can grow by oligotrophy, using extremely

low levels of nutrients (oligo=few) on silica gel or

glass. They seem to grow by scavenging trace amounts

of volatile organic compounds from the atmosphere

(Wainwright 1993).

Fungi that cannot be cultured

To close this chapter we should record that several fungi

still cannot be grown in laboratory culture. They have

been termed obligate parasitesbut now more com-

monly are termed biotrophic parasites(Chapter 14).

Many of them are extremely important in environ-

mental and economic terms, including the ubiquitous

arbuscular mycorrhizal fungi (Glomeromycota), rust

fungi (Basidiomycota), powdery mildew fungi

(Ascomycota), and downy mildews (Oomycota). All of

these produce nutrient-absorbing haustoriaor equiv-

alent structures in host cells (Chapter 14).

It remains to be seen if some of these fungi will ever

be grown in axenicculture (i.e. separate from their

hosts). However, significant progress has been made in

culturing the rust fungi, starting with Puccinia grami-

nis(black stem rust of wheat) and several other rust

species (Maclean 1982). P. graminiswas found to grow

slowly, and only after a prolonged lag phase. Its linear

extension rate on agar ranged from 30 to 300μm

day−^1 , compared with rates from 1 to 50 mm day−^1

for fungi that are commonly grown in laboratory con-

ditions. P. graministends to leak vital nutrients, such

as cysteine, into the growth medium, so a high spore

density is needed in the inoculum to minimize the

diffusion of nutrients away from individual sporel-

ings. Also, P. graminis produces self-inhibitors which

can result in “staling” of the cultures. This is true of

several fungi if they are grown on sugar-rich media, and

it causes a progressive reduction and eventual halting

of growth. It can be overcome by using relatively

weak media. Some other rust fungi need relatively

high concentrations of CO 2 – for example, Melampsora

liniwhich causes flax rust. If attention is paid to all these

points then several rust fungi can be maintained in

laboratory culture. However, in the process of being

cultured (or of becoming culturable) some of them

change irreversibly to a “saprotrophic” form that can-

not reinfect plants.

General texts

Booth, C. (1971)Methods in Microbiology, vol. 4. Academic

Press, London.