Fig. 2.9), normally grows on soil or dung, but also grows
on cold-stored meat, which it degrades by producing
proteases. Perhaps the best examples of psychrophilic
fungi are the “snow moulds” such as Sclerotinia bore-
alisand Typhula idahoensiswhich are found in the cold,
northern states of the USA, where they cause serious
damage to cereal crops or grass turf if there is prolonged
snow cover. When the snow melts in spring, the
cereal or grass leaves have rotted and are covered with
sclerotia, which persist until the next snow cover,
when they rot the plant tissues again. Up to 50%
of the winter-sown cereal crops can be destroyed
by these fungi each year. Psychrophilic Pythiumspp.
cause similar problems in Japan, whilst in Britain it
is more common to see cereals or turf damaged by
Monographella nivalis. This fungus is only weakly para-
sitic, but it invades and rots the plant tissues when their
resistance is lowered by prolonged low temperature
and low light. Late-season applications of nitrogenous
fertilizer can predispose turf to attack because nitrogen
promotes lush growth, rendering the plants susceptible
to winter damage.
The physiological basis of temperature
tolerance
There is no single feature that determines the different
temperature ranges of fungi. Instead, there are a range
of factors that contribute to temperature tolerance in
different organisms. The one common theme is that
the ability to grow in the more extreme environments
involves adaptation of the whole organism, and the
temperature limits will be set by the first cellular
component or process that breaks down. Probably for
this reason, the cellular complexity of all eukaryotes,
including fungi, limits their upper temperature to
about 60 – 65°C, whereas the simpler cellular organiza-
tion of bacteria and archaea enables some to grow
at 80°C or higher. The lower temperature limits
for microbial growth are set by factors such as the
reduced rates of chemical reactions at low temperatures,
the increased viscosity of cellular water at subzero
temperatures, and excessive concentrations of cellular
ions leading to protein inactivation. As Robinson
(2001) stated: “the lower growth temperature limit of
psychrophiles is fixed, not by the cellular properties of
cellular macromolecules, but instead by the physical
properties of aqueous solvent systems inside and out-
side the cell.” There are no substantiated reports of any
microorganism growing below −12°C.
Studies on bacteria, yeasts, and filamentous fungi
have revealed a general phenomenon – that changes
in temperature lead to changes in the fatty acid com-
position of the membrane lipids. These changes help
to ensure that membrane fluidity is optimal for the
functioning of membrane transporters and enzymes.
This phenomenon is termed homeoviscous adaptation.
In some psychrophilic yeasts and filamentous fungi
the fatty acids and membrane phospholipids are
more unsaturated than in mesophiles, and the degree
of unsaturation increases at lower temperatures.ENVIRONMENTAL CONDITIONS 145
Fig. 8.4Aspergillus fumigatus, a thermotolerant mitosporic fungus. (a,b) Flask-shaped phialides (p) produce conidia (c) on
the inflated, club-shaped heads of conidiophores. (c) Clinical specimen, stained to show the hyphae of A. fumigatusin
an aspergilloma of the lungs.
(a) (b) (c)