Saturated fatty acids are less fluid than unsaturated fatty
acids at any given temperature (compare the fluidity
of margarine, which has a high content of unsaturated
fatty acids, and butter which has a high content of
saturated lipids). Consistent with this, an abundance
of polyunsaturated fatty acids has been reported
in the snow mould, Monographella nivalis. And, the
thermophilic fungus Thermomyces lanuginosus has
been found to have a twofold higher concentration of
linoleic acid (unsaturated) when grown at 30°C than
at 50°C.
Compounds such as the disaccharide trehalose
(see Fig. 7.6) often occur in high concentrations in
psychrophilic or psychrotrophic organisms, and fungi
are reported to accumulate this sugar in response to
low temperatures. Trehalose is thought to act as a gen-
eral stress protectant in the cytosol, and is known
to stabilize membranes during dehydration. Polyols
(polyhydric alcohols) such as glycerol and mannitol (see
Fig. 7.6) also tend to accumulate in response to stress
conditions, and mannitol can act as a cryoprotectant.
The enzymes and ribosomal components of thermo-
philes are reported to be more heat-stable than those
of mesophiles when extracted and tested in cell-free
systems. This has been shown for thermophilic yeasts
as well as for bacteria. The heat stability of enzymes is
conferred by increased bonding between the amino acids
near the enzyme active site, including bonds other than
the heat-labile hydrogen bonds. Heat-stabilizing factors
in the cytosol can also contribute to the thermostability
of enzymes. In recent years, attention has focused
on heat shock proteins, which can be synthesized at
elevated levels in response to a brief (e.g. 1 hour)
exposure to high temperatures, such as 45 –55°C. They
function as stress proteins, like the equivalent cold shock
proteins or proteins produced in response to oxygen
starvation. In fact, they are ubiquitous, being found
in organisms of all types, and they are also present in
normal conditions. They act like chaperones, helping
to ensure that the cell’s proteins are correctly folded
and that damaged proteins are destroyed. However, it
is not clear that they have any specific relevance to the
normal temperature ranges of fungi.
Finally, it may be asked whether thermophilic fungi
have a particularly high rate of metabolism and a cor-
respondingly high rate of substrate conversion into
fungal biomass. In other words, do thermophilic
fungi benefit specifically from being able to grow at
high temperatures? The relevant growth parameter is
the specific growth rate, “μμ”, defined in Chapter 4.
Comparisons of thermophiles (e.g. Thermomyces lan-
uginosus) and mesophiles (e.g. Aspergillus niger) show
no difference in the specific growth rate. So, it seems
that thermophilic fungi occupy their high-temperature
environments because they are specifically adapted
to do so, but they are no more efficient in substrate
utilization than are the mesophiles.Hydrogen ion concentration and
fungal growthThe responses of fungi to culture pH need to be
assessed in strongly buffered media, because otherwise
fungi can rapidly change the pH by selective uptake or
exchange of ions. Mixtures of KH 2 PO 4 and K 2 HPO 4 are
commonly used for this purpose. It is then found that
many fungi will grow over the pH range 4.0 –8.5, or
sometimes 3.0 –9.0, and they show relatively broad pH
optima of about 5.0 –7.0. However individual species
vary within this “normal” range, as shown by the
three representative examples in Fig. 8.5.
Several fungi are acid-tolerant, including some
yeasts which grow in the stomachs of animals and some146 CHAPTER 8
Fig. 8.5(a–c) pH growth response curves of three representative fungi in laboratory culture (Pythium oligandrumis a
member of the cellulose-walled Oomycota).