mycelial fungi (Aspergillus, Penicillium, and Fusariumspp.)
which will grow at pH 2.0. But their pH optimum in
culture is usually 5.5 – 6.0. Truly acidophilicfungi,
able to grow down to pH 1 or 2, are found in a few
environments such as coal refuse tips and acidic mine
wastes; many of these species are yeasts. Among
filamentous fungi, the most cited example of an
acidophile isAcontium velatumwhich was isolated on
laboratory media containing 2.5 N sulfuric acid. This
fungus can initiate growth at pH 7.0 but it rapidly
lowers the pH of culture media to about 3.0 which is
probably close to its optimum.
Strongly alkaline environments, with a pH of about
10, are found in soda lakes and alkaline springs.
The fungi that colonize these environments include
specialized species of filamentous fungi, such as Clado-
sporium, Fusarium, and Penicillium, and some yeasts, but
they are alkali-tolerant rather than alkalophilic. Some
truly alkalophilic Chrysosporiumspecies that grow up
to pH 11 have been isolated from birds’ nests. These
fungi are specialized degraders of keratin – the protein
found in skin, nails, feathers, and hair (Chapter 16).
The physiological basis of pH tolerance
In all cases that have been investigated, the fungi that
grow at extremes of pH are found to have an internal,
cytosolic pH of about 7. This intracellular pH can be
assessed crudely in extracts of disrupted cells. But, the
most accurate modern methods involve the insertion
of pH-sensitive electrodes into hyphae, or loading
hyphae with pH-sensitive fluorescent dyes that are
permeabilized through the plasma membrane. These
dyes show peaks of fluorescence at two wavelengths,
and the relative size of the two peaks changes with pH,
enabling changes of less than 0.1 pH unit to be mea-
sured accurately. The findings suggest that the fungal
cytosol has strong buffering capacity. Even when the
external pH is changed by several units, the cytosolic
pH changes by, at most, 0.2–0.3 units. Fungal cells
could achieve this homeostasis in several ways – for
example, by pumping H+ions out through the cell
membrane to counteract the inflow of H+in acidic
environments, by exchange of materials between the
cytosol and the vacuoles (which normally have acidic
contents), and by the interconversion of sugars and
polyols such as mannitol (Chapter 7) which involves
the sequestering or release of H+.
Because the cytosolic pH is so tightly regulated,
any perturbation of cytosolic pH can act as an intra-
cellular signal leading to differentiation or change of
growth polarity, etc. There are several examples of this
in plant and animal cells. One example relating to fungi
(although, strictly, it relates to the Oomycota) stems
from the fact that the cleavage of zoospores in the
sporangia of Phytophthora(Chapter 5) can be induced
experimentally by a cold shock. By using a fluorescent
pH-indicator dye, Suzaki et al. (1996) found that the
cytosolic pH was raised transiently from 6.84 to 7.04
by this treatment, but no zoospore cleavage occurred
if the sporangia had been microinjected with a buffer
of pH 7.0, to prevent any change in cytosolic pH.Ecological implications of pHThe effects of pH are much easier to investigate in
laboratory conditions than in nature, because pH is
not a unitary factor. In other words, a change of pH
can affect many different factors and processes. For
example, pH affects the net charge on membrane
proteins, with potential consequences for nutrient
uptake. It also affects the degree of dissociation of
mineral salts, and the balance between dissolved car-
bon dioxide and bicarbonate ions. Soils of low pH can
have potentially toxic levels of trace elements such
as Al^3 +, Mn^2 +, Cu^2 +, or Mo^3 +ions. Conversely, soils of
high pH can have poorly available levels of essential
nutrients such as Fe^3 +, Ca^2 +, and Mg^2 +. Nevertheless, in
general the pH–growth response curves in laboratory
culture seem to be relevant to natural situations. For
example, Pythiumspp. are generally intolerant of very
low pH but occur in soils above pH 4 –5, consistent with
the data for Pythium oligandrumin Fig. 8.5. Similarly,
Stachybotrys chartarum is found predominantly in
near-neutral and basic soils, again consistent with the
data in Fig. 8.5.
Fungi can alter the pH around them and thus to some
degree create their own environment. The form in
which nitrogen is made available can be a key factor
in this respect. If nitrogen is supplied in the form of
NH 4 +ions, which almost all fungi can use in labor-
atory culture or in nature, then H+ions are released in
exchange for NH 4 +and the external pH can be lowered
to a value of 4 or less, leading to growth inhibition
of the more pH-sensitive fungi such as Pythiumspp.
Conversely, the uptake of NO 3 −can cause the external
pH to rise by about 1 unit. Fungi also release organic
acids (Chapter 7) which can lower the external pH. Some
aggressive tissue-rotting pathogens of plants, such as
Athelia rolfsiiand Sclerotinia sclerotiorum, release large
amounts of oxalic acid in culture or in plant tissues,
lowering the pH to about 4.0. This seems to contribute
significantly to pathogenicity, because these fungi
also secrete pectic enzymes with acidic pH optima.
Oxalic acid can combine with Ca^2 +in the plant tissues,
removing Ca^2 +from the pectin in plant cell walls,
so the walls are more easily degraded by the pectic
enzymes (Chapter 14).
Relatively small pH gradients can help to orientate
fungal growth, as Edwards & Bowling (1986) found by