untitled

152 CHAPTER 8

from H 2 O 2 in normal conditions pose little threat if
catalase is active.

Carbon dioxide

All fungi need carbon dioxide in at least small
amounts for carboxylation reactionsthat generate
fatty acids, oxaloacetate, etc. (Chapter 6). Fungi that
grow in anaerobic conditions often have a high CO 2
requirement, whereas several aerobic fungi can be
inhibited by high concentrations of CO 2.
In normal aerobic respiration, glucose is converted
to CO 2 and water according to the familiar empirical
equation:

C 6 H 12 O 6 +6O 2 →6CO 2 +6H 2 O

However, oxygen and carbon dioxide behave differ-
ently from one another in solution, and this can
have significant effects on fungal growth. CO 2 dissolves
in water to form carbonic acid, which dissociates to
bicarbonate ions in a pH-dependent manner. At pH 8,
the equilibrium is approximately 3% CO 2 (equivalent
to carbonic acid) with 97% HCO 3 −(bicarbonate ion).
But at pH 5.5 the equilibrium is approximately 90%
CO 2 and 10% HCO 3 −. Studies of fungi in laboratory
culture at different pH levels suggest that fungi are more
sensitive to the bicarbonate ion than to CO 2 as such.
Even so, it can be questioned whether CO 2 (or bicar-
bonate) is a major growth inhibitor in nature. CO 2 is
much more soluble than is oxygen in water, and
when the different diffusion coefficients of oxygen and
CO 2 are taken into account (the coefficient for CO 2 is
actually lower) it can be calculated that CO 2 diffuses
about 23 times more rapidly than O 2 in water. Thus,
in normal aerobic respiration, when a fungus generates
one mole of CO 2 for every mole of O 2 consumed, the
oxygen will be depleted in a water film before the CO 2
level reaches a level of even 1%. In short, fungi that
grow in undisturbed water, or even 1 millimeter below
the surface of an agar plate, are likely to experience
significant oxygen depletion. This is one of the major
reasons why liquid culture media – and many industrial
fermenter-based systems like those for the production of
Quorn™ mycoprotein – need to be vigorously aerated
(Chapter 4).

Water availability and fungal growth

General principles

All fungi need the physical presence of water for

uptake of nutrients through the wall and cell membrane,

and often for the release of extracellular enzymes.

Fungi also need intracellular water as a milieu for

metabolic reactions. However, water can be present in

an environment and still be unavailable because it is

bound by external forces. So, in order to understand

how the availability of water affects the growth of fungi,

we need to establish some basic principles.

The sum of all the forces that act on water and tend

to restrict its availability to cells is termed the water

potential, denoted by φand defined in terms of energy

(negative MegaPascals), where one MPa is equivalent

to 9.87 atmospheres, or 10 bar pressure. As familiar

reference points, ultra-pure water has a potential of

0 MPa, normal sea water has a potential of about

−2.8 MPa, and most plants reach “permanent wilting

point” in soils of about −1.5 MPa. The units are neg-

ative because the environment exerts a pull on water.

The total water potential consists of the sum of

several different potentials: osmotic potential(solute

binding forces, denoted by φπ), matric potential

(physical binding forces, denoted by φm), turgor

potential(φp), and gravimetric potential(φg). So, by

summing all these forces:

φ(water potential) =φπ+φm+φp+φg

It follows that, in order for a fungus to retainits exist-

ing water, it must generate a potential equalto the

external water potential φ, and in order to gainwater

from the environment a fungus must generate a (neg-

ative) potential greaterthan φ(Papendick & Mulla

1986).

How fungi respond to water potential

Most fungi are highly adept at obtaining water,

even in environments that exert a significant water

stress. However, the water moulds (Oomycota such

as Saprolegnia and Achlya spp.) are exceptions to

this. They have little ability to maintain their turgor

against external forces, probably because they grow only

or predominantly in freshwater habitats. It used to be

thought that fungi needed to remain turgid in order

to grow, but hyphae of the water moulds can continue

to grow even when they have lost turgor. This is prob-

ably because the extension of hyphal tips is achieved

by continuous extension of the cytoskeletal com-

ponents, similar to the extension of pseudopodia in

amoeboid organisms. Nevertheless, even the water

moulds need to be turgid in order to penetratesolid

surfaces (Harold et al. 1996; Money 2001). Since the

penetrating ability of hyphal tips is one of the key fea-

tures of mycelial fungi, a fungus that lacks turgor is

essentially crippled.

Almost all fungi of soil and other terrestrial habitats

can grow readily in media of −2 MPa. If the water stress