FUNGAL NUTRITION 113
starch (by producing amylase), some can utilize lipids
(by producing lipases) or proteins (by producing pro-
teases), and some can grow on cellulose, hemicelluloses,
pectic compounds (Chapter 14), or chitin. Often, these
enzymes occur as complexes, consisting of two or
more enzymes that attack the polymer at different
sites, acting synergistically. We will see this in the case
of cellulose breakdown, later in this chapter. At the
extreme end of the spectrum in Fig. 6.1, a few highly
specialized fungi degrade the most structurally complex,
cross-linked polymers. For example, lignin is degraded
by enzymes of a few wood-rotting Basidiomycota termed
“white rot fungi” (Chapter 11). Similarly, “hard ker-
atin” in the horns and hoofs of animals is degraded by
a few keratinophilicfungi, including species related
to the dermatophytic pathogens of humans (Chapter
16).
To summarize this section, fungi exploit a wide
range of organic nutrient sources, but in all cases they
depend on the uptake of simple, soluble nutrients
which diffuse through the wall and enter the cells
via specific transport proteins. These only allow the
passage of small molecules such as monosaccharides,
amino acids, and small peptides of two or three amino
acids. Even disaccharides such as sucrose and cello-
biose (a breakdown product of cellulose) may need to
be degraded to monosaccharides before they are taken
up by most fungi. Any larger molecules have to be bro-
ken down by extracellular enzymes (depolymerases)
which are secreted by the fungus.
Fig. 6.3(a) Typical current profile around an individual hypha of a growing fungus (Achlyasp.). (b) Interpretation of the
current, involving proton export through ATPase-driven proton pumps behind the hyphal tip, and uptake of protons
at the hyphal tip through symport proteins that simultaneously transport amino acids. ((a) From Gow 1984. (b) Based
on Kropf et al. 1984.)
Fungal adaptations for nutrient captureAn electrical dimension to hyphal growthThe growing tips of fungi generate an electrical
field around them. It can be mapped by placing a
tiny vibrating electrode in the liquid film around a
hypha, resulting in a trace like that shown in Fig. 6.3.
Typically, the exterior of the hypha is more electro-
negative at the apex than further back, showing that
current (which is positive by convention) enters at
the tip and exits in the subapical regions. In most
fungi that have been examined in sufficient detail- for example Achlya (Oomycota) and Neurospora
(Ascomycota) – the current seems to be carried by pro-
tons (H+) because it corresponds with a gradient of pH
along the hyphal surface and the current still occurs
when other candidate ions are reduced or eliminated
from the growth medium. However, in marine fungi
the current might be carried by K+. For several years it
was speculated that the electrical field might be the
driving force for apical growth, perhaps by affecting the
activities of membrane proteins or the cytoskeleton.
However, this now seems unlikely because the direc-
tion of current flow can be reversed without affecting
tip growth (Cho et al. 1991). Instead, the electrical field
seems to be intimately involved in nutrient uptake.
The uptake of organic nutrients is an energy-
dependent process. Ions (e.g. H+) are pumped to the