Introduction to Fungi, Third Edition

24
Basidiomycete yeasts
24.1 Introduction
Fungi living predominantly or exclusively as
yeasts are encountered in three classes of
Basidiomycota, namely the Heterobasidiomycetes
(Chapter 21), Urediniomycetes (Chapter 22)
and Ustilaginomycetes (Chapter 23). We shall
discuss basidiomycete yeasts together in the
present chapter because these organisms,
although taxonomically diverse, are unified by
many biological features.
24.1.1 Ecology
Little is known about the ecology of basidiomycete
yeasts. They occur in marine and freshwater
habitats, the soil and the plant rhizosphere, and
especially on above-ground plant surfaces such
as tree bark, leaves, flowers and fruits. A certain
degree of specificity of yeast species relative to
plant species or organs of a given plant host has
been observed (Phaff, 1990). Basidiomycete yeasts
may be found in all climatic zones from the
arctic to the tropics. They generally exist as
saprotrophic phylloplane organisms. When
nutrients become available, there may be a
steep increase in the population density of
these yeasts. Many yeasts isolated from soil
have their origin in vegetation which becomes
incorporated into humus after leaf fall.
Basidiomycete yeasts are not noted as plant
pathogens, but some species can infect animals.
Cryptococcus neoformans is one of the most serious
fungal pathogens of humans (pp. 661 665).
Members of another group (Malassezia spp.) live
commensally on the skin of humans and other
mammals, causing superficial dermatomycoses
under suitable conditions (p. 671).
Basidiomycete yeasts therefore share many
ecological features with their ascomycete counterparts,
and they are easily isolated following
similar procedures, i.e. by plating out soil
suspensions, leaf washings or filters bearing
water samples onto standard agar media
augmented with antibacterial antibiotics
(p. 262). One good way to isolate ballistoconidium-forming
yeasts is to attach a piece of
vegetation to the underside of a Petri dish lid,
permitting ballistosporic yeasts to shower their
spores onto the agar medium. Within 2 3 days,
yeasts such as Sporobolomyces spp. will grow and
can be isolated in pure culture. Yeasts are
preserved in an active state on agar slopes
at 4°C. Lyophilized preparations can also be
made from vegetative cells of many species,
and these remain viable for several years.
Many basidiomycete yeasts, the so-called
‘red yeasts’, are coloured yellow, orange, pink
or red due to the presence of carotenoids (see
Fig. 24.8). Red yeasts are found in all three classes
containing basidiomycete yeasts, but they are
rare among ascomycetes (see p. 253 for the
Lalaria state of Taphrina). Carotenoid production
can be of commercial value, as in Phaffia
rhodozyma which produces astaxanthin, an
important food and feed pigment (p. 665).
Other commercial applications of basidiomycete
yeasts are as potential producers of lipid, e.g. as
cocoa butter substitute (Ratledge, 1997), and as
biocontrol agents against storage rots of fruits