Introduction to Fungi, Third Edition

ZYGOMYCETES: MUCORALES
167
species of Absidia may also infect domestic
animals.
A number of species have been used in the
production of oriental foods such as sufu,
tempeh and ragi (Nout & Aidoo, 2002) and
some are used as starters in the saccharification
of starchy materials before fermentation to
alcohol (Hesseltine, 1991). In modern biotechnology,
many mucoralean fungi are employed in
biotransformation processes (for references, see
Kieslich, 1997). Further, a number of species are
oleaginous, i.e. they are able to synthesize and
accumulate lipids to over 20% (dry weight) of
their biomass. Because these lipids (principally
triacylglycerides) may be enriched in polyunsaturated
fatty acids (PUFAs), oleaginous members
of the Mucorales are of current biotechnological
interest (Certik & Shimizu, 1999).
Extensive studies of nutrition and physiology
have been made. A wide variety of sugars can be
used, and whilst starch can be decomposed by
some species, cellulose is generally not utilized.
Under anaerobic conditions, ethanol and numerous
organic acids are produced. Many Mucorales
need an external supply of vitamins for growth
in synthetic culture. Thiamine is a common
requirement, and the amount of growth of
Phycomyces has been used as an assay for the
concentration of thiamine.
Zycha et al. (1969) have given a general
account of the taxonomy of the Mucorales,
including keys to genera and species. Benny
et al. (2001) recognized 13 families and 57 genera.
Classification and identification are based
largely on the morphology of the anamorph.
However, DNA sequence comparisons indicate
that several families and even some larger
genera are polyphyletic (O’Donnell et al., 2001),
meaning that the traditional family-level classification
scheme is artificial. We retain it here
because it presents an accessible framework
of morphological features within which the
Mucorales can be understood, and because
convincing alternative schemes have not yet
been put forward.
7.2.1 Growth and asexual reproduction
The mycelium is coarse, coenocytic and
richly branched, the branches tapering to fine
Fig 7.2 Mucor mucedo. (a) Mycelium and young
sporangiophores with globules of liquid attached. (b) Immature
sporangium with the columella visible through the sporangial
wall. (c) Dehisced sporangium showing the columella, the frill
representing the remains of the sporangial wall, and
sporangiospores.
points (Fig. 7.2). Later, septa may appear.
Thick-walled mycelial segments (chlamydospores)
may be cut off by such septa (Benjamin,
1979) and in certain species, e.g. Mucor racemosus,
the presence of chlamydospores in sporangiophores
may be a useful diagnostic feature
(Fig. 7.14b). In anaerobic liquid culture, especially
in the presence of CO 2 , several species of
Mucor (e.g. M. rouxii) grow in a yeast-like instead
of a filamentous form (Fig. 7.3) but revert to
filamentous growth in the renewed presence
of O 2 . The cell walls of Mucorales are chemically
complex (Ruiz-Herrera, 1992; Gooday, 1995).
Chitin microfibrils are present but are often