Environmental Microbiology of Aquatic and Waste Systems

4.1 Taxonomy of Microorganisms in Aquatic Environments 81

Taxonomy of Fungi

The classification of the fungi is based mainly on
morphology of the hyphae, the structures housing the
sexual structures, or the structure to which the sexual
spores are attached (Samson and Pitt 1989 ; Guarro
et al. 1999 ). The principal diagnostic characteristics
are shown in Fig. 4.22 and are as follows:
(a) Septation of the hyphae
The septation, or lack thereof, of the hyphae is
important in classifying fungi. Non­septate or
coenocytic hyphae are found in Phycomycetes
(Zygomycota). All other fungal groups have
septate hyphae. Examples of Phycomycetes are
Mucor spp. and Rhizopus spp., the bread mold.
(b) The nature of the asexual spores of aquatic Phyco­
mycetes
Aquatic fungi are found among Phycomycetes.
The asexual cells of many aquatic Phycomycetes
are motile and are flagellated and help in the iden­
tification of the organisms (see Figs. 4.22 and
4.2 3 ). Many aquatic Phycomycetes, classified as
Oomycetes, by some authors are pathogens of
plants and fish. Phytphthora infestans which
caused the famous potato blight and subsequent
famine in Ireland which led to massive Irish

immigration to the US belong to this group. Others

are Plasmopara viticola (the cause of downy

mildew of grapes), Plasmopara halstedii (sunflower

downy mildew), and Saprolegniales spp., or water

molds, which cause diseases of fish and other

aquatic vertebrates. Some authors argue that the

Oomycetes are so different from other fungi (so­

called true fungi) that they should not be classified

with them. The majority of authors classify them

with the Phycomycetes, their peculiarities not­

withstanding (see Table 4.8).

(c) The presence of asci

An ascus or sac (plural, asci) which contains

ascosposcores (sexual spores typically eight in

number housed in an ascus) is diagnostic of

Ascomycetes (Ascomycota). The trivial name of

this group of fungi is sac fungi.

(d) The presence of Basidiomycetes

The presence of basidiospores, typically four in

number attached to a basidium, (a club­like struc­

ture) identifies Basidiomycetes. Some of the best

known Basidiomycetes (Basidiomycota) are mush­

rooms. A microscopic examination of the “gills”

on the underside of the mushrooms reveals the

basidia carrying the basidiospores.

Fig. 4.22 Diagnostic features of fungi (From Guarro et al.
1999 )
Zygomycota (Phycomycetes): (a) coenocytic hypha; (b)
zygospore; (c) sporangiophore; (d) sporangiospores.
Basidiomycota (Basidiomycetes): (e) basidiomate; (f) basid­
ium; (g) basidiospore; (h) hypha with clamp connections.

Ascomycota (Ascomycetes): (i) ascommata; (j) ascus;

(k) ascospores; (l) septate hyphae.

Deuteromycota (Deuteromycetes): (m) pycnidium; (n) conid­

iophore; (o) conidiogenous cells; (p) conidia.

Oomycota (regarded as aquatic Phycomycetes) : (q) zoo­

spore; (r) gametangia; (s) oospores