A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 169

works. Anabaena species are also able to
produce a range of toxins in water - includ-
ing microcystins, anatoxin-a, saxitoxins and
lipopolysaccharides (Chorus and Bartrum,
1999). Cyanophyta. Figs. 1.3, 1.4, 1.5, 2.7
and 4.24a-4.24c

(b) Trichomes embedded in obvious and
extensiveirmmucilage............ Nostoc

100 μm

Cells of Nostoc are similar to Anabaena but
are embedded in firm, extensive, mucilage
which may be leathery in texture and
coloured straw or brown. In older colonies,
the trichomes tend to be situated towards

20 μm

Figure 4.23 Aphanizomenon . Top: Typical view of
planktonic raft in lake phytoplankton. Bottom: SEM
detail of individual filaments.

are of uniform width throughout with a layer
of amorphous and frequently inconspicuous
mucilage on their surface. The filaments may
be straight, curved or coiled depending upon
the species. Some species produce gas vac-
uolesandcanformblooms.Cells(3.5-14μm
wide) are rounded or barrel-shaped, giving
the filament the appearance of a string of
pearls. Heterocysts are spherical and may not
be frequent in nitrogen-rich waters. Akinetes
(rounded or cylindrical) are produced adja-
cent to the heterocysts and are frequent in
natural populations. Many species can only
be identified if the akinetes are present. An
extremely common species in the plankton
of standing waters where it frequently forms
blooms. Anabaena can occur in lakes, ponds
and ditches. It has been reported (Palmer,
1962) as potentially producing taste and
odour in water used for drinking purposes.
The occurrence of dense planktonic popula-
tions can cause problems of filter clogging
when the water passes through a treatment

100 μm 20 μm

50 μm

Figure 4.24a Anabaena. Top left: Large planktonic
colony. Top right: Detail of olive-brown cells in colony.
Bottom: Small planktonic colony showing filament
embedded in mucilage, and having a single heterocyst
(arrow). Indian ink preparation. Comparison of the two
colonies shows the colour variation that can occur in this
alga.