A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 179

typical blue-green colour and the smaller
species are easily confused with bacteria.
Common in the plankton or along the mar-
gins of lakes and ponds where it can occur
on wet rocks. Cyanophyta. Plate IX. Fig. 1.3.

67 (66) (a) Cells of colony densely crowded within
themucilage................. Microcystis

Cells of Microcystis are spherical to sub-
spherical (very slightly elongate) and are
usually gas vacolate. They form large globu-
lar to irregular mucilaginous colonies (often
containing hundreds of cells) a millimetre
or more in size that are planktonic and are
often responsible for nuisance water blooms.
Colonies may be globular, more elongate or
irregular and with holes (clathrate) and the
mucilage is distinct and fairly firm. During
periods of non-active growth, the colonies
may rest on the bottom and in that state may
loose their gas vacuoles. Colonies increase
in number by fragmentation. Cells 2.5-6 μm
diameter. During the actively growing sea-
son, it is planktonic and may form blooms
in eutrophic waters. Frequently reported as
producing taste and odours in drinking water
(Palmer, 1962), large planktonic populations
can also clog filtration systems in water treat-
ment works. Microcystis is well known for
producing toxins such as microcystins and
lipopolysaccharides in water (Chorus and
Bartrum, 1999). Cyanophyta. Figs. 1.3, 2.18,
2.21 and 4.34.

25 μm

100 μm

Figure 4.34 Microcystis . Indian ink preparations.
Top: Detail from edge of colony showing granular algal
cells within copious mucilage Bottom: Typical appear-
ance of entire colony, loosely extending in all directions.
The colour of this alga varies from olive brown (top) to
fresh blue-green (bottom).

(b) Cells more spaced out within the
mucilage so that the colonies appear less
dense . ..................... A phanocapsa

approximately spherical 1-5.5 μmindiame-
ter depending upon species. Common in the
plankton. Cyanophyta. Plate IX. Fig. 4.35. Aphanocapsa forms globular amorphous

gelatinous colonies that are usually free-
floating but can be terrestrial. Cells are much
more spaced out than in a Microcystis colony
and also sometimes occur as pairs within
the colony as a result of cell division. The
mucilage at the outer margin of the colony
is not as distinct as in Microcystis. Cells

68 (56) (a) Cells arranged in colonies of definite
shape . ............................... 69

(b) Cells either individual, in pairs or
in aggregations with no definite shape
.................................... 106