Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 5

Planktonic algae are frequently characterised in
relation to discrete size bands - picoplankton ( 2

μm), nanoplankton (2-20 μm), microplankton
(20-200 μm) and macroplankton (

<

b

a

200 μm). Each
size band is characterised by particular groups of
algae (Table 1.1).
In the benthic environment, the size range of
attached algae is even greater - ranging from small
unicells (which colonise freshly exposed surfaces) to
extended filamentous algae of the mature periphy-
ton community. Filaments of attached algae such as
Cladophora , for example, can extend several cen-
timetres into the surrounding aquatic medium. These
macroscopic algae frequently have small colonial
algaeandunicellsattachedasepiphytes(Fig.2.28),so
there is a wide spectrum of sizes within the localised
microenvironment.

>

d

c

f

e

g

Diversity of shape

h

The shape of algal cells ranges from simple single
non-motile spheres to large multicellular structures
(Fig. 1.2). The simplest structure is a unicellular
non-motile sphere (Fig. 1.2b), which may become
elaborated by the acquisition of flagella (Fig. 1.2c),
by a change of body shape (Fig. 1.2a) or by the
development of elongate spines and processes
(Fig. 1.2d). Cells may come together in small
groups or large aggregates but with no definite shape
(Figs. 1.2d and 1.2e), or may form globular colonies
that have a characteristic shape (Figs. 1.2f and 1.2g).
Cells may also join together to form linear colonies
(filaments), which may be unbranched or branched
(Figs. 1.2h and 1.2i).
Although motility is normally associated with the
possession of flagella, some algae (e.g. the diatom
Navicula and the blue-green alga Oscillatoria ) can
move without the aid of flagella by the secretion of
surface mucilage. In many algae, the presence of sur-
face mucilage is also important in increasing overall
cell/colony size and influencing shape.
Size and shape, along with other major phenotypic
characteristics, are clearly important in the classifica-
tionandidentiicationofalgalspecies.Atafunctional
and ecological level, size and shape are also impor-
tantintermsofsoluteandgasexchange,absorptionof
light, rates of growth and cell division, sedimentation

i

Figure 1.2 General shapes of algae. Non-motile uni-
cells: (a) Selenastrum ;(b) Chlorella . Motile unicells: (c)
Chlamydomonas. Non-motile colony: (d) Scenedesmus ;
(e) Asterionella . Motile colony: (f) Pandorina ;(g)
Volvox. Unbranched filament: (h) Spirogyra . Branched
filament: (i) Cladophora .

in the water column, cell/colony motility and grazing
by zooplankton (Sigee, 2004).

1.2 Taxonomic variation - the
major groups of algae

Freshwater algae can be grouped into 10 major
divisions (phyla) in relation to microscopical
appearance (Table 1.2) and biochemical/cytological
characteristics (Table 1.3). Some indication of