Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 18

algae also have a number of distinctive cytological
aspects.

Flagella, when present, occur in pairs. These are
equal in length without tripartite tubular hairs and
have a similar structure (isokont). Some genera
have four or even eight flagella, but this is unusual.
Chloroplasts vary in shape, size and number. In
unicellular species, they tend to be cup-shaped
(Fig. 4.55), but in filamentous forms may be annu-
lar (Fig. 4.19), reticulate (Fig. 4.17), discoid or
ribbon-like (Fig. 4.13). They have a double outer
membrane, with no enclosing periplastidal endo-
plasmic reticulum.

50 μm

Figure 1.5 Mid-summer blue-green algal bloom in a
eutrophiclake.SEMviewoflakeepilimnionphytoplank-
ton sample (August), showing the dense population of
filamentous Anabaena flos-aquae which totally domi-
nated the algal bloom (chlorophyll- a concentration 140
μgml −1 ). Copious mucilage associated with this alga is
seen as numerous fine strands, formed during the dehy-
dration preparation process. See also Figs. 2.8 (seasonal
cycle) and 4.24 (live Anabaena ).

Production and storage of the photosynthetic
reserve (starch) occur inside the plastid, with gran-
ulesfrequentlyclusteredaroundthepyrenoid.Inall
other eukaryote algae, the storage material occurs
mainly in the general cytoplasm.

In motile species, an eyespot is frequently
present, appearing red or orange (Fig. 4.38) in fresh
specimens. The cell wall in green algae is made of
cellulose.

organisms to large globular colonies and extensive fil-
amentous growths. They are characterised by a fresh
green coloration due to the presence of chlorophylls-
a and - b , which are not obscured by accessory pig-
ments such as β-carotene and other carotenoids. In
cases where algae have excessive light exposure,
the carotenoid pigments may be photoprotective and
occur at high levels, obscuring the chlorophylls and
giving the alga a bright red colour. This is seen
with the lagellate Haematococcus (Fig. 4.54), which
frequently colours bird baths and other small pools
bright red, and with the snow alga Chlamydomonas
nivalis where areas of frozen landscape are similarly
pigmented.

1.4.2 Morphological diversity

Green algae are the most diverse group of algae,
with about 17,000 known species (Graham and
Wilcox, 2000). This diversity is reflected by the
variation in morphology, with organisms being
grouped into unicellular, colonial or filamentous
growth forms (Table 1.7). The level of greatest
morphological and reproductive complexity is rep-
resented by Charalean algae (e.g. Chara, Nitella ),
which can reach lengths of over a metre, have
whorls of branches at nodes along the length of
the thallus (Chapter 4, Plate 1) and have been fre-
quently confused with aquatic higher plants such as
Ceratophyllum .
In the past, this morphological diversity provided
the taxonomic basis for green algal classification
(Bold and Wyne, 1985) - with orders primarily being
defined largely on a structural basis. These included
the orders Volvocales (flagellate unicells and simple

1.4.1 Cytology

In addition to their characteristic pigmentation
and other biochemical features (Table 1.3), green