Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 8

the ecological and taxonomic diversity of these
groups is given by the number of constituent species
(Table 1.2) for freshwater and terrestrial algae in the
British Isles (taken from John etal ., 2002), with green
algae and diatoms far outnumbering other groups -
reflecting their widespread occurrence and ability
to live in diverse habitats. Diatoms in particular
(over 1600 species) are ecologically successful,
both as planktonic and benthic organisms. In
addition to the above groups, John et al . (2002)
also list other phyla - Raphidophyta (2 species),
Haptophyta (5), Eustigmatophyta (3), Prasino-
phyta (13) and Glaucophyta (2). Although these
minor phyla have taxonomic and phylogenetic
interest, they have less impact in the freshwater
environment.
In terms of diversity, freshwater algae also have a
major division into prokaryotes (blue-green algae)
and eukaryotes (remaining groups) based on cell
size, ultrastructure, antibiotic resistance and gen-
eral physiology. Even within the eukaryote groups,
fundamental differences in phenotype and molec-
ular characteristics indicate evolutionary deriva-
tion from a range of ancestral types (polyphyletic
origins).

appear colourless. Facultative heterotrophs (photo-
organotrophs, mixotrophs) retain plastids and show
green pigmentation. Even within a 'normal' ecologi-
cal situation, the colour of a particular alga can show
considerable variation (see, for example, Anabaena ,
Fig. 4.24).
Apart from colour, the other obvious characteris-
tics viewed under the light microscope are overall
size, whether the organism is unicellular or colo-
nial and whether it is motile (actively moving) or
non-motile. Within different groups, algae may be
largely unicellular (euglenoids, dinoflagellates, cryp-
tophytes), multicellular (brown algae) or a mixture of
the two (other groups). Motility (single cells or entire
colonies) is also an important feature, with some
algal groups being entirely flagellate (dinoflagellates,
cryptophytes) while others are a mixture of flagellate
and non-flagellate organisms (green algae, xantho-
phytes). Other groups of algae are entirely without
flagella, but are able to move by buoyancy regula-
tion (blue-greens), gliding movements on substratum
(blue-greens, diatoms) or are entirely non-motile (red
and brown algae).

Resistant spores

While the above description of microscopical appear-
ance relates particularly to actively growing vegeta-
tive cells, it should be remembered that during winter
months the algae of temperate water bodies typically
occur as thick-walled resistant spores. Colour is often
obscured in these structures or entirely altered, mak-
ing taxonomic identification difficult. Examples of
resistant spores are illustrated for blue-green algae
(e.g. Anabaena akinetes - Figs. 2.7 and 4.24c), green
algae (e.g. Haematococcus cysts - Fig. 4.54) and
dinoflagellates (e.g. Ceratium cysts - Fig. 2.7).

1.2.1 Microscopical appearance

The colour of freshwater algae is an important aspect
of their classification (Table 1.2), and ranges from
blue-green (Cyanobacteria) to grass green (Chloro-
phyta), golden brown (Chrysophyta, Bacillario-
phyta), brown (Phaeophyta) and red (Rhodophyta).
Variations in colour are shown in Fig. 1.3 and in the
colour photographs of Chapter 4. The use of colour
as a taxonomic marker can be deceptive, however,
sincethenormalbalanceofpigmentsmayvary.Green
algae living on snow, for example, may have a pre-
ponderance of carotenoid pigments - forming a 'red
bloom' (Hoham and Duval, 2001).
The presence of chlorophylls and associated pig-
ments is also variable (Sigee, 2004). Obligate het-
erotrophs, entirely dependent on uptake of organic
moleculesbyorganotrophyorphagotrophy,mayhave
completely lost their chloroplasts (e.g. Fig. 1.11) and

1.2.2 Biochemistry and cell structure

Major biochemical features of freshwater algae
include pigmentation, food reserves and external cov-
ering (Table 1.3). Different groups have distinctive
combinations of chlorophylls and carotenes, while
only three groups (blue-greens, cryptomonads and
redalgae)havephycobilins.Allpigmentedalgaehave