Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 14

Oscillatoriales. Filamentous algae, lacking hetero-
cysts and akinetes. These relatively simple algae
occur as planktonic (some bloom forming) or ben-
thic aggregations. In some cases, they form dense
mats on mud or rocky substrata which secondarily
detach as metaphyton into the main body of water
(Fig. 2.1).

movementsofilamentousalgaesuchas Oscillatoria )
on solid substrata.

1.3.2 Morphological and taxonomic diversity

Blue-green algae are remarkable within the prokary-
ote kingdom for showing the range of size and
form noted earlier, with some organisms forming
quite large, complex, three-dimensional colonies.
Limited differentiation (Figs. 4.24a-c) can occur
within colonies, with the formation of heterocysts
(nitrogen-fixing cells) and akinetes (thick-walled
resistant cells). In filamentous forms the cells may
be the same width along the filament or in some
genera they may narrow or taper towards the end,
even forming a distinct hair-like structure in the case
of Gloeotrichia (Fig. 4.22). Clear branching occurs
in the most complex forms, with a fundamental
distinction between 'true branching' as in Stigonema
(Fig. 4.20) and 'false branching' as in Tolypothrix
(Fig. 4.21). True branching involves division of a
single cell within the filament, giving rise to two
or more daughter cells which themselves form
branches. In contrast, false branching involves
lateral extension of the filament without the single
cell division and daughter cell development as
above.
Freshwater blue-green algae can be divided into
four main groups (Table 1.5) in relation to general
morphology, presence/absence of specialised cells
and the nature of branching in filamentous forms.
These four groups form the basis for current taxon-
omy of this phylum, as adopted by John et al . (2002)
and Komarek et al . (2003a, b).

Nostocales. A diverse group of filamentous
algae, planktonic or benthic, with heterocysts and
akinetes but not showing true branching. Fila-
ments may be unbranched or show false branch-
ing. These algae are able to form large colonies by
lateral association of filaments (bundles), 3D tan-
gles or as radiating filaments from the centre of the
sphere.

Stigonematales. Filamentous algae, with hetero-
cysts and akinetes and showing true branching.
Structurally the most complex blue-green algae,
with some thalli (e.g. Fischerella ) differentiated
into multiseriate/uniseriate basal filaments and
uniseriate erect branches. Largely benthic algae,
with genera such as Stigonema commonly attached
to substrata in standing and flowing waters, detach-
ing to form planktonic masses.

The range of size and form noted in Table 1.5 indi-
cates a wide morphological diversity, which is useful
in taxonomic identification. The relative importance
of molecular versus morphological characteristics
in relation to taxonomy reflects the debate as to
whether these organisms should be treated as bac-
teria (cyanobacteria - Stanier et al ., 1978) or algae
(blue-green algae - Lewin, 1976). Although they
fundamentally resemble bacteria in their prokaryote
features, they also differ from bacteria in carrying out
photosynthesis coupled to O 2 evolution, and in the
complexity of their morphology. Although current
taxonomy is based primarily on phenotypic charac-
ters (see above), ultrastructural and molecular data
provide useful supplementary information (Komarek
et al ., 2003a, b). Current molecular analyses support
the separation of non-heterocystous and heterocys-
tous genera (Rudi et al ., 2000) and are particularly
relevant in the case of unicellular blue-greens,
where morphological features are not adequate. The

Chroococcales. The simplest blue-green algae,
occurring essentially as solitary cells (no filamen-
tous forms), typically enclosed by a thin layer of
mucilage. The cells may remain as single cells or
be aggregated into plate-like or globular colonies.
Typically planktonic with some colonial forms
(e.g. species of Microcystis ) forming massive
surface blooms containing individual colonies that
are recognisable with the naked eye. They typically
lack specialised cells, though one group (typified
by Chamaesiphon ) forms exospores.