Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 13

eastern USA. Litaker et al . (2003) used unique
sequences in the internal transcribed spacer (ITS)
regionsITS1andITS2todevelopPCRassayscapable
of detecting Pfiesteria in natural river assemblages.
Thesehavebeensuccessfullyusedtodetectthepoten-
tially harmful organism in the St. Johns River system,
Florida (USA).

inclusions such as carboxysomes, cyanophycin gran-
ules, polyphosphate bodies and glycogen particles
(Fig. 1.4). Photosynthetic pigments are associated
with thylakoid membranes, which are typically dis-
persed throughout the peripheral protoplasm.
As with other prokaryotes, cell walls are made up
of a peptidoglycan matrix with an inner lipopolysac-
charide membrane and are generally quite thin.
Blue-green algae resemble Gram-negative bacteria
in having an additional outer cell membrane. Some
species also have a mucilage layer outside the cell
wall which may be dense or watery, structured or
unstructured. The outside layers of the cell wall
can sometimes become stained straw coloured or
brownish from iron and other compounds in the
surrounding water as in Scytonema and Gloeocapsa .
The fundamental bacterial nature of these organ-
isms distinguishes them from all other algae and
determines a whole range of features - including
molecular biology, physiology, cell size, cell struc-
ture and general morphology.

1.3 Blue-green algae

Blue-green algae (Cyanobacteria) are widely occur-
ring throughout freshwater environments, ranging in
size from unicellular forms such as Synechococcus
(barely visible under the light microscope; Figs. 2.16
and 4.31) to large colonial algae such as Microcystis
(Fig.4.34)and Anabaena (Fig.4.24a).Largecolonies
of the latter can be readily seen with the naked eye
and show a simple globular or filamentous form with
copious mucilage. The balance of photosynthetic pig-
ments present in blue-green algae (Table 1.3) varies
with light spectrum and intensity, resulting in a range
of colours from brown to blue-green (Fig. 4.24a).
Phycobilin pigments are particularly prominent, with
phycocyanin tending to predominate over phycoery-
thrinatlowlightlevels,givingthecellstheblue-green
colour typical of these algae. It is thus an advantage
to observe material from shaded as well as better
illuminated situations wherever possible.

Gas vacuoles

Insome species, gas vacuoles may be formed, appear-
ing under the light microscope as highly refractive or
quite dark structures. Gas vacuoles aid buoyancy in
planktonic species, allowing the cells to control their
position in the water column. Cells may then con-
gregate at a depth of optimal illumination, nutrient
concentration or other factor for that species. This is
not necessarily at the surface - where the light inten-
sity may be too great and cause photoinhibition or
permanent cell damage. Movement up and down in
the water column can enhance nutrient uptake as it
allows the cells to migrate to depths where essential
nutrients, for example phosphates, are more abun-
dant and then up towards the surface for light energy
absorption. When gas vacuole-containing species are
present in a sample of water they often tend to float to
the surface (this can cause problems when preparing
the sample for cell counts; see Chapter 2).
In addition to gas vacuole-mediated movements
of planktonic blue-greens within the water column,
other types of motility also occur - including gliding

1.3.1 Cytology

Prokaryote status

The prokaryote nature of these algae is indicated by
the small size of the cells (typically 10 μmindiame-

ter) and by the presence of central regions of nucleoid
DNA (not enclosed by a nuclear membrane). These
nucleoid regions (Fig. 1.4) can be observed both by
light microscopy (as pale central areas within living
cells) and by transmission electron microscopy (as
granularregionsinchemicallyixedcells)-wherethe
absence of a limiting membrane can be clearly seen.
Although these algae lack the cytological complexity
of eukaryotes (no membrane-bound organelles such
as mitochondria, plastids, microsomes and Golgi
bodies), they do have a range of simple granular

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