Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 23

About one-third of the euglenoids are pho-
tosynthetic and classed amongst the algae. The
rest are colourless, being either heterotrophic or
phagotrophic, and are usually placed in the Protozoa.
In photosynthetic organisms, pigmentation is closely
similar to that in green algae (Table 1.3), but the
variable presence of carotenoid pigments means that
these organisms can routinely vary in colour from
fresh green (Fig. 4.51) to yellow-brown. In some sit-
uations, the accumulation of the carotenoid astaxan-
thin gives cells a bright red colouration. This is seen
particularly well in organisms such as Euglena san-
guinea , which forms localised blooms in ponds and
ditches.

several chloroplasts per cell, which vary in appear-
ance from discoid to star-, plate- or ribbon-shaped.
Cytological features that distinguish euglenoids
(Table 1.3) include the following:

The presence of an anterior flask-shaped depress-
ion or reservoir within which the flagella are
inserted. Although two flagella are present only
one emerges from the reservoir into the surround-
ing medium, the second being reduced and con-
tained entirely within the reservoir. An eyespot is
often present and is located close to the reservoir.

The production of paramylon as storage reserve.
This β-1,3-linked glucan does not stain blue-black
with iodine solution and is found in both green and
colourless forms.

1.5.1 Cytology

Euglenoids are typically elongate, spindle-shaped
organisms (Figs. 1.6, 4.51) and usually contain

The presence of a surface coat or pellicle that gives
the cell a striated appearance. This occurs just
below the plasmalemma and is composed of inter-
lockingproteinstripsthatwindhelicallyaroundthe
cell. In some the pellicle is flexible (allowing the
cell to change its shape), while in others the pellicle
is completely rigid giving a permanent outline to
the cell. Genera such as Trachelomonas have cells
surrounded by a lorica, the anterior end of which is
a narrow, flask-shaped opening through which the
flagellum emerges.

Eyespot

Pocket
(reservoir)

Contractile
vacuole

Paramylon

Reproduction is asexual by longitudinal division,
with sexual reproduction being completely unknown
in this group of organisms.

Plastid

Nucleus

1.5.2 Morphological diversity

Almost all euglenoids are unicellular, with colonial
morphology being restricted to just a few organ-
isms where cells are interconnected by mucilaginous
strands. One of these is Colacium , a stalked euglenoid
that is widespread on aquatic substrates - but is par-
ticularly commonly attached to zooplankton such as
Daphnia . In the sessile state, this organism does not
have emergent flagella on the cells - which occur
at the end of the branched mucilaginous attachment

Figure 1.6 Diagrammatic view of Euglena , showing
major cytological features. Graham & Wilcox, 2000.
Reproduced with permission from Prentice Hall. See also
Fig. 4.51 (live Euglena ).