Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 28

Relatively few species occur in this group, which
is also characterised by the presence of two large
plates (valves) covering a major portion of the cell.

Dinokont cells, where the flagella emerge from
the middle of the cell (e.g. Ceratium , Peri-
dinium ). Dinokont cells have a distinct symmetry
(Fig. 1.8) with dorsoventral division into epitheca
and hypotheca. These are separated by an equa-
torial groove (cingulum), with a further small
groove - the sulcus, extending posteriorly within
the hypocone. The two are flagella inserted in the
ventral region of the cell. Beating of the flagella -
one of which is extended while the other is con-
tained in the equatorial groove - gives the cell a
distinctive rotatory swimming motion. The term
'dinoflagellate' is derived from the Greek word
dineo , which means 'to whirl'.

Dinoflagellates do not show the range of morphol-
ogy, from unicellular to colonial forms, seen in other
algal groups. There are some unusual non-flagellate
amoeboid, coccoid and filamentous forms, however,
which reveal their phylogenetic relationship to main-
stream dinoflagellates by the characteristic biflagel-
late structure of their reproductive cells - known as
dinospores (zoospores).

25 μ m

Figure 1.9 Morphology of Ceratium . Top: Diagram-
matic views of a matured cell (left) and cyst formation
(right). Bottom: Central region of a matured cell (SEM
preparation), showing details of equatorial groove and
thecal plates with pores. See also Figs. 2.7 (cyst), 2.16
(iodine-stained sample), 2.18 (cell death) and 4.56 (live
cells).

1.7.3 Ecology

In freshwater environments, dinoflagellates are typi-
cally large-celled organisms such as Ceratium , Peri-
dinium and Peridiniopsis . Their large size reflects the
high nuclear DNA levels (large amounts of geneti-
cally inactive chromatin) and correlates with a long
cell cycle and low rate of cell division. These features
are typical of organisms that are K-strategists, domi-
nating environments that contain high populations of
organisms living under intense competition (Sigee,
2004).
Dinoflagellates are meroplanktonic algae, present
in the surface waters of lakes and ponds at certain
times of year. The annual cycle is characterised by
two main phases.

1.7.2 Morphological diversity

These biflagellate unicellular organisms are of two
main types, depending on the point of insertion of the
flagella.

Desmokont dinoflagellates, where the two flag-
ella emerge at the cell apex (e.g. Prorocentrum ).