Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 39

1

s

2

3

4

Figure 1.15 Diatom morphology:
(1) pennate diatom with raphe; (2)
pennatediatomwithpseudoraphe;(3)
sulcus (s) on Melosira ; (4) spines on
margin of Stephanodiscus ;(5)valve
shapes: (a) isopolar, (b) heteropolar,
(c) asymmetrical, (d) symmetrical.

b

a

5

c

d

In planktonic diatoms, simple filamentous (e.g. Aula-
coseira - Fig. 4.9) and radial (e.g. Asterionella -Figs.
1.16 and 4.42) colonies have evolved, but there is no
formation of spherical globular colonies that are a
key feature of other algal groups.

Colonialformsofdiatomsarisebymeansofsimple
linkages, which are of three main types.

Mucilage pads. These occur at the ends of cells
to join diatom cells in various ways. Asterionella

Table 1.13 Centric and Pennate Diatoms.

Characteristic Centric Diatoms Pennate Diatoms

Symmetry Radial Bilateral

Examples Stephanodiscus (Fig. 4.58)

Cyclotella (Fig. 4.62)

Pinnularia (Figs. 1.13 and 4.71)
Navicula (Fig. 4.73)

Gliding Motility Non-motile Some (raphid) diatoms are actively

motile.

Plastids Many discoid plastids Two large plate-like plastids

Sexual reproduction
(Hasle and Syvertsen,
1997)
Gamete formation Independent formation of gametes from

parent cells

Pairing of parental cells prior to gamete
production

Egg cells Oogamous - production of one or two

eggs per parent cell

Isogamous

Sperm cells 4-128 sperms per parent cell. Each with

a single flagellum bearing two rows
of mastigonemes.

Few amoeboid, non-flagellate sperm
cells.

Ecology Mainly planktonic, typical of open

water

Planktonic, epiphytic and benthic forms