Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 35

The cell wall of diatoms is very dense. These
organisms are only able to stay in suspension in
turbulent conditions, limiting the development of
extensive planktonic diatom populations to well
mixed unstratified waters.

with the higher resolution of the scanning electron
microscope and is important in species identification.
Diatomsoccurasnon-lagellatesinglecells,simple
colonies or chains of cells, and are widely distributed
in both marine and freshwater environments. Their
success in colonising and dominating a wide range
of aquatic habitats is matched by their genetic diver-
sity - with a worldwide total of 285 recorded genera,
encompassing 10,000-12,000 species (Round et al .,
1990; Norton et al ., 1996). Diatoms are also very
abundant in both planktonic and benthic freshwater
environments, where they form a major part of the
algal biomass, and are a major contributor to primary
productivity.

The formation of the diatom cell wall is dependent
onanadequatesupplyofsolublesilica(silicicacid)
in the surrounding water. The diatom spring bloom
of temperate lakes strips out large quantities of
silica from the water, reducing concentrations to a
level that becomes limiting.

Unlike other types of cell wall material, silica is
rigid and unable to expand. This means that daugh-
ter cells are unable to enlarge and progressive cell
divisions result in a gradual decrease in cell size.
Ultimately this decrease reaches a critical level,
at which point sexual reproduction is required to
completely shed the original cell wall and form
new, large daughter cells.

1.10.1 Cytology

Distinctive cytological features (Table 1.3) include:

Plastids with periplasmic endoplasmic reticulum
and the presence of girdle lamellae.

Chrysolaminarin and lipid food reserves, present
outside the plastid.

Cell wall structure

The frustule is composed of two distinct halves - the
epitheca and hypotheca - which fit together rather
like the lid and base of a pill-box (Fig. 1.12). At
cell division, two new walls are formed internally
at the cell equator 'back to back', and become the
hypothecae of the two daughter cells. The original
epitheca and hypotheca of the parent diatom become
theepithecaeofthetwodaughtercells,sotheepitheca
is always the oldest part of the frustule.
In both centric and pennate diatoms the epitheca
consists of two main parts - a circular disc (the epi-
valve) with a rim (the mantle) plus an attached band
or girdle (the epicingulum). The hypotheca corre-
spondinglyconsistsofahypovalve,hypovalvemantle
and hypocingulum. When the epitheca and hypotheca
fit together to form the complete frustule, the
hypocingulum fits inside the epicingulum to give an
overlap.
Because of frustule morphology (Fig. 1.12),
diatoms can be observed from either a girdle (side)
view or a valve (face) aspect - with girdle and valve
views of live diatoms being clearly distinguishable

A distinctive cell wall, the frustule, composed
of opaline silicon dioxide (silica) together with
organic coatings.

Silica cell wall

The presence of silica in the diatom cell wall can
be detected by cold digestion in a strongly oxidising
acid to remove organic matter (see Section 2.5.2).
If the cell wall resists this treatment it is probably
silica. The cell wall of diatoms differs from that
of other algae in being almost entirely inorganic in
composition. It has evolved as an energy-efficient
structure, requiring significantly less energy to man-
ufacture than the cellulose, protein and mucopeptide
cellwallsofotheralgae(FalkowskiandRaven,1997).
This gives diatoms a major ecological advantage at
times when photosynthesis is limited (e.g. early in
the seasonal cycle), but has a number of potential
disadvantages.