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(Bacillariophyta) are one of the most distinctive and successful
groups of unicellular algae, occurring throughout the world in
marine, brackish and fresh waters, as well as in damp subaerial
habitats. They are usually estimated to contribute about 25-35 % of
the world’s productivity (in terms of carbon fixation) and to be
represented by 50 – 200 thousand species depending upon one’s species
concept. Their most distinctive characteristic is the possession of
elaborate, siliceous cell walls, features of which are used to define
and classify species. Because silica resists degradation, diatoms are
regularly preserved in both freshwater and marine sediments, and in
conjunction with knowledge of their ecological specificity, such
records have been used to infer lake or ocean histories. In
particular, diatoms in lake sediments have been used to deduce
changes in pH and nutrient status, as well as climate change. They
are also used extensively to infer water quality in contemporary
Diagram of frustule, labelling valves and bands.
cell walls (frustules) are essentially bipartite structures, with an
older and a younger half (valve), each with a series of linking bands
(girdle bands or cingula), often likened to a petri dish or pill-box
1). Once formed, the
siliceous components are rigid and cell expansion is only possible by
the addition of extra girdle bands associated with the younger valve.
Valves vary in shape
as well as in the type and
arrangement of pores, slits and other processes of the silica wall,
and these features have formed the basis of their identification and
cells are often approximated to geometric shapes and described in
relation to their major axes and planes, radial, apical, transapical
and valvar (Fig.
2). Because they are
three-dimensional structures, shape will differ depending on the
aspect presented to the viewer, either valve or girdle view.
Occasionally cells are seen end-on, but this is unlikely in fixed or
cleaned material, since cells tend to lie with their largest face on
Fig. 2A Axes
of a centric diatom. a-a radial, b-b valvar
B Axes of a
pennate diatom. a-a apical, b-b valvar, c-c transapical.
may be divided into two major groups based on valve symmetry (Fig.
that are circular with essentially radial symmetry, the centric
diatoms, and those that are more elongate, with primarily bilateral
symmetry, the pennate diatoms. The latter group is further
sub-divided on the presence or absence of a paired slit system (the
raphe) in the valve, associated with motility, into the araphid
Raphid diatoms are subdivided into biraphid and monoraphid taxa. Monoraphid
diatoms have a
functional raphe system on one valve only, the R valve. The other valve
(P valve) has a solid
raphe sternum since the raphe slit has been filled in with silica
during morphogenesis. From the point of view of identification, these
are useful groupings, however, they do not necessarily reflect the
phylogenetic relationships of the diatoms; neither the centric,
araphid nor monoraphid diatoms are now considered monophyletic.
Fig. 3A A
centric diatom showing radial symmetry.
B A pennate
diatom showing bilateral symmetry.
of the raphe slits,
and the presence or absence of associated ribs or struts
are important features by which taxa can be identified and
classified. In some groups of diatoms, e.g. Nitzschia, Hantzschia,
the raphe slits are positioned along one
the valve face, often raised along a ridge (keel) and spanned
internally by struts (fibulae),
which may appear as refractive lines or dots.
In another group of diatoms, Surirellales, the fibulate raphe slits
are positioned such that one runs along each long margin of the valve
face, interrupted at both poles by “nodules” (e.g. Surirella
Ontogenetically, one corresponds to the central nodule (the origin of
the raphe system), the other to two fused polar nodules.
addition to being perforated by pores and / or slits, some diatoms
are ornamented with spines,
often around the margin of the valve at the junction of the valve
face and the valve mantle, or they may possess hollow processes
(strutted or labiate processes) associated with the production of
chitin fibres or mucilage fultoportula.
These features may facilitate the formation of colonies.
Specialised areas of simple pores (apical
pore fields) often allow the secretion of mucilage
pads or stalks
for attachment to a substratum or to other
cells in a colony. Some
species have the ability to live as motile individuals or to attach
to a substratum, depending on the circumstances.
Aspects of diatom life
Fig. 4 Diatom cell division. Diagram shows
in girdle view. The parent valves and girdle bands lie outside the
forming daughter valves. Each is enclosed in a silicalemma, inside
its respective protoplast, but lies close to its sibling and the
surface topography of one valve may be modified by that of its
cell division in diatoms requires the formation of one new valve for
each daughter cell, the new valve being deposited in a membrane-bound
vesicle within the daughter protoplast, released to the exterior on
sibling valves are usually formed in close juxtaposition to each
other, so that their surface topographies may interact, e.g. where
there is a depression on one sibling valve a raised area is formed on
the other (e.g. Cyclotella
Other species may link sibling valves by the formation of interlocking
spines, as in Fragilaria and related genera.
of the rigidity of the cell walls, new valves may be slightly smaller
than the parent valves and, over time, the average size of a diatom
population will decrease. This has a number of consequences. One of
these is that diatoms must be able to escape the inevitable size
diminution if they are to survive. This is usually achieved via
sexual reproduction in which gametes are formed and fuse outside the
confines of the parent frustules, forming a zygote that expands to
the species maximum, then laying down new silica valves to form new
vegetative cells. Another consequence of size reduction, particularly
for pennate diatoms, is that smaller cells may be mistaken for other
taxa because valve outline and proportions may differ from that of
larger cells (Fig.
5). This is due to
a greater reduction in valve length than width, sometimes accompanied
by a change in apical shape. Sometimes apices in smaller individuals
are simpler, but not for all species.
||Fig. 5 Size reduction
Examples of changes in valve outline and perceived
shape with size reduction.
b. Gomphonema gracile / Gomphonema parvulum
courtesy of Dawn Rose);
c. Dickiea subinflatoides / Dickiea
d. Navicula capitata.
protoplast is enclosed within the frustule, the interphase
configuration of nucleus, chloroplast (s) and other cytoplasmic
components being characteristic for species and genera (see live
usually golden-brown in colour, however, they can vary from very pale
to dark chestnut brown, and a few appear greenish, even when healthy.
Centric diatoms usually have several to many, simple, rounded
chloroplasts arranged around their cell periphery (e.g. Melosira
varians). Raphid pennate diatoms on the other hand, have
one, two or four (rarely more) chloroplasts per cell, varying in
shape and configuration within the cell according to genus and
family, usually with a conspicuous central cytoplasmic bridge in
which the nucleus is located (e.g. Navicula
lanceolata). Araphid pennate diatoms may have one or two
large chloroplasts per cell, like raphid diatoms, or many small
chloroplasts, like centric diatoms. Chloroplast number and
arrangement can be particularly informative for many naviculoid
genera. Most diatom chloroplasts contain pyrenoids,
more refractive proteinaceous bodies, which may be rounded or
angular, one to many per chloroplasts
visibility depends upon their size,
shape and position. The rounded pyrenoids of Gomphonema and Cymbella
are often easily seen in valve view at the
the chloroplast, the rod-shaped pyrenoids of Navicula are
rarely seen because they are only visible in girdle view. Colony
formation and mode
attachment are sometimes indicative of the presence of
particular structures, e.g. linking spines, if cells are linked valve
face to valve face, or pore fields if cells are linked by pads of
mucilage or attached to substrata by pads or stalks.
seeks to put the correct name to a specimen. It does not depend upon
any knowledge of the evolutionary or systematic relationships of the
species, and can use any feature that is helpful in discriminating
between specimens, regardless of its distribution across the group.
Therefore the taxa selected by choosing a particular character state
are not necessarily closely related. Correct identification in this
CD-ROM rests on the unique combination of character states of the
species in question.
diatoms are identified based on the morphology of the siliceous wall
(frustule) although protoplast features can also be used. For both,
however, it is first essential to establish which view
of the diatom is visible. Cells will normally lie in either valve or
girdle view, depending upon which has the larger and/or flatter
surface. Girdle views can usually be recognised by being more or less
rectangular, with the girdle bands being thinner in cross-section
than the valves. Some girdle views are curved, biconvex or
occasionally undulate, but all are usually “squared-off” in some
way. There are very few diatoms whose valve view has right angles (or
is approximately right-angled).
the SEM has revealed much about diatom ultrastructure, we have tried
to describe characters as they are seen in LM, to make the key
accessible to those who lack knowledge of diatom ultrastructure.
There has therefore been no attempt to describe all the structural
details of different taxa; more information on this can be obtained
from conventional publications (e.g., Round et al. 1990), which
also provide details of diatom life histories, etc.
purposes of this CD-ROM, elongate valve shapes are first considered
without reference to the apices, i.e. any protracted
apices are ignored.
Apex shape is considered independently. Similarly, shapes of axial
considered independently, the former extending as far as the central
raphe endings in raphid diatoms, the latter being opposite the gap
between the central raphe endings. In diatoms without a raphe system,
the axial area is along the apical axis and the central area is in
the middle of the valve.
presence, absence and position of features such as raphe
slits are important. It should be noted that some diatoms have
a raphe system on one valve only (monoraphid
diatoms), so the valves of a cell differ. We have designed
this key so that the raphid and araphid valves can be identified
independently. In other words it is not necessary to know that they
represent different parts of the same species to reach a correct