Diatoms
Bacillariophyta comes from the Latin word bacillus that means "little
stick" or "rod" and the Greek word phyta that means "plant".
Bacillariophyta are commonly referred to as diatoms. The single cells,
colonies or filaments are microscopic and usually yellow to light brown
in colour. Most diatoms are autotrophic but a few are obligate
heterotrophs (they must absorb organic carbon) because they lack
chlorophyll altogether. Those with chloroplasts contain the
photosynthetic pigments chlorophyll a and c and fucoxanthin. The
storage products are chrysolaminarin and oil droplets, the latter aid in
buoyancy. Diatoms are easily recognised by their distinctive siliceous
cell walls (called frustules) which have the form of a petri-dish or box.
The frustule is usually sculptured with pores and striations, the pattern
being used to identify and classify the many different species. Most
diatoms are classified within two major morphological groups, the
centric and pennate diatoms.
Centric diatoms exhibit radial symmetry,
while pennate diatoms are bilaterally symmetrical about a longitudinal
axis. Centric diatoms are non-motile, while some pennate diatoms
possess a slit-like structure, called a raphe, along the surface of one
or both valves. Through the secretion of polysaccharides, the raphe
allows the cell to perform gliding movements when in contact with a
substrate. Except for male gametes, diatoms lack flagella. The
primary means of reproduction is asexual, by cell division. A wide
variety of diatoms are common in freshwater and marine habitats
where they live free-floating or attached to a substrate. Diatoms are
extremely important components of phytoplankton.
Diatomaceous earth (almost pure deposits of
diatom frustules) has a variety of uses, such as filtration and
insulation. Diatoms also have many industrial and commercial
applications in products such as foods, filters, paints, and cosmetics.
Centric diatoms are non-motile, while some pennate diatoms possess a slit-like structure, called a raphe, along the surface of one
or both valves. Through the secretion of polysaccharides, the raphe
allows the cell to perform gliding movements when in contact with a
substrate. Except for male gametes, diatoms lack flagella. The
primary means of reproduction is asexual, by cell division. A wide
variety of diatoms are common in freshwater and marine habitats
where they live free-floating or attached to a substrate. Diatoms are
extremely important components of phytoplankton. Besides being the largest contributors to global primary production and forming the base of aquatic food webs, they are used as powerful ecological tools to investigate past conditions (fossils) and monitor environmental changes over time.
Bacillariophyta comes from the Latin word bacillus that means "little
stick" or "rod" and the Greek word phyta that means "plant".
Bacillariophyta are commonly referred to as diatoms. The single cells,
colonies or filaments are microscopic and usually yellow to light brown
in colour. Most diatoms are autotrophic but a few are obligate
heterotrophs (they must absorb organic carbon) because they lack
chlorophyll altogether. Those with chloroplasts contain the
photosynthetic pigments chlorophyll a and c and fucoxanthin. The
storage products are chrysolaminarin and oil droplets, the latter aid in
buoyancy. Diatoms are easily recognised by their distinctive siliceous
cell walls (called frustules) which have the form of a petri-dish or box.
The frustule is usually sculptured with pores and striations, the pattern
being used to identify and classify the many different species. Most
diatoms are classified within two major morphological groups, the
centric and pennate diatoms. Centric diatoms exhibit radial symmetry,
while pennate diatoms are bilaterally symmetrical about a longitudinal
axis.
Characteristics: The cells of Aulacoseira are mostly longer than broad. These capsule-like cells are cylindrical in appearance; hence the valves are more commonly encountered in girdle view. Cells are round in valve view typical of the Centrales. In some there is a sulcus or ring-like incision around the mid-region, the girdle being smooth. The cell wall or frustule is punctate, coarsely or faintly so. These punctae or surface pits are arranged in straight or spiralled rows. Spines are usually present on the end walls. It is these elongate spines which join the cells together to form cylindrical filaments. Filaments may be straight, curved or coiled. The chloroplasts of this genus are numerous and plate-like in shape with a greenish golden-brown colour.
Dimensions: Cells are 4-20 μm in diameter.
Ecology: Free-floating (planktonic) or attached to rock and aquatic plants (benthic) in slightly eutrophic streams, as well as dams and large slowflowing rivers. Cells are attached and lie end to end in relatively long unbranched filaments.
Notes: Aulacoseira is abundant and forms dark brown strands on many surfaces in streams and large rivers. Species in this genus were previously referred to as Melosira Agardh, but many differences in valve ultra-structure between Melosira taxa led to the resurrection of the genus Aulacoseira. The commonly reported Melosira varians Agardh remains in that genus. The species are extremely variable and it is possible to find both long and narrow and short and broad celled filaments mixed together in the same sample. However, the sculpturing of fine punctae, resembling a nutmeg grater, and the terminal spines (one long and several short) are highly characteristic.
Problems: Blooms, filter clogging, taste and odour.
Characteristics: Valves are usually linear to linear-lanceolate, sometimes
tending towards elliptical. The cells have more or less capitate ends and may
be swollen in the centre. A gap (pseudoraphe) is evident between the
transverse markings. The cells contain two chloroplasts, often with three or
more pyrenoids.
Dimensions: Cells are 10-170 μm long and 2-5 μm wide.
Ecology: Fragilaria is an extremely diverse genus including species that
occur both as benthic forms in streams and lakes, as well as in the plankton
of lakes when they may be attached side to side to form chain or ribbon-like
colonies. Fragilaria species tend to be associated with circumneutral to
slightly alkaline fresh waters.
Notes: Fragilaria can be distinguished from Synedra Ehrenberg on the basis
of their capacity to form chains; in addition they are typically smaller and less
robust than Synedra. Fragilaria valves can be difficult to identify under the
light microscope as they are often oriented in girdle view.
Problems: Blooms, filter clogging, taste and odour.
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Characteristics: The long, narrow cells are joined by pads of mucilage at one end to form radiating star-shaped colonies of 4, 8 or 16 individuals. Individual cells are straight and narrow with rounded ends. The cells are somewhat expanded at each end (heteropolar), the inner ends at the "hub" of the colony are somewhat larger. The pseudoraphe is narrow and often discerned with difficulty, the transverse striae are very fine and difficult to see (not usually visible under the light microscope). Usually two or more chloroplasts are found per cell. Dimensions: Cells are 40-130 μm long and 1.3-6 μm wide. Colonies are 60-320 μm in diameter. Ecology: Asterionella is abundant in the plankton of lakes, especially those that are mesotrophic or eutrophic. The common species are usually found in hard water lakes. Populations can frequently reach bloom proportions. During population growth, silica is absorbed from the water and can become limiting causing numbers to decline.
Notes: There is also a form of Tabellaria Ehrenberg ex Kützing which has
stellate colonies similar in size and shape to Asterionella, but whose cells are
recognised by their longitudinal septa and by the knee-like swellings at the
centre of each cell.
Problems: Taste, odour and filter clogging.
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