Microbes and Plants - Freshwater Ecology: Concepts and Environmental Applications - page 134

Bacillariophycae, the Diatoms
The diatoms are extremely important
primary producers in lakes, streams, and
wetlands. They are often dominant in
plankton tows during the spring in
oligotrophic-mesotrophic lakes and in the
benthos of lakes, streams, and wetlands year
round.
The key defining characteristic of di-
atoms is the silicon opalescent-glass cell
wall called the frustule. This frustule has
two halves, and the halves fit together to
make an elongate, pennate (Fig. 8.6), or cir-
cular centric form. Centric forms are com-
mon in the plankton, and pennate forms
are common in the benthos. The frustules
may be attached to form chains or filaments
of many cells. The frustules are resistant to
dissolution, so they may remain in the sed-
iments for some time. This attribute makes
them a valuable tool in paleolimnology (the
study of the ecological history of lakes) and
in forensic medicine (Sidebar 8.3).
Diatoms are useful in paleolimnological
studies because they sink and accumulate in
the sediments and leave a record of the com-
munity structure of planktonic diatoms. If
conditions in the lake change, the diatom
community also changes. Isotopes can be
used to date sediments with depth to link
ecological change inferred from diatom frus-
tules to a temporal sequence. These tech-
niques have been used to show that acid pre-
cipitation is the result of industrialization and
other important aspects of the history of
lakes, such as fluctuation in salinity or trophic
state. Ruth Patrick, one of the leading envi-
ronmental researchers in the United States,
has made diatoms and their use in environ-
mental studies her specialty (Biography 8.1).

Sidebar 8.2.
Cyanobacterial Toxins

Cyanobacteria are among several groups of
toxic primary producers that can be found in
freshwater. Cyanobacteria produce at least
two general types of toxin, neurotoxins and
hepatotoxins. These toxins can be responsible
for a variety of problems, including illness of
humans who drink water containing the toxins,
death of dialysis patients dialyzed with water
containing the toxins, dermatitis from skin con-
tact, potential long-term liver damage from con-
taminated water supplies, and animal deaths
from drinking water containing cyanobacterial
blooms (Falconer, 1999; Codd et al., 1999a;
Chorus et al., 2000). Twenty-five genera contain-
ing 40 species of cyanobacteria have been con-
firmed to have members that produce toxins
(Codd, 1995; Carmichael, 1997). The neurotoxins
act very rapidly (also known as very rapid death
factors) and are responsible for the deaths of
domestic animals that drink from water contain-
ing high concentrations of them (Carmichael,
1994). The neurotoxins are lethal at very low con-
centrations; the notorious toxin dioxin is 10-60
times less toxic than the cyanobacterial aphan-
toxin (Kotak et al., 1993). The neurotoxins include
anatoxin-a, anatoxin-a(s), saxitoxin, and neosax-
itoxin (the first two are unique to cyanobacteria).
Some cyanobacterial genera containing species
that are known to produce neurotoxins include
Anabaena, Aphanizomenon, and Oscillatoria. It
is difficult to know if a species is producing a
toxin in a particular lake because different
strains of each species can produce different
amounts of toxins.
Hepatotoxins kill animals by damaging the
liver, including the associated pooling of blood.
These toxins are in a family of at least 53 related
small peptides. There is concern that these com-
pounds lead to increased rates of liver cancer
(Carmichael, 1994). The Canadian government
implemented a recommended water quality
guideline of 0.5 g liter 1 microcystin-LR (the
most common hepatotoxin) as a result of this
threat, and other countries will likely follow suit
(Fitzgerald et al., 1999; Codd et al., 1999b).
Genera with species known to produce hepa-
totoxins include Microcystis and Nodularia.
These genera pose a threat to drinking water
quality because they commonly form large
blooms in nutrient-rich drinking water reservoirs
during summer. In the treatment of algal blooms

Dinophyceae, the Dinoflagellates
The dinoflagellates are commonly
found in lakes and occasionally in streams.
They are unicellular and free swimming,
and they are subsequently found in the phy-
toplankton. Species can also be found in
wetlands and ponds. They can have cellu-
lose plates or armor covering their body
(Fig. 8.8). One flagellum encircles the cell,