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 Dinoflagellates - These are very important algae in both freshwater and marine
ecosystems. They possess flagellar motility and are capable of active migration in the water
column. During times of stratification, they can exert diel changes of position, presumably in
association with light intensity or nutrients. Examples in freshwater include Peridinium and
Ceratium. In marine systems blooms of these form the famous 'red tides' that occur with
increasing frequency and intensity in recent times. The most common 'red tide'-forming genus is
Gymnodinium. In marine systems these are well-known for their ability to form powerful toxins
which when concentrated in shellfish can cause paralytic shellfish poisoning (PSP). There will be
more discussion of this later in this section. Freshwater species are seasonally abundant and are
an important component of the aquatic ecosystem. These are also capable of both
photosynthetic food production as well as ingestion of food particles although the mechanism of
ingestion does not involve highly organized structures as in Euglena.
Red algae - These have very complex life cycles, unique pigments and storage products,
and varied micro- and macroalgal growth forms. Red algae are never found in the plankton, and
are seldom, if ever, found in lakes and reservoirs. In freshwaters, the best-known of these is
Batrachospermum, found in freshwater streams in the spring. Marine systems have numerous
examples of red algae, some of which are used as a food source by humans. At this time these
are not among the HAB forming organisms of freshwaters.
Other groups - There are other unique but inconspicuous algal types in freshwaters. For
example a single species, Gonyostomum semen, is commonly found in stratified, productive
lakes and reservoirs. Although it may be common, perhaps dominant, in a microenvironment
near the thermocline, it is seldom collected and accounted for because of its fragility. Lacking a
cell wall and possessing trichocysts, these cells tend to be destroyed by normal collection and
preservation methods. Other unique algal species, like Gonyostomum, await further research to
account for their contribution to the aquatic ecosystem.
Cyanobacteria: the Blue -Green Algae. One group of algae that commonly
contribute to nuisance growths is technically considered not to be algae by many scientists. The
so-called "blue-green algae" are actually prokaryotic organisms whereas the species named and
described in the previous paragraphs are eukaryotic organisms. The distinctions between these
two types of organisms are many and they are important to the environmental effect they can
have on aquatic systems. As stated earlier the prokaryotic "blue-green algae" do not have
cellular organelles (membrane bound structures like the nucleus, chloroplast, or mitochondrion).
The "blue-green algae" actually have more features in common with bacteria, which also are
prokaryotic organisms. For this reason the "blue-green algae" are correctly referred to as
Cyanobacteria. This section will refer to these organisms as cyanobacteria although it is still
acceptable to refer to them as "blue-green algae" and there are still numerous references to that
terminology in the literature. The cyanobacteria include many taxa that have a variety of growth
forms. They can occur as single small cells or in aggregations of small single cells. They also
grow in filaments (or similar structures called trichomes) but they are usually microscopic. The
large aggregations of these forms can easily be seen without the aid of a microscope such as
1.2.43
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