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Algae
The word
algae represents a large group of different organisms from different
phylogenetic groups, representing many taxonomic divisions. In general
algae can be referred to as plant-like organisms that are usually
photosynthetic and aguatic, but do not have true roots, stems, leaves,
vascular tissue and have simple reproductive structures. They are
distributed worldwide in the sea, in freshwater and in moist situations on
land. Most are microscopic, but some are quite large, e.g. some marine
seaweeds that can exceed 50 m in length.
The algae have chlorophyll and can manufacture their own food through the
process of photosynthesis. Recently they are classified in the kingdom of
protiste, which comprise a variety of unicellular and some simple
multinuclear and multicellular eukaryotic organisms that have cells with a
membrane-bound nucleus.
Almost all the algae are eukaryotes and conduct photosynthesis within
membrane bound structure called chloroplasts, which contain DNA. The
exact nature of the chloroplasts is
different among the different lines of algae.
Cyanobacteria are organisms traditionally included among the algae, but
they have a prokaryotic cell structure typical of bacteria and conduct
photosynthesis directly within the cytoplasm, rather than in specialized
organelles.
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Types of algae
The main phylogenetic groups of algae are
[1], [2]:
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Diatoms:
unicellular organisms of the kingdom
protista, characterized by a silica shell of often intricate and
beautiful sculpturing. Most diatoms exist singly, although some join to
form colonies. They are usually yellowish or brownish, and are found in
fresh- and saltwater, in moist soil, and on the moist surface of plants.
Fresh-water and marine diatoms
appear in greatest abundance early in the year as part of the phenomenon
known as the spring bloom, which occurs as a result of the
availablity of both light and (winter-regenerated) nutrients.
They reproduce asexually by cell division. When
aguatic diatoms die they drop to the bottom, and the shells, not being
subject to decay, collect in the ooze and eventually form the material
known as diatomaceous earth. Diatoms can occur in a more compact form as
a soft, chalky, lightweight rock, called diatomite. Diatomite is
used as an insulating material against both heat and sound, in making
dynamite and other explosives, and for
filters, abrasives, and similar
products. Diatoms have deposited most of the earth’s limestone, and much
petroleum is of diatom origin. The
surface mud of a pond, ditch, or lagoon will almost always yield some
diatoms.
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Chlorophyta:
division of the kingdom of protista consisting of the photosyntetic
organism commonly known as green algae.
The various species can be unicellular, multi-cellular,
coenocytic (having more than one nucleus in a cell), or colonial.
Chlorophyta are largely aguatic or marine, a few types are terrestrial,
occurring on moist soil, on the trunks of trees, on moist rocks and in
snow banks. Various species are highly specialized.
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Euglenophyta:
small phylum of the kingdom protista,
consisting of mostly unicellular aguatic algae. Some euglenoids contain
chloroplasts with the photosynthetic pigments; others are heterotrophic
and can ingest or absorb their food. Reproduction occurs by longitudinal
cell division. Most live in freshwater. The most characteristic genus is
Euglena, common in ponds and pools, especially when the water has
been polluted by runoff from fields or lawns on which fertilizers have
been used. There are approximately 1000 species of euglenoids.
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Dinoflagellata:
large group of flagellate protistis. Some
species are heterotrophic, but many are photosynthetic organisms
containing chlorophyll. Various other pigments may mask the green
of these chlorophylls. Other species are
endosymbionts of marine animals and protozoa, and play an important part
in the biology of coral reefs. Other dinoflagellates are colorless
predators on other protozoa, and a few forms are parasitic.
Reproduction for most dinoflagellates is asexual,
through simple division of cells following mitosis. The dinoflagellates
are important constituents of plankton, and as such are primary food
sources in warmer oceans. Many forms are phosphorescent; they are
largely responsible for the phosphorescence visible at night in tropical
seas. There are approximately 2000 species of dinoflagellates.
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Chrysophyta:
large group of eukariotyes algae commonly called golden algae,
found mostly in freshwater. Originally they were taken to include all
such forms except the diatoms and multicellular brown algae, but since
then they have been divided into several different groups based on
pigmentation and cell structure. In many
chrysophytes the cell walls are composed of cellulose with large
quantities of silica. Formerly classified as plants, they contain the
photosynthetic pigments chlorophyll a and c. Under some
circumstances they will reproduce sexually, but the usual form of
reproduction is cell division.
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Phaeophyta:
phylum of the kingdom protista consisting
of those organisms commonly called brown algae. Many of the
world's familiar seaweeds are members of phaeophyta. Like the
chrysophytes brown algae derive their color from the presence, in the
cell chloroplasts, of several brownish carotenoid pigments, as
fucoxathin. With only a few exceptions, brown algae are marine, growing
in the colder oceans of the world, many in the tidal zone, where they
are subjected to great stress from wave action; others grow in deep
water. There are approximately 1500 species of
phaeophyta.
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Rhodophyta: phylum of
the kingdom protista consisting of the photosynthetic organisms commonly
known as red algae. Members of the division have a characteristic
clear red or purplish color imparted by accessory pigments called
phycobilins. The red algae are multicellular and are characterized by a
great deal of branching, but without differentiation into complex
tissues. Most of the world's seaweeds belong to this group. Although red
algae are found in all oceans, they are most common in warm-temperate
and tropical climates, where they may occur at greater depths than any
other photosynthetic organisms.
Most of the coralline algae, which secrete
calcium carbonate and play a
major role in building reefs, belong here. Red algae are a traditional
part of oriental cuisine. There are 4000
known marine species of red algae; a few species occur in freshwater.
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Cyanobacteria:
phylum of prokaryotic aguatic bacteria that
obtain their energy through photosynthesis. They are often referred to as
blue-green algae, even though it is now known that they are not
related to any of the other algal groups, which are all eukaryotes.
Cyanobacteria may be single-celled or colonial. Depending upon the species
and environmental conditions, colonies may form filaments, sheets or even
hollow balls. Some filamentous colonies show the ability to differentiate
into three different cell types.
Despite their name, different species can be red, brown,
or yellow; blooms (dense masses on the surface of a body of water) of a
red species are said to have given the Red Sea its name.
There are two main sorts of pigmentation.
Most cyanobacteria contain chlorophyll a, together with various
proteins called phycobilins, which give the cells a typical blue-green to
grayish-brown colour. A few genera, however, lack phycobilins and have
chlorophyll b as well as a, giving them a bright green
colour.
Unlike bacteria, which are heterotrophic decomposers of the wastes and
bodies of other organisms, cyanobacteria contain the green pigment
chlorophyll (as well as other pigments), which traps the energy of
sunlight and enables these organisms to carry on photosynthesis.
Cyanobacteria are thus autotrophic producers of their own food from simple
raw materials. Nitrogen-fixing cyanobacteria need only
nitrogen and
carbon
dioxide to live: they are able to fix nitrogen gas, which cannot be
absorbed by plants, into ammonia (NH3),
nitrites (NO2)
or nitrates (NO3), which can be absorbed by plants and
converted to protein and nucleic acids.
Cyanobacteria are found in almost every conceivable habitat, from oceans
to fresh water to bare rock to soil. Cyanobacteria produce the compounds
responsible for earthy odors we detect in soil and some bodies of
water. The greenish slime on the side of your damp flowerpot, the wall of
your house or the trunk of that big tree is more likely to be
cyanobacteria than anything else. Cyanobacteria have even been found on
the fur of polar bears, to which they impart a greenish tinge. In short,
Cyanobacteria have no one habitat because you can find them almost
anywhere in the world.
Related topics
For more books and
reading information see our website:
Eutrophication books overview
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