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Fundamentals of Biology
Kate Bruffey
January 11, 2000
Green Algae
The group of organisms collectively referred to as
"Algae" were at one time included entirely in the kingdom of Plantae.
However, with the present-day kingdom classification, all the eukaryotic algae
(cells having distinct nuclei) are now in the kingdom Protista, with the
exception of Blue-green algae, Cyanobacteria and Prochlorophyta, which are in
the kingdom Monera with the other prokaryotic organisms (cells having no
distinct nuclei). Nevertheless, some phycologists still consider algae to
be plants because they contain chlorophyll and carry out photosynthesis.
Before describing the specifics of green algae (Chlorophyta) it is important
that one has a general understanding of all algae.
Algae are very important in the balance between food
producing and consuming organisms. Algae are autotrophs (food producing)
and provide food for countless species of water- dwelling animals. Algae
also make it possible for animals to exist on land. As algae carry out
photosynthesis, they release oxygen into the atmosphere. They are so
plentiful that they produce 90% of the world's atmospheric oxygen.
Algae vary in size and shape from microscopic hard-shelled
forms to rubbery kelps that grow as long as 230 ft. Most algal cells are
supported by an inner wall of cellulose. Layers of cells are held together
by a jelly like substance called pectin. Some algae are unicellular and
move with flagella; others are multicellular and are nonmotile.
Algae are common in freshwater lakes, streams, oceans, as
well as damp habitats such as damp rock faces, tree trunks, moss hammocks or
damp soil. A few even grow within the pores of rocks in deserts, relying
upon the night time dew for their source of moisture. Others grow on
melting snow or attach to the under surface of floating ice. They grow on
other plants, wood, turtles, water fleas, and even inside plants and animals.
Algae are classified into five groups according to the
pigments they contain. These five groups are; golden algae; fire algae;
green algae; brown algae and red algae. Regardless of their color all
algae contain a green pigment called chlorophyll. Most also contain a
second type of chlorophyll. Algae are also commonly classified by the form
in which they store food and by their means of reproduction.
The shapes of algae are also used in classification.
Even though many algae are only made up of one cell, they can have different
shapes, such as stars, needles, pyramids, cubes, round balls, eggs, long
threads, vases and worms. Colonies may be shaped as a hollow ball, a
diamond, a cube, a star and a flat plate. Multicellular algae may be
shaped like small brushes, palm trees, leaves, whips, tubes and flat ribbons.
There are three ways algae may form other plants like
themselves: 1) Asexually, 2) sexually where the parent plant releases gametes.
They join together and grow into new parent cells. A zygote is formed by
two gametes joining together. 3) Another types of sexual reproduction some
algae utilize, forms swimming cells called zoospores that move about on the
water. These grow into two types of short threads or filaments which
produce the gametes.One thread produces eggs and the other produces sperms.
These join together to the bottom of the ocean and become a small
leaf-shaped plant. In time, it grows into a large plant.
Now to the specifics of green algae. Chlorophyta,
the green algae, is one of the largest algal phyla and one of the most diverse,
from common pond scum to the bright green sea weeds. The 7000 species of
green algae range from microscopic single cells, long strings and filaments,
flat plants (the common sea lettuce) and even hollow tubes to some multicellular
organisms reaching 25 ft long. Green algae, unlike some other groups of
algae, contain the same three pigments found in land plants: Chlorophyll a,
Chlorophyll b and a type of carotene. Like many land plants, green
algae store food as starch. Some groups of green algae produce oil as well
as starch. The similarities between plants and green algae fossils have
led some evolutionists to suggest that plants evolved from green algae some 2
billion years ago.
Green algae may occur as single cells (either motile of
nonmotile), in colonies (more often nonmotile) and as multicellular filaments.
The unicellular forms assume an almost endless variety of shapes. Colonial
forms may be loose aggregates of single cells or may have these cells arranged
in a characteristic pattern. Some filamentous types bear a superficial
resemblance to higher plants. The motile unicellular organisms are free
swimming, moving by means of whip-like flagella (usually two in number).
Even the nonmotile, species may produce motile reproductive cells (zoospores).
Unicellular Group: Chlamydomonas
inhabit fresh water pools. They have two flagella which they lose in
reproduction. Chlamydomonas reproduce asexually which involves the
division into 2-8 daughter cells within the cell wall and membrane of the mother
cell. The flagella are released and they form 2 flagella on each of the
daughter cells just before they are released from the mother cell; Chlorella
is a small unicellular algae that is used predominately in studies of the
cellular processes and in the study of algae as a food source. They are
known for living inside animals, and for being the fastest multiplying green
algae that has been studied. Chlorella also contains vitamins, fats and starches
but it has not been made to taste good; Desmids are often mistaken as
diatoms but they are a plankton and a free-floating algae. They are
usually unicellular but sometimes are joined to form a filament-like colony.
They are often pinched in the middle so that they look like two cells that are
attached, but they are two symmetrical halves. The cell walls have unusual
patterns, which make desmids one of the most interesting freshwater algae; Protococcus
is a unicellular green algae but it may form into clumps. It is most
commonly found in damp forests, forming slippery film on rocks and green dust on
tree trunks. Protococcus reproduces asexually. The cell divides by
binary fission, which in doing so, produces two genetically identical daughter
cells.
Multicellular Groups: Oedogonium
reproduces both sexually and asexually. Sexually, the Oedogonium produces
an egg within another egg which is called oogonium. The antherida produces
a sperm which enters the antheridia and fertilizes the egg. It results in
a zygote, which forms a hard protective wall and can remain inactive for several
months. Before the zygote's wall breaks open meiosis occurs and four
flagellate zoospores are formed. Asexually, it forms a single cell,
multiflagellate zoospore within a cell; Spirogyra is a multicellular green
alga that grows in freshwater pools. Its cells form a slender
filament that look transparent. Each of the chloroplasts, within the
filament, contain a small protein body called a pyrenoid, which stores starch.
Spirogyra can reproduce asexually in two ways. The cells can go through
binary fusion which causes the filament to grow lengthwise. If the
filament is broken it grows on its own. This process helps disperse the
algae. The sexual reproduction of Spirogyra involves the process of
conjugation. Two filaments form connecting tubes and the content of one
cell flows into the other. The wall thickens around the zygote forming a
zygospore that can survive harsh conditions; Ulothrix is a filament
that can reproduce sexually and asexually. When it reproduces
sexually, it produces 8-64 isogametes inside a cell. Each of the gametes
have two flagella, which help them swim together and unite when released from
the mother cell. When the cells unite, they form a zygote which later
becomes a zygospore. Asexually, The Ulothrix reproduces by forming 4-8
zoospores in a cell. The zoospore contains 4 flagella which help them swim
away to form new colonies; Ulva is most commonly known as sea lettuce.
It has a life cycle that involves two distinct forms of the organism. The
two forms may look alike, but they are genetically different. One of the
forms is haploid (meaning they have [n] chromosomes). The haploid form of
the organism is called gametophyte because it produces gametes. When two
gametes fuse they form a diploid zygote. All of the cells that are
developed form that zygote are diploid. The resulting diploid form is called a
sporophyte because its cells undergo meiosis and therefore will produce spores.
Each haploid spore will develop into a haploid gametophyte. The alteration
between the sporophyte and the gametophyte stages in the life cycle is called
alteration of generations. All plants and many types of algae go through
this process of alteration of generations. This life cycle is widespread
because it has great survival value. The species benefit from the
recombination of parents' traits through the fusion of gametes and from the
opportunity to reproduce by the less risky process of forming spores. The
gametophytes and the sporophytes look identical in this specie, but in other
plants, the two forms may look very different.
Colonial Group: Volvox
organisms are made up of individual cells held together by strands of cytoplasm.
Colonial algae are different from multicellular organisms because their cells do
not have specialized functions. Cells in a colony can reproduce more
rapidly and readily than single cells because the mating cells are always
nearby. The size of the colony protects the members from the organisms
that feed on a single cell. Volvox is one of the most beautiful colonies.
The colony is a hollow ball formed by hundreds of thousands of bright green
cells. The whole colony spins slowly through the water by the synchronized
beating of the cells' flagella.
Since algae give off large accounts of oxygen and are a
source of food for marine animals and some land animals, extensive research has
been conducted in the suitability of green algae for providing oxygen and food
in the area of space exploration as well as its use in atomic submarines.
People could breathe the oxygen the plants give off. In turn the plants
could use the carbon dioxide the people exhale. The algae would combine
the carbon dioxide with the nitrogen gas to make their plant food.
Chlorella has been found to be over half protein and has all vitamins but
vitamin c as well as fats and starches, and can reproduce in 2 ½ hours to
double its weight. One strain of Chlorella would take only 3 to 5 cubic
feet per person in a spacecraft and provide enough oxygen to keep one person
alive and feed him the exact amount of food needed to live. Research has also
provided information regarding food manufacture, vitamin production, oxygen
yields and growth rates under various conditions. Algae have also been
found to eat human wastes.
In addition to space and submarine research, algae has
been used in laboratories to study poisons, to determine nutritional or food
requirements; to learn more about living processes and the causes of death.
It has also been important in biofiltration, the use of microscopic plants to
remove chemicals from polluted water. Iodine, calcium, and phosphorus are
chemicals that can be removed from polluted water by algae, which concentrate
them in or on our bodies.
Another area of focused research has been as a major food
supply for the starving people of the world. Crops of algae need less
space than any other crop. There is no waste. It reaches maturity in
a few hours, so the harvest is very short and can be year round as long as there
is sunlight for the algae to grow. The major drawback is that the
countries that need it most do not have the money necessary to purchase the
special equipment for algal farming. They also lack the scientists and
engineers to get the system under way. Countries like the United states
could do it very easily because the resources are available here. However,
it is not done because we do not need food. We are able to grow
conventional crops to supply not only our country but many other countries with
the food needed. Smaller countries such as Japan, China and Israel have
started producing algae on a small scale. When food becomes scarce, the
research and production of algae will intensify.
Although algae is important as a source of food and
oxygen, they can have negative effects, as when large populations produce an
unpleasant taste and odor in drinking water or clog filtration equipment.
In freshwater lakes and ponds polluted by nitrates and phosphates, algae
populations sometimes increase suddenly in an "algal bloom", forming a
dense, smelly scum and drastically decreasing the oxygen supply available to
other life forms. However, it is apparent the positives outweigh the
negatives.
Bibliography
"Algae". Microsoft Encarta 1996 Encyclopedia.
"Algae". Young Students Learning Library, Electric Library
Personal Edition. 1996.
Biology. Jovanovich Harcourt Brace Inc. 1986.
"Chlorophyta". Department of Botany, NMNH, Smithsonian Institute.
1997.
"Chlorophyta". The Columbia Encyclopedia, Fifth Edition. 1993.
Hickman, Michael. "Algae". The 1998 Canadian Encyclopedia.
Hills, Dr. Christopher , Nakamura, Dr. Hiroshi. Food from Sunlight Planetary
Survival for Hungry People. University of the Trees Press. 1978.
Kavaler, Lucy. The Wonders of Algae. The John Day Company. 1961.
Pinkston Jr., Williams. Biology for Christian Schools, Second
Edition. Bob Jones University Press. 1991.
Schlichting Jr., Harold E. , Schlichting, Mary Southworth. Algae.
Steck-Vaughn Company. 1971. |