Marine Biology
Unit 2
MARINE ALGAE
Unit summary & Lecture notes
Summary
The most primitive of plants, algae are worldwide in distribution and are the major
aquatic plant life in both seawater and freshwater. The simplest forms are tiny and
unicellular, but some of the more complex forms, like kelp, can reach a length of more
than 100 feet. Algae form the very base of the entire oceanic food pyramid, and provide
the oxygen required by animals that live in the sea.
Unicellular Algae
I.
The unicellular algae show plant-like and animal-like characteristics.
Algae are eukaryotic, photosynthetic organisms. However, some are
known to travel around via their flagella and eat food particles like animals.
Because of these characteristics, unicellular algae are included in the
kingdom Protista.
a. Diatoms are unicellular organisms that live mostly as part of the
plankton. These microscopic organisms consist of cells
characteristically enclosed by cell walls made largely of silica (SiO2).
1. Diatoms are efficient photosynthetic factories, producing
much-needed food, as well as oxygen for other forms of life.
2. They are very important open water primary producers in
temperate and polar regions.
3. In fact, billions of diatom cells in the ocean account for a hefty
share of the food and oxygen produced on planet earth.
4. The glassy frustules (shells) or dead diatoms eventually settle
on the bottom of the sea floor. Here they form thick deposits
of siliceous material known as diatomaceous ooze that covers
the ocean floor.
b. Dinoflagellates make up another large group of planktonic, unicellular
organisms important in the economy of the oceans. Their most
outstanding characteristic is the possession of two unequal flagella.
1. They are mostly marine and are more common in the tropics.
2. Although most dinoflagellates have chlorophyll, many can also
ingest food particles.
3. Dinoflagellates are responsible for huge blooms called red
tides. These red tides can be devastating to marine life in the
area, including humans who may feed on infected shellfish.
4. Dinoflagellates are also noted for the production of light, a
phenomenon known as bioluminescence.
5. Zooxanthellae, the photosynthetic symbiotic algae that reside
in some invertebrate tissues, are actually dinoflagellates.
Unit 2 – Marine Algae
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Marine Biology
Multicellular Algae: The Seaweeds
I.
Seaweed, a generic term for algae that grow in the ocean, refers to the
macroscopic members of three algae divisions: Chlorophyta, Phaeophyta,
and Rhodophyta. Seaweeds are far more complex than unicellular algae,
although some forms may consist of single cells or simple filaments.
Seaweeds still lack the complex structures and reproductive mechanisms
characteristic of the higher, mostly terrestrial plants.
II.
Seaweeds show a wide range of growth forms and complexity of
structures. Being algae, seaweeds lack the true leaves, stems, and roots
of the higher plants. The complex body is known as a thallus. The thallus
can take many forms depending on the specie’s particular environment
and photosynthetic needs
III.
The general structure of seaweed is analogous to land plants, but being
algae, seaweed lack the true leaves, stems, and roots of the higher plants.
The complete body is known as the thallus, whether it is a filament, a thin
leafy sheet, a crusty cushion, or a giant kelp.
a. The blades are leaf-like flattened portions of the thallus. They increase
the surface area and are the main photosynthetic regions of the algae.
b. The pneumatocysts are gas-filled bladders attached to the base of the
blade to keep the blade nearer the surface. The mixture of gases in
the pneumatocysts is generally identical to the atmospheric
composition of gases (N2, O2, CO2, CO, H2).
c. The stipe, found in some seaweeds, provides support for the blades. It
is long and tough in the large kelps.
d. The holdfast is a structure that looks like roots. The holdfast anchors
the thallus to the bottom. Holdfasts are particularly well developed in
the kelps. They are not involved in any significant absorption of water
and nutrients as are true roots.
(See diagram of Macrocystis as an example of general structure)
IV.
Three types of seaweeds are recognized: the green, brown, and red
algae. Although their pigment composition is used in their classification,
their actual color may not mean much because it tends to vary a great
deal.
a. Division Chlorophyta: the green algae. Most are restricted to
freshwater and terrestrial environments. Only around 10% of the
estimated 7,000 species are marine. Some of these species are the
dominant plant form in environments with wide variations in salinity
such as bays, estuaries, and in isolated tide pools on rocky coasts.
Most green algae are unicellular or filamentous; many are microscopic.
Since their pigments (chlorophylls) are the same as those in the higher
plants, it is thought that land plants evolved directly from the green
algae. Common genera of Chlorophyta include: Ulva (sea lettuce),
Caulerpa, Codium (dead man’s fingers), and Halimeda.
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Marine Biology
b. Division Rhodophyta: the red algae. There are more species of marine
red algae than of green and brown algae combined. They are
characterized by having red pigments called phycobilins, which mask
chlorophyll. Red algae are found in most marine environments. Their
thallus ranges in complexity from filamentous to dense clumps to
species with large blades as long as 2 m. The coralline red algae are
important in several marine environments. They deposit calcium
carbonate around their cell walls. Warm-water coralline algae are
actively involved in the formation and development of coral reefs.
c. Division Phaeophyta: the brown algae. Almost all of the 1500 known
species of this olive green to dark brown algae are marine. Brown
algae are often the dominant primary producers on temperate and
polar rocky coasts. They include the most complex and the largest of
the seaweeds known as the kelps. Most kelps are found in great
abundance below the low tide level in temperate latitudes and in the
Arctic. In these environments, they are a most important element of
the marine life, providing food and shelter for many organisms. A few
of the more recognizable species of brown algae include:
1. Sargassum, the Sargasso weed, has spherical air bladders
that keep the small, leaf-like blades afloat at the sea surface.
Most species grow on rocks, but at least two float offshore in
huge masses. They give the Sargasso Sea, an area in the
Atlantic north of the West Indies, its name.
2. Nereocystis, the bullwhip kelp, consists of a long, whip like
stipe up to 30 m in length with a large spherical
pneumatocyst at the upper end.
3. Laminaria consists of a single large blade that may reach 3m
in length. This species has great economic importance in
that it is harvested for food in several parts of the world.
4. Macrocystis, the giant kelp, is the largest of the kelp. Its
massive holdfast, which is attached to a hard bottom, may
weigh several kilograms. Several long stipes grow from the
holdfast, from which elongate blades develop. At the base
of each blade a pneumatocyst eventually develops, which is
filled with gas, thereby helping keep the blades close to the
surface. Individual plants as long as 100 m have been
recorded. It has been estimated that such kelp plants can
grow 50 cm or more per day for short periods, which make
this species the fastest growing plant on the planet.
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Marine Biology
Stipe
Blade
Pneumatocyst
Holdfast
Macrocystis pyrifera
V.
In the colder waters of the north and south Atlantic and Pacific Oceans,
kelp beds, or forests are formed when many plants, each with many fastgrowing and intertwined stripes form dense and very productive regions.
Kelp beds are harvested like giant wheat fields for the extraction of several
natural products (discussed below). These regions have enormous
primary productivity, analogous to a terrestrial tropical rain forest in their
complexity. Kelp forests are the basis of recreation and fisheries on North
America’s west coast. Kelp forests are the primary habitat for many
species of fish and marine mammals such as the sea otter and California
sea lion.
VI.
Harvested seaweed is used in many ways. The most obvious is as a food
source. The farming, or mariculture, of seaweed is big business in China,
Japan, and Korea. Two important starch-like chemicals found in
seaweeds are alginates (from brown algae) and carrageenan (from red
algae). These compounds are used as stabilizers and emulsifiers in the
manufacture of dairy products such as ice cream, cheese, and toppings.
They are also used as a thickener and emulsifier in the manufacture of
various products, from shampoo, shaving cream and cosmetics, to
plastics and pesticides.
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