1. 20-3 Plantlike Protists: Unicellular Algae
Photo Credit: Andrew Syred/Science Photo Library/Photo Researchers, Inc.
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Plantlike Protists
Plantlike protists contain chlorophyll and carry out photosynthesis.
Plantlike protists are commonly called “algae.”
Algae are sometimes classified with the plants.
There are seven major phyla: three multicellular and four unicellular.
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Plantlike Protists
The four phyla of unicellular algae are:
Euglenophyta
Chrysophyta
Bacillariophyta
Pyrrophyta
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4. Chlorophyll and Accessory Pigments
Allow algae to harvest and use energy from sunlight.
Life in water poses a problem for photosynthesis.
Much of light’s energy is absorbed by water.
Seawater absorbs large amounts of red and violet wavelengths, so light becomes dimmer and bluer in deeper waters.
Chlorophyll a is most efficient at capturing red and violet light, so in deep waters it is ineffective for photosynthesis.
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5. Chlorophyll and Accessory Pigments
One trait used to classify algae is the type of photosynthetic pigments they contain.
Some algae have evolved different forms of chlorophyll—a, b, and c—that absorb different wavelengths of light to adapt to the condition of their watery environment.
Accessory pigments absorb light at different wavelengths than chlorophyll, giving algae a variety of colors.
Accessory pigments pass the energy they absorb to the photosynthetic machinery of the algae.
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6. Phylum Euglenophyta: Euglenophytes
Euglenophytes are plantlike protists that have two flagella but no cell wall.
They are closely related to the zooflagellates.
The phylum is named for the genus Euglena, which contains organisms found in ponds and lakes worldwide.
A typical euglena is about 50 micrometers long, and an excellent swimmer.
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Euglenophytes
Pellicle
Euglena
Contractile vacuole
Carbohydrate
storage bodies
Chloroplast
Nucleus
Eyespot
Euglenophytes are plantlike protists that have two flagella but no cell wall. The green structures inside the euglena shown are chloroplasts, which allow the organism to carry on photosynthesis. Like paramecia, euglenas expel excess water through a contractile vacuole.
Gullet
Flagella
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Euglenophytes
Two flagella emerge from a gullet in the cell.
The longer of the flagella spins so it pulls the organism rapidly through the water.
Euglenophytes are plantlike protists that have two flagella but no cell wall.
Gullet
Flagella
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Euglenophytes
Near the gullet is a reddish pigment known as the eyespot, which helps find sunlight to power photosynthesis.
Euglenas can also live as heterotrophs, absorbing nutrients from decayed organic matter.
Eyespot
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Euglenophytes
Euglenas store carbohydrates in small storage bodies.
Euglenas reproduce asexually by binary fission.
Carbohydrate
storage bodies
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Euglenophytes
Euglenas do not have cell walls. Instead, they have an intricate cell membrane called a pellicle.
The pellicle folds into ridges, each supported by microtubules.
It is tough and flexible, allowing euglenas to crawl through mud when there isn’t enough water.
Pellicle
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Chrysophytes
Chrysophytes
Members of the phylum Chrysophyta are a diverse group of plantlike protists including yellow-green algae and golden-brown algae.
Their chloroplasts contain bright yellow pigments that give the phylum its name.
Chrysophyta means “golden plants.”
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Chrysophytes
The cell walls of some chrysophytes contain the carbohydrate pectin rather than cellulose, and others contain both.
Chrysophytes store food in the form of oil rather than starch.
They reproduce both asexually and sexually.
Most are solitary, but some form threadlike colonies.
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Diatoms
Diatoms
Members of the phylum Bacillariophyta.
Diatoms produce thin, delicate cell walls rich in silicon (Si)—the main component of glass.
The walls are shaped like the two sides of a petri dish or flat pillbox, with one side fitted snugly into the other.
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Dinoflagellates
Dinoflagellates
Members of the phylum Pyrrophyta.
About half of the dinoflagellates are photosynthetic; the other half live as heterotrophs.
Dinoflagellates have two flagella that fit in grooves between two thick plates of cellulose that protect the cell.
Most dinoflagellates reproduce asexually by binary fission.
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Dinoflagellates
Many dinoflagellates are luminescent.
When they are agitated, they give off light.
Some areas of the ocean ate so filled with dinoflagellates that the movement of a boat’s hull will cause the dark water to shimmer with blue light.
This luminescence gives the phylum its name: Pyrrophyta, meaning “fire plants.”
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17. Ecology of Unicellular Algae
Plantlike protists are important to freshwater and marine ecosystems because they make up the base of the food chain in many aquatic ecosystems.
They make up a considerable portion of phytoplankton found in marine ecosystems.
Phytoplankton constitute the population of small, photosynthetic organisms found near the surface of the ocean.
Phytoplankton carry out half of Earth’s photosynthesis. In addition, they provide nourishment for many organisms.
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18. Ecology of Unicellular Algae
Algal Blooms
Many protists are vital in recycling sewage and other wastes.
When waste is excessive, algae may grow into enormous masses known as blooms, which deplete water of nutrients and the cells die in great numbers.
Their decomposition robs water of oxygen, killing fish and invertebrate life and disrupting the equilibrium of the aquatic ecosystem.
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Red Tides
Red Tide Great blooms of certain dinoflagellates have occurred in recent years on the east coast for unknown reasons. These blooms are known as “red tides.” These species produce a potentially dangerous toxin, and when filter-feeding shellfish eat the dinoflagellates they become filled with toxin. Eating shellfish infected by red tides can cause serious illness, paralysis, and even death in humans and fish.
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