Plantlike Protists

Plantlike protists are classified into seven phyla based on type of pigments, form of food storage, and cell-wall composition. The characteristics of the seven phyla are summarized in Table 25-2.

Phylum Chlorophyta (Green Algae)

The phylum Chlorophyta (klaw-RAHF-uh-tuh) contains more than 17,000 identified species of protists called green algae. Green algae have an amazing number of body forms, ranging from single cells and colonies to filamentous and multicellular forms.

Green algae share several characteristics with plants. Both green algae and plants have chlorophylls a and b. They both have carotenoids, which are accessory pigments that capture light energy and transfer it to chlorophyll a. Green algae and plants store food as starch and have cell walls made up of cellulose. The similarities between green algae and plants suggest that they may have had a common ancestor or that ancient green algae gave rise to land plants.

Most species of green algae are aquatic. Some species, such as the Protococcus shown in Figure 25-9, inhabit moist terrestrial environments, such as soil and tree trunks. Some green algae live as symbiotic partners with invertebrates, such as corals. Other green algae live with fungi as a part of organisms called lichens.

figure 25-9

Many green algae, such as Protococcus, inhabit moist environments on land, such as the shady, often north-facing sides of tree trunks.

figure 25-9

Many green algae, such as Protococcus, inhabit moist environments on land, such as the shady, often north-facing sides of tree trunks.

figure 25-10

Giant kelp (Macrocystis pyrifera) forms kelp gardens along the Pacific coast and provides habitat for a rich variety of life, including crustaceans, fishes, sharks, and sea otters.

figure 25-10

Giant kelp (Macrocystis pyrifera) forms kelp gardens along the Pacific coast and provides habitat for a rich variety of life, including crustaceans, fishes, sharks, and sea otters.

Word Roots and Origins stipe from the Latin stipes, meaning "log" or "trunk of a tree"

figure 25-11

Red algae, such as this Coral/ina species, are smaller than most brown algae, but members of both phyla are often referred to as seaweeds.

figure 25-11

Red algae, such as this Coral/ina species, are smaller than most brown algae, but members of both phyla are often referred to as seaweeds.

Phylum Phaeophyta (Brown Algae)

The phylum Phaeophyta (fee-AHF-uh-tuh) contains approximately 1,500 species of multicellular organisms called brown algae. Brown algae contain chlorophylls a and c and fucoxanthin (FYOO-koh-ZAN-thin), an accessory pigment that gives the algae their characteristic brown color. Brown algae store food as laminarin, a carbohydrate whose glucose units are linked differently than those in starch.

Brown algae are mostly marine organisms, and they include plantlike seaweeds and kelps. They are most common along rocky coasts where ocean water is cool. A few species of brown algae, such as Sargassum, can be found far offshore, where they form dense, floating mats.

Some of the largest algae known are classified in the phylum Phaeophyta. The large brown alga shown in Figure 25-10 is Macrocystis pyrifera, a giant kelp that thrives in intertidal zones and reaches 60 m in length. The thallus is anchored to the ocean bottom by a rootlike holdfast. The stemlike portion of the alga is called the stipe. And the leaflike region, modified to capture sunlight for photosynthesis, is the blade.

Phylum Rhodophyta (Red Algae)

The 4,000 species in the phylum Rhodophyta (roh-DAHF-uh-tuh) are known as red algae. Red algae contain chlorophyll a and accessory pigments called phycobilins. Phycobilins play an important role in absorbing light for photosynthesis. These pigments can absorb the wavelengths of light that penetrate deep into the water. As a result, phycobilins allow red algae to live at depths where algae lacking these pigments cannot survive. Some species of red algae have been found at depths of nearly 270 m, which is about three times deeper than organisms from any other algal phyla have been found.

A few species of red algae live in fresh water or on land, but most red algae are marine seaweeds. Despite their common name, not all red algae are reddish in appearance. The depth at which red algae live determines the amount of phycobilins they have. The Corallina alga shown in Figure 25-11 displays typical red algae pigmentation and body shape.

Phylum Bacillariophyta (Diatoms)

The phylum Bacillariophyta (BAS-uh-ler-ee-AHF-uh-tuh) contains as many as 100,000 species of unicellular protists called diatoms. Diatoms have cell walls, or shells, consisting of two pieces that fit together like a box and lid. Each half is called a valve, and the shells contain silicon dioxide. There are two basic types of diatoms. Centric diatoms have circular or triangular shells and are most abundant in marine environments. Pennate diatoms have rectangular shells and are most abundant in freshwater environments. Diatoms are an abundant component of phytoplankton and important producers in freshwater and marine food webs. In addition, diatoms release atmospheric oxygen.

Phylum Dinoflagellata (Dinoflagellates)

More than 2,000 species of organisms called dinoflagellates make up the phylum Dinoflagellata (DIE-noh-FLAJ-uh-LAH-tuh). Dinoflagellates, such as those shown in Figure 25-12, are small, usually unicellular organisms bearing two flagella of unequal length. The flagella are oriented perpendicular to each other. Dinoflagellates have cell walls made of cellulose plates that look like armor when seen under a microscope.

Most dinoflagellates are photosynthetic autotrophs, but a few species are colorless and heterotrophic. Photosynthetic dinofla-gellates usually have a yellowish green to brown color due to large amounts of carotenoids, as well as chlorophylls a and c.

Some species of dinoflagellates, such as those in genus Noctiluca, can produce bioluminescence, the production of light by means of a chemical reaction in an organism. Other species produce toxins and red pigments. When the populations of these species explode, they turn the water brownish red, resulting in a phenomenon known as red tide. Red tides are fairly common in the Gulf of Mexico. Red tide toxins can kill large numbers of fish. When shellfish, such as oysters, feed on red tide dinoflagellates, they also consume the toxins, which are dangerous to humans who eat the shellfish.

Phylum Chr ysophyta (Golden Algae)

The phylum Chrysophyta (kruh-SAHF-uh-tuh) contains about 1,000 species of golden algae. Most golden algae live in fresh water, but a few species are found in marine environments. Golden algae cells form highly resistant cysts that enable them to survive beneath the frozen surfaces of lakes during winter and dry lake beds during summer. Two flagella of unequal length are located at one end of each cell.

Most golden algae appear yellow or brown because of the presence of carotenoids. Golden algae also have chlorophylls a and c and store much of their surplus energy as oil. Golden algae likely played a role in the formation of petroleum deposits.

Phylum Euglenophyta (Euglenoids)

The phylum Euglenophyta (YOO-gluh-NAHF-uh-tuh) contains about 1,000 species of flagellated unicellular algae called euglenoids. Euglenoids are both plantlike and animal-like. Many are autotrophic, like plants, but they lack a cell wall and are highly motile, like animals. Euglenoids contain chlorophylls a and b and carotenoids. Most euglenoids live in fresh water, but a few occupy moist environments, such as soil or the digestive tracts of certain animals.

Euglena, shown in Figure 25-13, are abundant in fresh water, especially water polluted by excess nutrients. Euglena lack a cell wall, so they are flexible and can change their shape. Euglena have an elastic, transparent pellicle made of protein just beneath their cell membrane. Eyespots help Euglena sense their environment. Euglena also have a contractile vacuole that expels excess water. Although usually photosynthetic, Euglena raised in the dark do not form chloroplasts and become heterotrophs.

figure 25-12

Flagella spin dinoflagellates, such as these members of genus Ceratium, through the water like tops. Only one of the two flagella is clearly visible in this picture. The second flagellum lies in the girdle at the middle of the organism. (450x)

figure 25-12

Flagella spin dinoflagellates, such as these members of genus Ceratium, through the water like tops. Only one of the two flagella is clearly visible in this picture. The second flagellum lies in the girdle at the middle of the organism. (450x)

figure 25-13

Members of the genus Euglena contain chloroplasts like a plant but move around like an animal. Euglena are common in ponds. They are propelled by a long flagellum. An eyespot guides them toward light.

Long flagellum r

Short flagellum-

Contractile -vacuole

Nucleus-

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