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The plant-like protists, or algae, are all photosynthetic autotrophs. These organisms form the base of many food chains. Other creatures depend on these protists either directly for food or indirectly for the oxygen they produce. Algae are responsible for over half of the oxygen produced by photosynthesizing organisms. Many forms of algae look like plants, but they differ in many ways. Algae do not have roots, stems, or leaves. They do not have the waxy cuticle plants have to prevent water loss. As a result, algae must live in areas where water is readily available. Algae do not have multicellular gametangia as the plants do. They contain chlorophyll, but also contain other photosynthetic pigments. These pigments give the algae characteristic colors and are used to classify algae into various phyla. Other characteristics used to classify algae are energy reserve storage and cell wall composition.

Members of the phylum Euglenophyta are known as euglenoids. These organisms are both autotrophic as well as heterotrophic. There are hundreds of species of euglenoids. Euglenoids are unicellular and share properties of both plants and animals. They are plant-like in that they contain chlorophyll and are capable of photosynthesis. They do not have a cell wall of cellulose, as do plants; instead, they have a pellicle made of protein. Euglenoids are like animals in that they are motile and responsive to outside stimuli. One particular species, Euglena, has a structure called an eyespot. This is an area of red pigments that is sensitive to light. An Euglena can respond to its environment by moving towards areas of bright light, where photosynthesis best occurs. In conditions where light is not available for photosynthesis, euglenoids can be heterotrophic and ingest their food. Euglenoids store their energy as paramylon, a type of polysaccharide.

Members of the phylum Bacillariophyta are called diatoms. Diatoms are unicellular organisms with silica shells. They are autotrophs and can live in marine or freshwater environments. They contain chlorophyll as well as pigments called carotenoids, which give them an orange-yellow color. Their shells resemble small boxes with lids. These shells are covered with grooves and pores, giving them a decorated appearance. Diatoms can be either radially or bilaterally symmetrical. Diatoms reproduce asexually in a very unique manner. The two halves of the shell separate, each producing a new shell that fits inside the original half. Each new generation, therefore, produces offspring that are smaller than the parent. As each generation gets smaller and smaller, a lower limit is reached, approximately one quarter the original size. At this point, the diatom produces gametes that fuse with gametes from other diatoms to produce zygotes. The zygotes develop into full sized diatoms that can begin asexual reproduction once more. When diatoms die, their shells fall to the bottom of the ocean and form deposits called diatomaceous earth. These deposits can be collected and used as abrasives, or used as an additive to give certain paints their sparkle. Diatoms store their energy as oils or carbohydrates.

The dinoflagellates are members of the phylum Dinoflagellata. These organisms are unicellular autotrophs. Their cell walls contain cellulose, creating thick, protective plates. These plates contain two grooves at right angles to each other, each groove containing one flagellum. When the two flagella beat together, they cause the organism to spin through the water. Most dinoflagellates are marine organisms, although some have been found in freshwater environments. Dinoflagellates contain chlorophyll as well as carotenoids and red pigments. They can be free-living, or live in symbiotic relationships with jellyfish or corals. Some of the free-living dinoflagellates are bioluminescent. Many dinoflagellates produce strong toxins. One species in particular, Gonyaulax catanella, produces a lethal nerve toxin. These organisms sometimes reproduce in huge amounts in the summertime, causing a red tide. There are so many of these organisms present during a red tide that the ocean actually appears red. When this occurs, the toxins that are released reach such high concentrations in the ocean that many fish are killed. Dinoflagellates store their energy as oils or polysaccharides.

The phylum Rhodophyta consists of the red algae. All of the 4,000 species in this phylum are multicellular (with the exception of a few unicellular species) and live in marine environments. Red algae are typically found in tropical waters and sometimes along the coasts in cooler areas. They live attached to rocks by a structure called a holdfast. Their cell walls contain thick polysaccharides. Some species incorporate calcium carbonate from the ocean into their cell walls as well. Red algae contain chlorophyll as well as phycobilins, red and blue pigments involved in photosynthesis. The red pigment is called phycoerythrin and the blue pigment is called phycocyanin. Phycobilins absorb the green, violet, and blue light waves that can penetrate deep water. These pigments allow the red algae to photosynthesize in deep water with little light available. Reproduction in these organisms is a complex alternation between sexual and asexual phases. Red algae store their energy as floridean starch.

The 1,500 species of brown algae are the members of the phylum Phaeophyta. The majority of the brown algae live in marine environments, on rocks in cool waters. They contain chlorophyll as well as a yellow-brown carotenoid called fucoxanthin. The largest of the brown algae are the kelp. The kelp use holdfasts to attach to rocks. The body of a kelp is called a thallus, which can grow as long as 180 ft (60 m). The thallus is composed of three sections, the holdfast, the stipe, and the blade. Some species of brown algae have an air bladder to keep the thallus floating at the surface of the water, where more light is available for photosynthesis. Brown algae store their energy as laminarin, a carbohydrate.

The phylum Chlorophyta is known as the green algae. This phylum is the most diverse of all the algae, with greater than 7,000 species. The green algae contain chlorophyll as their main pigment. Most live in fresh water, although some marine species exist. Their cell walls are composed of cellulose, which indicates the green algae may be the ancestors of modern plants. Green algae can be unicellular, colonial, or multicellular. An example of a unicellular green alga is Chlamydomonas. An example of a colonial algae is Volvox. A Volvox colony is a hollow sphere of thousands of individual cells. Each cell has a single flagellum that faces the exterior of the sphere. The individual cells beat their flagella in a coordinated fashion, allowing the colony to move. Daughter colonies form inside the sphere, growing until they reach a certain size and are released when the parent colony breaks open. Spirogyra and Ulva are both examples of multicellular green algae. Reproduction in the green algae can be both sexual and asexual. Green algae store their energy as starch.

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