Characteristics of Algae
Algae is a convenient term used to describe eukaryotic organisms that contain chloroplasts and use chlorophyll to capture and convert light energy into food. Unlike plants, algae do not have leaves, roots, flowers or seeds. Most algae live in water or are symbiotic organisms.
Classifying the large number of diverse species of algae (singular—alga) into discrete subgroups is difficult. All algae have a nucleus (eukaryotes) and photosynthesis occurs within chloroplasts. Some are unicellular, some are colonial, and some are multicellular. Some have cell walls, some do not. Some are mobile, some are not. In fact, the term algae is an overarching catch-all grouping, not the name of a specific phylum. The first algae groupings were defined by the famed Irish botanist William Henry Harvey (1811–1866). Harvey studied many species of marine algae and grouped them into four divisions based upon their color. He called his four divisions Red Algae, Brown Algae, Green Algae and Diatoms.
With the advent of high-powered microscopes and detailed chemical analysis the lines have blurred as to what types of algae belong in each division. Different reference books may include the same species of algae under different subgroups due to the use of different criteria during the classification process. One of the simplest methods of classifying algae is to distinguish unicellular from multicellular and then divide each group by color. Using this classification scheme creates three divisions of unicellular algae—green, fire and golden, and three divisions of multicellular algae—green, red and brown.
Euglenophyta are green, unicellular, freshwater algae that have flagella. The flagella allow euglena to move in response to light and other environmental stimuli. Many euglenas are strict photoautotrophs, meaning they use light energy, water, and nutrients from the water around them to produce the energy they need to survive. However, some euglenas are mixotrophs. They use photosynthesis when sunlight is available but are heterotrophs (consumers) when light is not present. One difference between euglena and other algae is that euglena lack a true cell wall; instead they have a protein covering called a pellicle around the cell membrane. Energy can be stored by euglena for later use. It is stored as a type of carbohydrate called paramylon.
Fire algae, or Pyrrophyta, contain the very important marine algae called dinoflagellates. Fire algae are unicellular, mostly marine algae, with flagella. The prefix pyrrho- comes from the Greek work pyrrhos, which means fire. Dinoflagellates contain two green pigments, chlorophylls a and c, plus reddish accessory pigments that provide the red color to this type of algae. Many species of fire algae also contain bioluminescent pigments that “glow” when the algae are disturbed. When large masses of marine algae are unsettled by ships at night, the ocean can seem to glow due to these bioluminescent algae. Dinoflagellates are also responsible for the phenomena known as red tide, which occurs when an overabundance of dinoflagellates causes the ocean to appear red. Toxins produced by the algae kill many fish during a red tide event. Fire algae have cells walls composed of cellulose, two flagella for mobility, and they store energy in the form of starch.
The final group of unicellular algae is the group that contains diatoms. Often golden in color, hence the term golden algae, or Chrysophyta, diatoms are very different than the other algae. Chryso- is derived from the Greek word for gold. Diatoms have a unique cell wall composed of silica. The cell wall is composed of two “halves” called tests that fit together. These silica tests contain many holes and resemble a pasta strainer. Diatoms also have a unique way of storing energy. Instead of producing a starch or carbohydrate, diatoms produce an oil-like liquid. The golden color in diatoms occurs within two elongated membrane-bound organelles called plastids. The tests of dead diatoms are used as pool filters and in many cosmetic products.
Green algae, or Chlorophyta, are thought to be the ancestors of green plants. Chlorophyta and plants are sometimes grouped together into a new kingdom called Viridiplantae. Chloro- is derived from the Greek word for green. Chlorophyta may be multicellular, colonial, or unicellular. The primary photosynthetic pigments are chlorophyll a and b. All green algae have cell walls made of cellulose and most have a flagellum or two at some stage in their lives for mobility. Energy captured during photosynthesis is stored as starch. Most Chlorophyta live in fresh water or in moist environments. Several species of green algae are symbiotic, including lichens found growing on many trees in the United States.
Red algae, or Rhodophyta, also use chlorophyll a for photosynthesis but rather than chlorophyll b they use chlorophyll d as their second photosynthetic pigment. Rhodo- is derived from the Greek word for rose. The accessory pigments that give red algae their color include orange carotenoids, yellow xanthophylls and red phycobilins. The cell walls of red algae are made of cellulose, agar and carrageenin. Carrageenin is used in the manufacture of food such as ice cream while agar is used as a thickener in desserts. Energy captured during photosynthesis is stored as a type of starch called Floridian starch. Unlike green algae, red algae do not have flagella. They do have a complex reproductive cycle called the alteration of generations. Seaweeds are a type of multicellular red algae. One type of seaweed, nori, is used to make sushi.
Brown algae, or Phaeophyta, are the final group of multicellular algae. Once again the division name is derived from the Greek word for its color as the prefix phaeo- comes from the Greek word for brown. Brown algae are typically found in the ocean. Beside chlorophyll a and chlorophyll c, brown algae also contain the brown pigment fucoxanthin. It is fucoxanthin that masks the green chlorophyll giving this group its characteristic brown color. Giant kelp are part of the brown algae group. These large, multicellular, plant-like algae can be up to 100 meters long. In order to achieve such enormous size, brown algae have evolved differentiated tissues that function in specific ways to ensure the survival of the algae. The holdfast is similar to a root in that it secures the algae to the bottom. The stipe provides support for the leaf-like blades. Blades increase the amount of surface area for photosynthesis. Like red algae, brown algae reproduce using an alteration of generations. The large kelp forest called the Sargasso Sea, which is located in the middle of the Atlantic Ocean, contains (or consists of) 14,000 square miles of floating brown algae.
The purpose of this laboratory is to explore the many diverse characteristics of algae. Both live and preserved specimens will be observed.
Methylene blue solution, 0.01%, 1 mL
Methyl cellulose solution, 1 mL
Diatomaceous earth, 0.1 g
Microscope slides, depression type
Pipets, graduated, 2
†Prepared slides (shared)
Create a concept map for each division of algae to highlight the similarities and differences between different groups.
Methylene blue will stain skin and clothing. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.
- Observe each prepared microscope slide using a compound microscope. Begin each observation using the low power objective before switching to higher power to observe the internal and external features of each alga. Sketch each type of alga on the Characteristics of Algae Worksheet. Label the following items in each sketch—nucleus, cytoplasm, chloroplasts and flagella, where appropriate. Record the magnification used.
- Observe each type of living specimen using a compound microscope. Place one drop of the appropriate algae culture in the well section of a clean depression slide. Note: Be careful to use the correct graduated pipet for each culture. Algae can be difficult to locate in a culture jar. It may take several attempts to locate specimens. The use of a stereoscope will make this easier. See specific notes for assistance in locating each type of algae.
- Euglenas are attracted to light and are usually green in color. Euglenas contain a brown spherical eyespot that is used to orient the cell toward light. The nucleus, chloroplasts, other organelles and flagella are clearly visible using a compound microscope. Locate euglena swimming throughout the culture. They are visible as green specks using a stereomicroscope. If necessary, add a drop of methyl cellulose solution to the depression well to slow the euglena down.
- Diatoms have a green or golden color. Each species of diatom has its own unique shape. Many species are likely living together within the culture. The golden plastids are easily visible in the living specimens but the tests may be difficult to distinguish from the rest of the cell. Diatoms can be found throughout the culture. Do not use methyl cellulose solution. After viewing the living diatoms, prepare a wet mount of diatomaceous earth. Each gram of diatomaceous earth is composed of millions of tests from long-dead diatoms. Very little material is necessary to make a preparation of this white powder.
- Volvox are large green colonial algae. They are visible without using a stereomicroscope as green balls throughout the culture. Volvox appear as hollow “Cush ball” colonies often with daughter colonies inside. There may be more than 500 biflagellate algae cells grouped together as one Volvox. Each individual cell within the colony contains its own chloroplast and nucleus. The collective beating of each cell’s flagella causes the entire colony to roll through the water. Eventually the mother colony will rupture, releasing numerous daughter colonies to the water. Do not use methyl cellulose solution.
- Spirogyra are found as long filaments of cylindrical cells with distinctive ribbon-like spiral chloroplasts. The chloroplasts lie within the cytoplasm between the inner cell wall and a very large vacuole. Toward the center of some cells a nearly transparent nucleus will be visible. Do not use methyl cellulose solution.
- Carefully observe each live algae and sketch the characteristics of the organisms. Label the following items on each sketch—nucleus, cytoplasm, chloroplasts and flagella (where appropriate).
- Add a drop of methyl cellulose solution, if needed (see specific notes), in order to make careful observations of the internal organelles and the flagella. Use a clean toothpick to stir the methyl cellulose into the drop of algae culture.
- Some organelles become more visible when stained. Carefully add one drop of methylene blue stain to the depression slide. Carefully mix the stain using a clean toothpick. Locate an organism and carefully observe the internal structures. Add any new features to the original sketches using a blue pencil.
- Rinse the depression slide with water and dry with lens paper between each use.