Algae are typically autotrophic nonvascular plants. Most live in water, either fresh or salt, but some species occur in almost any habitat where photosynthetic plants can be found, and some even have the ability to survive in the dark. Fossil evidence suggests that algae occurred in great diversity in the oceans long before there was life on land. In keeping with ancient origins, a complete treatment of algae would cover at least half a dozen different phyla.
Bluegreen algae (cyanobacteria) have already been covered in laboratory 17. This lab concentrates on one alga phylum, the Chlorophyta, or green algae. It also deals with lichens, which are composed of a sac fungus and either a green or bluegreen alga, and which are widespread in land environments.
Green algae include a wide range of growth forms, reproductive patterns, and habitat preferences. Most green algae are freshwater plants but some species may be found on surface soil, bark, as part of lichens, on snow, and in salt water. Many species of green algae are unicellular, many are filamentous, some form colonies, and some are complex multicellular plants with specialized tissues and division of labor. Reproduction may be asexual (fission, zoospores, fragmentation) or sexual with isogametes or heterogametes. Isogametes are physically identical, whereas heterogametes occur in two types, for example sperm and eggs. Green algae may be motile with flagella or non-motile (without flagella). It is thought that the green algae gave rise to the first primitive land plants.
What these diverse green algae have in common is a food chemistry like that of most land plants. That means they have chlorophyll A and B as primary pigments, carotenes and xanthophylls as accessory pigments, and store food as “ordinary” starch.
Figure 20-1. Protococcus.
This alga is normally found on bark and stone that stays relatively damp.
Scrape a little of the green scum off the bark to make a wet mount. Examine the slide and look for green alga cells among the bark debris. Then add iodine solution so that more cell parts may be distinguished. Label Figure 20-1.
Figure 20-2. Chlorella.
This small round green alga often occurs as a contaminant in growth media. It has been proposed as a possible source of protein and oxygen for long manned space flights.
Look for the cup-shaped chloroplast in the cells on the prepared slide and label Figure 20-2. Make a wet mount from the culture that is provided and see if you can find structural details within these tiny cells.
These green algae are important freshwater phytoplankton. There is great morphological variation among species, but most are single-celled algae with a bilateral symmetry that makes it seem like they are always in the middle of cell division when they actually are not. They have a central nucleus and a single two-lobed chloroplast.
Figure 20-3. Assorted Desmids.
Observe the slide of mixed desmids. Note the many diverse shapes and forms. In Figure 20-3, label the parts of the cell on the right. Make a wet mount if material is available and look at the real thing.
B.Motile Unicellular Algae—Chlamydomonas
Figure 20-4. Chlamydomonas.
These single-celled algae have two flagella and swim vigorously. They may or may not have pyrenoids (crystals of accumulated starch). They also may or may not have an orange colored eyespot at the anterior end where the flagella are attached.
If fresh material is available, prepare a hanging drop mount by carefully combining a drop of each of two different mating types (+ and -). Individuals of the two strains will join and eventually merge to form a zygote. After observing the mating you may use an iodine solution to kill and stain the cells to reveal more detail. Label Figure 20-4. You should be able to distinguish the cell wall, chloroplast, eyespot, flagella, nucleus, pyrenoid, and even two basal bodies where the flagella attach.
Many kinds of filamentous algae contribute to what scientists actually refer to as pond scum. Aquarium surfaces are also good sites to find these ubiquitous organisms.
Sexual reproduction is by conjugation, in which entire cell contents migrate through conjugation tubes into cells of an adjacent filament. A persistent zygospore forms in each receiving cell.
Observe the spiral shaped chloroplasts and their pyrenoids in the fresh or preserved material and/or the prepared microscope slides. Look for the conjugation tubes and zygospores. Label Figure 20-5.
Figure 20-5. Spirogyra. a. Vegetative Cells. b. Sequence of Changes during Conjugation (Usually Occur More or Less Simultaneously).
Look for the two different specialized sexual cells in this green alga. Antheridia are notably smaller cells that occur in groups and produce swimming sperm cells. An oogonium is larger than a normal cell, and contains an egg that matures into a persistent oospore after fertilization.
Find both kinds of sexual structures and label them on Figure 20-6. Make a wet mount and look for the reproductive parts.
Figure 20-6. Oedogonium. a. Filament with antheridium (male reproductive structure). b. Filament with oogonium (female reproductive structure).
Figure 20-7. Hydrodictyon.
1.Hydrodictyon (water net)
These algae (on demonstration) are colonial and can become large enough to be seen with the unaided eye. What appear to be cells are actually segments of filament containing many nuclei and chloroplasts scattered through the cytoplasm.
Observe the details under a microscope and label Figure 20-7.
Figure 20-8. Volvox. a. Small Colony. b. Mature Colony with Daughter Colonies inside. c. Cellular Detail of Colony Surface.
These hollow spheres are composed of many individual cells. There may be some cells present that are specialized for sexual reproduction.
Look at fresh or preserved material and at the prepared slides. Small daughter colonies are likely to be present within some of the larger colonies. Label Figure 20-8.
3.Acetabularia (mermaid's looking glass)
These green algae occur in warm ocean shallows such as on coastal Florida and in the Caribbean. The alga consists of a huge single one-nucleus cell that eventually produces the isogametes that comprise the parasol-shaped cap.
Observe the specimen on demonstration.
E.Multicellular Green Algae
Ulva (sea lettuce)
These algae have a life cycle in which diploid plants produce almost identical haploid offspring that in turn produce diploid plants (alternation of generations). Ulva is edible and is sometimes used as a condiment.
Observe the preserved material.
These freshwater algae can be abundant in hard water lakes. They impart a bitter taste to the water. The plants are often encrusted with lime deposits, and are largely free of the microscopic flora and fauna that usually occur on submerged plants. Look at the demonstration plants and slides of the oogonium and antheridium.
Cladophora is locally abundant in the Lake States. In contrast to Chara, Cladophora may indicate nutrient pollution. Plants are filamentous and branched. Their life cycle is similar to that of Ulva. Observe the plants on demonstration.
In a lichen, the photosynthetic capacity of unicellular algae combines with the mineral and water holding thallus (body) formed by a fungus mycelium to produce an autotrophic plant that occupies the same kinds of habitat as mosses! In most lichens the alga is just one of a few species of green or bluegreen. The most common is Trebouxia, a one-celled green alga. The lichen fungus, in contrast, is likely to be unique to each lichen species, but is
Figure 20-9. Crustose Lichen. a. Growth Form. b. Cross Section.
almost always from the Ascomycota.
The lichen thallus typically has a dense upper or outer cortex of fungal hyphae overlying a more loosely aggregated layer rich in algal cells. The lower surface of the thallus usually takes one of three different forms, each associated with a particular lichen growth form:
In crustose lichens the basal layer consists of loose hyphae that adhere to the substrate. The result is such tight adhesion that the lichen looks like a coat of paint, and cannot be removed intact without taking a part of the substrate.
In foliose lichens the basal layer resembles the compact surface layer, and adhesion to the substrate results from localized patches of hyphae. Foliose lichens can usually be detached intact from their substrate.
Figure 20-10. Foliose Lichen. a. Thallus, with Apothecia. b. Cross Section of Thallus Only.
Figure 20-11. Fruticose Lichen. a. Growth Form. b. Cross Section.
n fruticose lichens the basal layer surrounds a hollow core, for the thallus itself is upright and branched. Fruticose lichens either look like miniature shrubs or form cascading strands similar to Spanish moss.
Lichens produce acids that may etch the substrate beneath.
If sexual structures are present, they are those of the fungus, usually either apothecia or perithecia. The fungal spores they release germinate to produce hyphae, which must serendipitously come in contact with the correct alga to produce a new lichen. Soredia are small pieces of lichen produced on the thallus. Since they containing both fungal hyphae and algal cells they can produce new lichens if they are blown or washed to a favorable site. Simple fragmentation of the thallus may also produce lichen reproduction.
Some lichens are used for dyes (orcein, litmus, and cudbear), some for medicines (salves or antibiotics), and some as the primary food for reindeer. Under starvation conditions, lichens have been used as human food. Lichens are good air pollution indicators by their absence, for they are vulnerable to acid rain and heavy metals. Lichens also act as soil builders and binders.
Look at the diverse display of lichens and verify that you can recognize members of each growth form:
These lichens are often easily overlooked on their rock, soil or wood substrates. Apothecia, if present, are likely to be the only features that stand above the surface. In Figure 20-9 note the upper layer of dense fungus hyphae, the algae layer (dark round cells), and the looser mycelium that adheres to the substrate.
Figure 20-12. Physcia. Cross Section of Apothecium and Thallus.
1.Physcia (star lichen)
This lichen is common on the bark of tree trunks and twigs. Unlike many of the other foliose lichens on display, Physcia has conspicuous apothecia on the upper surface. Soredia may or may not be present depending upon the species of Physcia. Notice the difference in texture and color between dry and wet lichens. Make a wet mount of a cross section of an apothecium and look for asci and spores.
Examine the prepared slide of a lichen thallus and apothecia. Label Figure 20-12. Note the upper and lower layers of dense fungus hyphae, the discrete attachment point, the alga layer (dark round cells), and the loose fungus hyphal layer. Label the thallus, apothecium, ascus, and ascospores.
2.Other Foliose Lichens
Rock tripe (edible), lung lichen, Parmelia, star lichen, and other foliose lichens are on display. Try to figure out how to distinguish between them.
See Figure 20-11 and note the outer and inner layers of dense fungus hyphae sandwiching the algae layer (dark round cells).
The genus Cladonia
Figure 20-13. Detail of Lichen Soredium.
The most common fruticose lichens are species of Cladonia, which are often found on soil and dead wood. British soldiers and reindeer moss are members of this genus. Note the red fruiting structures of British soldiers.
Examine other fruticose lichens on demonstration. Try to distinguish awl lichen, pixie cup, ladder lichen, old man’s beard and any others that catch your attention.
Soredia are small sand-like granules consisting of fungus hyphae surrounding a few alga cells. That means they have everything needed to propagate a lichen. Soredia form on the surface of the lichen, break off easily, and can be carried far by wind or water.
Look for soredia under the demonstration microscope. See Figure 12 13.
complex multicellular plant
division of labor
antheridium (pl. antheridia)
oogonium (pl. oogonia)
alternation of generations
soredium (pl. soredia)
Answer Sheet, Laboratory 20