3.6.4. Ecological role

Some lichens are the food for animals, such as reindeer moss. “Reindeer moss”, actually a lichen, is a principal food source for large herbivores – namely, reindeer and caribou of the treeless arctic plains.

The algae found in lichens also occur as free-living species. The lichen fungi are generally found in nature only in the lichens. For these reasons lichens are generally classified according to the fungus species with which they are associated.

There are about 17,000 species of lichens, that is, of lichen fungi. Most are ascomycetes or Fungi Imperfecti, although a few are basidiomycetes. Algae from 30 different genera are found in symbiotic combination with these fungi.

3.6.5. Reproduction

Lichens reproduce most commonly by the breaking off of fragments containing both fungal hyphae and algae. New lichens are formed by the capture of an alga by a fungus. Sometimes the alga is destroyed by the fungus, in which case the fungus also dies. If it survives, a lichen is produced.

Summary and test questions

Algae are autotrophic members of the kingdom Protista. They produce most of the world’s atmospheric oxygen and provide the basis for much of its food supply. Most algae are aquatic. Asexual reproduction occurs by binary fission, fragmentation or the production of spores. Many species also reproduce sexually.

Almost all golden algae and fire algae are unicellular organisms. Diatoms are tiny golden algae with silica shells. Dinoflagellates, the most common fire algae, move by 2 flagella. Dinoflagellates show bioluminescence when disturbed and cause dangerous algal blooms called red tides.

Most green, brown, and red algae are multicellular organisms. Most brown algae live primarily in colder oceans. They include many seaweeds and kelps. Red algae grow primarily in tropical oceans. They grow at greater depths than other algae.

In the open air, plants are in danger of losing moisture because of evaporation. A protective outer coating called a cuticle is an adaptation that helps prevent evaporation. The cuticle also protects the plant from the relatively wide and abrupt temperature changes encountered on land.

Unfortunately, the cuticle also prevents the exchange of oxygen and carbon dioxide with the air. Small pores evolved on green parts of the plant that are not covered with cuticle. These pores, called stomata, allow the necessary & exchange of gases. In some plants, guard cells regulate the opening and closing of stomata in response to various environmental conditions.

Surrounding water supports algal cells. Plants form a complex carbohydrate called lignin. Lignin, combined with cellulose, forms an extremely tough material that supports soft plant tissues. By keeping plants exposed to the direct sunlight available on land, this support allows land plants to maximize opportunities for photosynthesis.

For aquatic plants, surrounding water allows flagellated sperm to swim to egg cells. In order to reproduce sexually, some land plants still need water for sperm to swim in. Other plants evolved structures that produce sperm that can travel through the air, in the wind or on the bodies of insects or animals. Many plants developed multicellular reproductive structures to protect the developing zygote and keep it from drying out.

About 400 million years ago, green algae-like organisms evolved those structures necessary to survive on land-cuticles, stomata, lignified cells, and multicellular reproductive organs. Plants then colonized the land and exploited a habitat where they had no competitors.

Modern land plants share certain characteristics with algae. For example, the life cycle of plants resembles that of some algae. Like most kinds of algae, the cell wall of plants is made primarily of cellulose. Land plants and some groups of algae store food as starch. Only green algae contain the two types of chlorophyll, a and b, found in modern land plants. Because of these chemical similarities, scientists claim that modern land plants evolved from forms of the green algae, Chlorophyta.

Lichens are combinations of fungi and algae which are morphologically and physiologically different from either organism as it exists separately. They are able to survive under adverse environmental conditions where neither partner could exist independently. The lichen represents a symbiotic relationship (mutualism) in which the fungus encloses the algal cells that give nourishment.

Test questions - 1

1. What are viruses?

2. What is lythic cycle?

3. What do you know about lysogenic cycle?

4. Describe transduction of viruses.

5. What kinds of virus infections do you know?

Test questions - 2

1. What are bacteria? Main groups.

2. Reproduction of bacteria.

3. What is botulism?

4. Photosynthetic bacteria.

5. Helpful and harmful bacteria.

6. How do bacteria look like?

7. Why do bacteria need in order to reproduce?

8. How did scientists find out that Archaebacteria are different from other kinds of bacteria?

9. What are three of special characteristics of Archaebacteria?

10. How are bacteria helpful for you?

11. What are some of the ways bacteria are harmful?

12. What can bacteria do to lakes, wastes and foods?

Test questions - 3

1. What kingdoms do algae depend on?

2. Structure of algae.

3. Which group of plants are algae?

4. How are many-celled algae important to other living things?

5. Economic importance of algae.

Test questions - 4

1. Ecological and Economic Roles of fungi.

2. What kind of fungi do you know?

3. Parasitic Sac Fungi:

4. Slime Molds. Description.

5. Adaptations to Life on Land.

Test questions - 5

1. What are lichens? What are peculiarities of lichens’ body?

2. Why are lichens considered as “pioneer species”?

3. How do lichens reproduce and get nutrients?

4. What is ecologic role of lichens?

5. What is practical application of the lichens?