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26 The Algae

 

CHAPTER OVERVIEW

This chapter discusses the characteristics of a diverse polyphyletic group of organisms known as the algae. They range from single cells to multicellular organisms over 75 meters in length. They are found in oceans and freshwater environments and are the major producers of oxygen and organic material. A few algae live in moist soil and other terrestrial environments. They do not constitute a unique kingdom; instead they are found within two of the five kingdoms previously discussed. An overview of their characteristics is presented, followed by discussion of each of the major groups of algae.

 

CHAPTER OBJECTIVES

After reading this chapter you should be able to:

! discuss the various habitats in which algae are found

! discuss the various morphological characteristics of the algae and the taxonomic relationships of this diverse polyphyletic group of organisms

! discuss asexual and sexual reproduction of the algae

! discuss the various classical divisions of algae and the differences in photosynthetic pigments and cell structures exhibited within these divisions

 

CHAPTER OUTLINE

I. Introduction

A. AlgaeCplants or protists that lack roots, stems, and leaves, but that have chlorophyll and other pigments for carrying out oxygenic photosynthesis

B. Phycologists (algologists)Cscientists who study algae

C. Phycology (algology)Cthe study of algae

II. Distribution of AlgaeCprimarily aquatic, with a few terrestrial organisms growing on moist surfaces

A. PlanktonicCsuspended in the aqueous environment

1. PhytoplanktonCalgae and other small aquatic plants

2. ZooplanktonCanimals and other nonphotosynthetic protists

B. BenthicCattached and living on the bottom of a body of water

C. NeustonicCliving at the air-water interface

D. Some algae are endosymbionts in protozoa, mollusks, worms, corals, and plants

E. Some associate with fungi to form lichens

III. Classification of Algae

A. Belong to seven divisions within two different kingdoms

B. Primary classification is based on cellular and not organismal properties

1. Cell wall (if present) chemistry and morphology

2. Storage of food and photosynthetic products

3. Types of chlorophyll molecules and accessory pigments

4. Number of flagella and their insertion location

5. Morphology of cells and/or thallus (body)

6. Habitat

7. Reproductive structures

8. Life history patterns

C. Molecular systems have reclassified the algae as polyphyletic with diverse origins and associations

IV. Ultrastructure of the Algal Cell

A. Surrounded by a thin, rigid cell wall (some have an outer matrix also)

B. When present, flagella are the locomotory organelles

C. Chloroplasts have thylakoids (sacs) that are the site of photosynthetic light reactions

D. Chloroplasts have a dense proteinaceous pyrenoid that is associated with the synthesis and storage of starch

E. The nucleus has a typical nuclear envelope with pores

V. Algal NutritionCcan be either autotrophic or heterotrophic

A. AutotrophicCrequire only light and inorganic compounds for energy; use CO2 as carbon source

B. HeterotrophicCuse external organic materials as source of energy and carbon

VI. Structure of the Algal Thallus (Vegetative Form)

A. Unicellular

B. Colonial

C. Filamentous

D. Membranous

E. Tubular

VII. Algal Reproduction

A. AsexualCoccurs only with unicellular algae

1. FragmentationCthallus breaks up and each fragment forms a new thallus

2. Spores formed in ordinary vegetative cell or in sporangium

a. Zoospores are flagellated motile spores

b. Aplanospores are nonmotile spores

3. Binary fissionCnuclear division followed by cytoplasmic division

B. SexualCoccurs in multicellular and unicellular algae

1. OogoniaCrelatively unmodified vegetative cells in which eggs are formed

2. AntheridiaCspecialized structures in which sperm are formed

3. ZygoteCfusion of sperm and egg

VIII. Characteristics of the Algal Divisions

A. Chlorophyta (green algae)

1. Contain chlorophylls a and b, and carotenoids; store carbohydrate as starch; cell walls are made of cellulose

2. Live in fresh and salt water, in soil, on and within other organisms

3. Have a variety of body typesCunicellular, colonial, filamentous, membranous, and tubular

4. Exhibit both asexual and sexual reproduction

5. Molecular classification places these with the land plants

6. ChlamydomonasCa representative unicellular green alga

a. Microscopic, rounded, with two flagella at anterior end

b. Single haploid nucleus

c. A large chloroplast with conspicuous pyrenoid for starch production and storage

d. Stigma (phototactic eyespot)

e. Contractile vacuoles act as osmoregulators

f. Asexual reproduction (zoospores) and sexual reproduction

7. ProtothecosisCa human and animal disease caused by the green alga, Prototheca moriformis

B. Charophyta (stoneworts/brittleworts)

1. Abundant in fresh and brackish waters; worldwide distribution

2. Some species precipitate calcium and magnesium carbonate from water to form a limestone covering (helps preserve members as fossils)

C. Euglenophyta (euglenoids)

1. Same chlorophylls (a and b) as Chlorophyta and Charophyta

2. Found in fresh and brackish waters, and in moist soils

3. Molecular classification indicates the euglenoids to be closely associated with the amoeboflagellates (flagellated protozoa)

4. Euglena

a. Elongated cell bounded by a plasma membrane

b. Stigma located near an anterior reservoir

c. A pellicle (articulated proteinaceous strips lying side-by-side) maintain a cellular shape that is somewhat flexible yet rigid enough to prevent excessive alterations

d. A large contractile vacuole collects water and empties it into the reservoir for osmotic regulation

e. Paired flagella at anterior end arise from reservoir base; only one beats to move the cell

f. Reproduction is by longitudinal mitotic cell division

D. Chrysophyta (golden-brown and yellow-green algae and diatoms)

1. Molecular classification associates these with the stramenophiles

2. Contain chlorophylls a and c1/c2, and the carotenoid fucoxanthin

3. Some lack cell walls, some have intricately patterned scales on the plasma membrane; diatoms have a distinctive two-piece wall of silica called a frustule

4. Zero, one, or two flagella (of equal or unequal length)

5. Unicellular or colonial

6. Reproduction is usually asexual, but occasionally sexual

7. Diatoms are photosynthetic, circular, or oblong cells with overlapping silica shells

a. EpithecaClarger half

b. HypothecaCsmaller half

8. Diatomaceous earthCdeposits of empty diatom shells

a. Used in detergents, polishes, paint removers, decolorizers, deodorizers, and fertilizers

b. Filtering agent

c. Component in soundproofing and insulating materials

d. Paint additive to increase night visibility of license plates and signs

E. Phaeophyta (brown algae)

1. Multicellular seaweeds; some species have the largest linear dimensions known in the eucaryotic world

2. Have branched filaments and more complex arrangements; some (kelps) are differentiated into flattened blades, stalks, and holdfast organs that anchor them to rocks

3. Sargassum forms huge floating masses

4. Contain chlorophylls a and c; carotenoids include fucoxanthin, violaxanthin, and J-carotene

5. Molecular classification associates these with the stramenophiles

F. Rhodophyta (red algae)

1. Some are unicellular, but most are multicellular, filamentous seaweeds

2. Comprises most of the seaweeds

3. Contain phycoerythrin (red pigment) and phycocyanin (blue pigment), and can therefore live in deeper waters

4. Their cell walls include a rigid inner part composed of microfibrils and a mucilaginous matrix consisting of sulfated polymers of galactose

5. One of the polymers, agar, is widely used as a gelling agent in bacterial culture media

6. Many also deposit calcium carbonate in their cell walls and contribute to coral reef formation

G. Pyrrhophyta (dinoflagellates)

1. They are unicellular, photosynthetic protists

2. Most are marine but a few are freshwater dwellers

3. Some are responsible for phosphorescence in ocean waters

4. Two perpendicular flagella cause organisms to spin

5. Contain chlorophylls a and c, carotenoids, and xanthophylls

6. ZooxanthellaeCsymbiotic dinoflagellates that have lost their cellulose plates and flagella and that live within the cells of other organisms

7. Responsible for toxic red tides; toxin is ingested by shellfish (which are unaffected by it) and passed to those who consume the shellfish


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