Saturday, January 28, 2012

The Algae


Chapter Overview

This chapter discusses the characteristics of the 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. 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:
1.       discuss the various habitats in which algae are found
2.       discuss the various morphological characteristics of algae
3.       discuss the taxonomic relationships of this diverse polyphyletic group of organisms
4.       discuss asexual and sexual reproduction of algae
5.       discuss the various classical divisions of algae and the characteristics used to establish the divisions
These are the most important concepts you are learning in this chapter:

  1. Most algae are found in freshwater and marine environments; a few grow in terrestrial habitats.
  2. The algae are not a single, closely related taxonomic group but, instead, are a diverse, polyphyletic assemblage of unicellular, colonial, and multicellular eucaryotic organisms.
  3. Although algae can be autotrophic or heterotrophic, most are photoautotrophs. They store carbon in a variety of forms, including starch, oils, and various sugars.
  4. The body of an alga is called the thallus. Algal thalli range from small solitary cells to large, complex multicellular structures.
  5. Algae reproduce asexually and sexually.
  6. The following classical divisions of the algae are discussed: Chlorophyta (green algae), Charophyta (stoneworts/brittleworts), Euglenophyta (euglenoids), Chrysophyta (golden-brown and yellow-green algae; diatoms), Phaeophyta (brown algae), Rhodophyta (red algae), and Pyrrhophyta (dinoflagellates).

Study Outline
  1. Introduction
    1. Algae-not a monophyletic group; instead the term is used to describe a group of organisms that lack roots, stems, and leaves, but that have chlorophyll and other pigments for carrying out oxygenic photosynthesis
    2. Phycologists (algologists)-scientists who study algae
    3. Phycology (algology)-the study of algae
  2. Distribution of Algae
    1. Primarily aquatic
      1. Planktonic-suspended in the aqueous environment
        1. Phytoplankton-algae and other small aquatic plants
        2. Zooplankton-animals and other nonphotosynthetic protists
      2. Benthic-attached and living on the bottom of a body of water
      3. Neustonic-living at the air-water interface
    2. Moist rocks, wood, trees, and soil
    3. Some are endosymbionts in protozoa, mollusks, worms, corals, and plants
    4. Some associate with fungi to form lichens
    5. Some are parasitic
  3. Classification of Algae
    1. In the Whittaker system
      1. Divided into seven divisions within two different kingdoms
      2. Primary classification is based on cellular properties
        1. Cell wall (if present) chemistry and morphology
        2. Storage food and photosynthetic products
        3. Types of chlorophyll 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
    2. Molecular systems have reclassified the algae as polyphyletic; they fall into five different lineages
      1. Plants
      2. Red algae
      3. Stramenopiles (this lineage also includes some protozoa)
      4. Alveolates (this lineage also includes some protozoa)
      5. Amoeboflagellates (this lineage also includes some protozoa)
  4. Ultrastructure of the Algal Cell
    1. Surrounded by a thin, rigid cell wall (some also have an outer matrix)
    2. Some are motile by flagella
    3. The nucleus has a typical nuclear envelope with pores
    4. Chloroplasts have thylakoids (sacs) that are the site of photosynthetic light reactions; may also have a dense proteinaceous pyrenoid that is associated with the synthesis and storage of starch
    5. Mitochondria can have discoid cristae, lamellar cristae, or tubular cristae
  5. Algal Nutrition
    1. Most are autotrophic-require only light and inorganic compounds for energy; use CO2 as carbon source
    2. Some are heterotrophic-use external organic materials as source of energy and carbon
  6. Structure of the Algal Thallus (Vegetative Form)
    1. Thallus-vegetative body of algae; can be unicellular or multicellular
    2. Algae can be unicellular, colonial, filamentous, membranous, or tubular
  7. Algal Reproduction
    1. Asexual-occurs only with unicellular algae
      1. Fragmentation-thallus breaks up and each fragment forms a new thallus
      2. Spores formed in ordinary vegetative cell or in sporangium
        1. Zoospores are flagellated motile spores
        2. Aplanospores are nonmotile spores
      3. Binary fission-nuclear division followed by cytoplasmic division
    2. Sexual-occurs in multicellular and unicellular algae
      1. Oogonia-relatively unmodified vegetative cells in which eggs are formed
      2. Antheridia-specialized structures in which sperm are formed
      3. Zygote-product of fusion of sperm and egg
  8. Characteristics of the Algal Divisions
    1. Chlorophyta (green algae)-molecular classification places these with plants
      1. Are extremely varied
        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, soil, and associated with other organisms
        3. Can be unicellular, colonial, filamentous, membranous, or tubular
        4. Exhibit both asexual and sexual reproduction
      2. Genus Chlamydomonas-Members of this genus are microscopic, rounded, with two flagella at anterior end; have single haploid nucleus, a large chloroplast with conspicuous pyrenoid for starch production and storage, a stigma (phototactic eyespot), and contractile vacuole (acts as osmoregulator); exhibit asexual reproduction (zoospores) and sexual reproduction
      3. Genus Chlorella-members of this genus are nonmotile, unicellular algae; are widespread in aquatic habitats and in soil; only reproduce asexually; lack flagella; have eyespots, contractile vacuoles, and a very small nucleus
      4. Genus Volvox-members of the genus exist as hollow spheres made up of a single layer of 500-60,000 flagellated cells; flagella beat in a coordinated fashion; some cells are specialized for reproduction
      5. Prototheca moriformis, which is common in soil, causes the disease protothecosis in humans and other animals
    2. 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 them as fossils)
    3. Euglenophyta (euglenoids)-molecular classification places these with amoeboflagellates
      1. Same chlorophylls (a and b) as Chlorophyta and Charophyta; found in fresh and brackish waters and in moist soils
      2. Genus Euglena-members of this genus:
        1. Have elongated cells bounded by a plasma membrane; inside the plasma membrane is a pellicle (articulated proteinaceous strips lying side-by-side), which is elastic enough to enable turning and flexing of the cell, yet rigid enough to prevent excessive alterations in cell shape
        2. Have a stigma located near an anterior reservoir
        3. Have a large contractile vacuole, which collects water and empties it into the reservoir for osmotic regulation
        4. Have paired flagella at anterior end that arise from reservoir base; only one beats to move the cell
        5. Reproduce by longitudinal mitotic cell division
    4. Chrysophyta (golden-brown and yellow-green algae and diatoms)-molecular classification places these with the stramenopiles
      1. Divided into three classes: golden-brown algae, yellow-green algae, and diatoms
      2. Contain chlorophylls a and c1/c2, and the carotenoid fucoxanthin
      3. Major carbohydrate reserve is chrysolaminarin
      4. 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; have zero, one, or two flagella (of equal or unequal length)
      5. Most are unicellular or colonial; reproduction is usually asexual, but occasionally sexual
      6. Diatoms are photosynthetic, circular or oblong cells with overlapping silica shells (epitheca-larger half and hypotheca-smaller half)
        1. Grow in aquatic habitats and moist soil
        2. Some are faculatative heterotrophs
        3. Vegetative cells are diploid and reproduce asexually with each daughter getting one old theca and constructing one new theca; this type of reproduction results in diatoms getting progressively smaller with each reproductive cycle; when diminished to 30% of original size, sexual reproduction occurs
    5. Phaeophyta (brown algae)-molecular classification places these with stramenopiles
      1. Multicellular seaweeds; some species have the largest linear dimensions known in the eucaryotic world
      2. Simplest species have branched filaments; more complex species (kelps) are differentiated into flattened blades, stalks, and holdfast organs that anchor them to rocks
      3. Contain chlorophylls a and c; carotenoids include fucoxanthin, violaxanthin, and b-carotene
    6. Rhodophyta (red algae)-molecular classification gives these a separate lineage
      1. Some are unicellular, but most are multicellular, filamentous seaweeds; comprise most of the seaweeds
      2. Carbohydrate reserve is floridean starch
      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 (agar); many also deposit calcium carbonate in their cell walls and contribute to coral reef formation
    7. Pyrrhophyta (dinoflagellates)-molecular classification places these with the alveolates
      1. Unicellular, photosynthetic protists
      2. Most are marine organisms but a few are freshwater dwellers; some are responsible for phosphorescence in ocean waters and for toxic red tides
      3. Their flagella and protective coats are distinctive
        1. Are clad in stiff, patterned, cellulose plates (thecae)
        2. Most have two perpendicular flagella that function in a manner that causes organism to spin
      4. Contain chlorophylls a and c, carotenoids, and xanthophylls
      5. Some can ingest other cells; some are heterotrophic; some are endosymbiotic, living within host cells where they lose their cellulose plates and flagella (zooxanthellae)

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