Survey of Protists: The Algae

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Survey of Protists, The Algae

Exercise 25
By the end of this exercise, students should be able to:

  1. Discuss the distinguishing features of different groups of algae.

  2. Appreciate the economic importance of algae.

  3. Outline the events of "alternation of generations" in green algae.

Domain Eukarya

  • Definition: Eukaryotes are organisms composed of cells that have membrane-bound nuclei.

  • The domain Eukarya is categorized into three main kingdoms: Fungi, Animalia, and Plantae, along with a diverse group of organisms known as protists.

  • Protists: This group includes all eukaryotes that do not exhibit the defining features of fungi, animals, or plants.

  • Protists typically inhabit moist environments and comprise simple eukaryotic forms such as amoebas and multicellular organisms like the brown alga, kelp.

Distinction Among Kingdoms

  • Fungi: These organisms have cell walls and are categorized as heterotrophic.

  • Heterotrophs: Definition - Organisms that must feed on organic matter produced by others because they lack the ability to synthesize organic compounds from inorganic substances. This means that they require organic nutrients produced by other organisms to survive.

  • Animals: These are multicellular organisms that consume food through ingestion and lack cell walls. Animals can respond rapidly to external stimuli.

Protista Subcategories

Protista is divided into three main categories:

  1. Plant-like protists: Example - Algae

  2. Fungus-like protists: Example - Slime molds

  3. Animal-like protists: Example - Protozoa

Algae

  • Definition: Algae are eukaryotic organisms capable of photosynthesis, typically lacking multicellular sex organs.

  • Distinguishing Characteristics:

    • Algae can be classified based on energy storage products, cell wall composition, and pigmentation, which determines their color.

    • Common classifications based on color include green algae, brown algae, and red algae.

Cellular Organization of Algae

  • Algal species can be categorized based on their cellular configuration:

    • Unicellular Algae: Exist as single, unattached cells that may or may not have motility.

    • Filamentous Algae: Form chains of cells attached end to end, which may be branched or unbranched.

    • Colonial Algae: Composed of groups of cells linked non-filamentously, such as spherical, flat, or three-dimensional forms. Colonies are clonal in nature, meaning that all cells are genetically identical.

    • Multicellular Algae: Less common in protists, characterized by differentiated cells with specialized functions and interdependence among those cells.

Specific Examples of Algae

  • Volvox (Colonial Green Alga):

    • Comprises many Chlamydomonas-like cells within a spherical structure.

    • Each cell possesses two flagella that extend from the colony’s surface, allowing synchronized movement through water.

    • Notably complex, some biologists classify Volvox as multicellular due to its specialized cell functions, including replication and gamete production.

Brown Algae (Supergroup Chromalveolata)

  • Description: Brown algae are primarily marine, complex organisms. They do not exist in unicellular or colonial forms.

  • They thrive in cooler waters and contain a pigment known as fucoxanthin, which gives them their characteristic brown color.

  • Brown algae range in size; some types can grow to lengths exceeding 50 meters, such as kelp, which has a thallus structure similar to land plants.

Notable Examples of Brown Algae

  • Macrocystis:

    • A type of kelp reaching lengths of up to 100 meters, with flat blades that float on water surfaces and a root system that anchors them below.

  • Sargassum:

    • This form of brown algae creates extensive floating masses, known as sargassum weed, in the Sargasso Sea, serving as microhabitats for various marine species.

  • Fucus (Rockweed):

    • Attaches to rocky intertidal zones via holdfasts and features reproductive structures called conceptacles containing multicellular sex organs: oogonia (female) producing eggs and antheridia (male) producing sperm.

Economic Significance of Brown Algae

  • Certain brown algae, such as Laminaria (marketed as “kombu”), are consumed as food, particularly in Eastern cuisines.

  • Brown algae are significant sources of alginic acid, which serves as a hydrophilic emulsifier in products like processed foods, cosmetics, and dripless paints.

Red Algae (Supergroup Archaeplastida)

  • Characterized by red pigments known as phycobilins in their plastids, red algae typically grow in warm marine waters.

  • Their thalli may be attached or free-floating, and they can take on filamentous or fleshy forms.

  • Commercial Importance: Examples include Irish moss (Chondrus crispus), which is harvested for carrageenan, a stabilizer found in food products such as salad dressings and dairy items.

Diatoms (Supergroup Chromalveolata)

  • Characteristics: Diatoms are unicellular and possess chlorophylls a and c along with xanthophyll pigments that impart a golden-brown hue.

  • They are crucial in oceanic food chains due to their vast numbers and rapid reproduction rates.

  • Cell Wall Composition: Diatoms have silica-based cell walls arranged in overlapping halves, akin to the halves of a petri dish. These walls contribute to the formation of diatomaceous earth, which can accumulate in deposits that may be several hundred meters thick.

Dinoflagellates (Supergroup Chromalveolata)

  • Defining Features: All dinoflagellates are unicellular and possess unique cellulose plates and two flagella situated in perpendicular grooves.

  • They are significant as primary producers in marine ecosystems, second to diatoms, and they include forms that are autotrophic or heterotrophic.

  • A notable phenomenon is the “red tide,” caused by blooms of the red-pigmented dinoflagellate Ptychodiscus brevis, which can deplete oxygen and produce harmful toxins lethal to fish.

Euglenoids (Supergroup Excavata)

  • These primarily freshwater unicellular forms feature chlorophylls a and b and lack rigid cell walls, instead relying on a pellicle made predominantly of protein, which enhances cell flexibility.

  • Euglenoids are motile and equipped with two flagella.

  • The chloroplasts of Euglena may contain a pyrenoid, a structure that aids in concentrating CO2.

  • They exhibit varied nutritional modes including autotrophy, heterotrophy, and saprophytism, depending on environmental conditions, thus highlighting the difficulty in classifying protists based on conventional criteria.