bio2106-1

Lecture 1 - ALGAE (Plant-like protists)

What are Algae?

  • Definitions by various authors:

    1. Fritsch, F. (1935): Defined algae as holophytic organisms and their numerous colorless derivatives, which do not achieve the higher level of differentiation characteristic of archegoniate plants.
    2. Smith, G. M. (1955): Defined algae as simple plants that utilize an autotrophic mode of nutrition.
    3. Chapman, V. J. (1962): Defined algae (includes seaweeds found in seashores and green skeins in stagnant waters) as among the simplest organisms in the plant kingdom.
    4. Prescott, G. W. (1969): Defined algae as chlorophyll-bearing organisms, along with their colorless relatives, characterized as thalloid, meaning they lack true roots, stems, and leaves.
    5. Singh, R. N. (1974): Defined algae as predominantly simple plants exhibiting a range of photosynthetic pigments and producing oxygen during photosynthesis.

  • Characteristics of Algae:

    • Algae comprise diverse groups that are typically autotrophic and can range from unicellular to multicellular forms.
    • The most complex marine algae are referred to as seaweeds.
    • Algae convert solar energy into biomass using photosynthesis, similar to plants, but lack many distinct organs found in terrestrial plants.
    • They do not possess structures associated with terrestrial flora such as true leaves (phyllids) or roots (rhizoids).

  • Nutritional Modes:

    • Predominantly autotrophic, some species are mixotrophic, getting energy through both photosynthesis and the uptake of organic carbon by:
    • Osmotrophy: Uptake of dissolved organic compounds by osmosis.
    • Myzotrophy: Simultaneous heterotrophic and autotrophic modes utilize both inorganic and organic carbon in the presence of light.
    • Phagotrophy: Engulfment of large food particles, as seen in amoebas guided by pseudopodia.

Structure of Algae

  • Cell Wall:

    • Most algal cell walls are constructed of cellulose, incorporating hemicellulose, mucilage, pectin, and substances like alginic acid, calcium carbonate, and silica in various combinations.

  • Protoplast:

    • Defined as the protoplasmic content of the cell, surrounded by a lipoproteinaceous cell membrane, which typically includes one or more spherical or ellipsoidal nuclei and cytoplasm.

  • Cell Membrane:

    • Composed of lipids and proteins; exhibits a fluid mosaic structure similar to other biological membranes.
    • It is thin, elastic, and selectively permeable, controlling material passage into and out of the cell.

  • Nucleus:

    • A well-organized spherical or elliptical structure surrounded by a double-layered nuclear membrane.
    • Internal structure includes chromatin reticulum within a matrix termed karyolymph, and may contain one or more nucleoli.
    • In eukaryotic algae, membrane-bound organelles (e.g., chloroplasts, mitochondria, Golgi apparatus, ER) are present, while in prokaryotic algae (like Cyanophyceae) the nucleus is not membrane-bound.

  • Chloroplasts:

    • Key features of algal cells, containing photosynthetic pigments and organized in a double-membrane structure.

  • Mitochondria:

    • Present in all algal cells except Cyanophyceae; consist of a double membrane envelope and contain an aqueous matrix of enzymes and solutes.

  • Endoplasmic Reticulum (ER):

    • A network of interconnecting tubules and cisternae traversing the cytoplasm.

  • Golgi Apparatus (Dictyosomes):

    • Found in all algal cells except blue-green algae; serves as an intermediary between the ER and plasma membrane, observable via electron microscopy.

  • Eye-Spot (Stigma):

    • Pigmented spots found in motile vegetative/reproductive cells, linked to light perception; located within thylakoids.

  • Vacuoles:

    • Present in nearly all algal cells, excluding Cyanophyceae; bounded by tonoplast.

  • Flagella:

    • Present in motile reproductive or vegetative cells except in Cyanophyceae and Rhodophyceae; vary in number (1-4 or more), length, arrangement, and presence of hairs.

Reproduction in Algae

  • Common Types:

    • Primarily binary fission in unicellular prokaryotic algae (e.g., Anacystis) for reproduction.
    • In filamentous/multicellular forms, accidental fragmentation may occur with each piece developing into a new organism (vegetative reproduction).
    • Understanding reproduction via specialized cells (excluding sex cells) is termed asexual reproduction.

  • Special Cases: Anabaena and Nostoc:

    • Cells store food materials and develop thick walls to form spores or akinetes, which can withstand adverse conditions and germinate when conditions improve.

  • Filamentous Algae (e.g., Ulothrix):

    • Reproduce by producing motile cells called zoospores through mitosis; zoospores possess 2-4 flagella, swim temporarily, then settle to form a vegetative filament.

  • Examples of Asexual Reproduction:

    • Chlamydomonas: Unicellular form producing zoospores; may develop into a palmella stage when environmental conditions become dry.
    • Oedogonium: Produces single zoospores with one nucleus and flagella.
    • Ulva: Produces multiple zoospores from a single cell with flagella.
    • Ectocarpus: Distinction in sporangia type:
    • Plurilocular Sporangia: Contains many cells producing multiple zoospores.
    • Unilocular Sporangia: Contains one cell producing a single zoospore.

Sexual Reproduction

  • Involves fusion of two opposite sex cells (gametes), resulting in a zygote.
  • Basic Features of Sexual Reproduction:
    • Gametes are haploid and often morphologically similar or different:
    • Isogamy: Both gametes are of the same size and morphology.
    • Anisogamy: Gametes are distinctly different, with the larger being female (minus type).
    • Oogamy: Larger female gametes (eggs) are non-motile, while male gametes (sperm) are motile and attracted by gamones (special hormones).
  • Gamete fusion forms a zygote, which can develop into a resting zygospore under unsuitable conditions.
  • Gametes formed in haploid thalli via mitosis or in diploid thalli via meiosis.

Ecology

  • Microscopic Algae:

    • Found in single cells or small colonies, mostly in open waters (termed phytoplankton).
    • Some species reside on rocks and larger aquatic plants in shallow habitats (periphyton).
    • Others are found in moist terrestrial environments (soil and rocks).

  • Ecological Role:

    • Microscopic algae serve as primary producers, forming the base of ecological food webs.
    • They fix solar energy into biological compounds within open waters—the only mechanism in oceans, ponds, and lakes.
    • Food Chain Interaction:
    • Phytoplankton consumed by zooplankton (small grazing animals, mostly crustaceans).
    • Larger zooplankton or small fish, known as planktivores, consume zooplankton.
    • Predators like larger fish (piscivores), birds, and marine mammals (seals, whales, etc.) depend on phytoplankton productivity.

  • Macroscopic Algae:

    • Observable without magnification; can be enormous (e.g., kelp) and serve as primary producers at the base of food webs.
    • Fewer herbivores consume macroscopic algae biomass; major interactions are with decomposers/detritivores.
    • Macroscopic algae contribute significantly to habitat structure, creating marine "forests" due to their size and complexity.

  • Mutualistic Relationships:

    • Some green algae engage in mutualistic associations, forming lichens with fungi or partnering with animals (e.g., Chlorella residing in protozoa, sponges, hydra).

Environmental and Temporal Dynamics

  • Species Segregation:
    • Algal species segregate along environmental gradients based on needs and tolerances.
    • Temporal changes in phytoplankton communities exhibit patterns based on nutrient requirements and competition.
    • In intertidal waters, stress from desiccation impacts algal distribution along the shorelines, especially regarding wave action and competition.
    • Brown and red algae demonstrate zonation along transects on rocky shores, with specific adaptations to physical stresses such as desiccation and wave action.