Introduction to botany  © punam kumar

 

CHAPTER 15:

Algae structure and reproduction

Introduction

Algae (singular:alga) are eukaryotes ("true-nucleus"). An informal grouping of primitive, mainly aquatic plants that possesses chlorophyll a as their primary photosynthetic pigment and can manufacture their own food through the process of photosynthesis. Plant body (thallus) do not have true roots, stems, leaves, vascular tissue and have simple reproductive structures.They never produce multicellular embryos inside the female reproductive organ. There are unicellular (e.g. planktons) and multicellular (e.g. seaweeds) algae found in freshwater (e.g. lakes and rivers), marine (e.g. sea) and terrestrial (e.g. moist pavements) habitats. < The study of algae is termed phycology or algology, and one who studies algae is known as a phycologist.

Classification of algae :- Algae belong to Domain-Eukarya and Kingdom - Protista (Protoctista). In 1959 American biologist R. H. Whittaker described a classification system of five primary kingdoms: plants, animals, fungi, protists, and bacteria. Because the Protista are so diverse in form, classification within the kingdom has proved difficult. The classification of the Protista is currently based largely on the structure and organization of the cell, the presence of organelles, and the pattern of reproduction or life cycles. The five-kingdom classification system divides the Protista into 27 distinct phyla. More recently, however, classifications based on comparisons of cell physiology and DNA sequences suggest that many protist phyla may be sufficiently large and diverse to be classified as kingdoms.Possible classifications are discussed, and a summary classification of the living world into kingdoms (Monera, Protista, Fungi, Animalia, Plantae) and phyla is suggested. This classification also suggests groupings of phyla into superphyla and form-superphyla, and a broadened kingdom Protista (including green algae, oomycotes and slime molds but excluding red and brown algae).“Phylum” and “division” represent the same level of organization; the former is the zoological term, the latter is the botanical term.


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The Plant-like Protista (The Algae) The greatest cellular and morphological diversity in the kingdom occurs within the algae. While algae have been traditionally regarded as simple plants, they actually belong to more than one domain, including both Eukaryota and Bacteria (see Blue-green algae), as well as more that one kingdom, including plants and protists. Although the majority of the group are unicellular, there are also species which are colonial, filamentous, coenocytic and multicellular. Algal protists are aquatic organisms which have chlorophyll a (like cyanobacteria and plants). They differ from other photosynthetic organisms in that they also have accessory pigments such as carotenoids, xanthophylls, phycobillins, and other forms of chlorophyll.

Morphological diversity of the green algae(Division Chlorophyta) :-There is a vast variation in the vegetative structures (thallus) of algae.

Chlorophyceae

:- The members of chlorophyceae exhibit greater diversity in form and structure.On the basis of thallus organisation, it can be of following types
  1. Unicellular algae:-These algae are single cells.All the vital function of life are performed by the single cell (occasionally the term acellular -used). The unicellular algae are all sizes and shapes. They range from small spherical cells to large irregular shaped cells. Most of the shape variations are designed to increase the surface to volume ratio of the cells.The unicellular forms are :-
    • Unicellular motile forms (with flagella):- The motile unicell may be spherical,oblong,pear-shaped or sometimes elongated bearing flagella eg. Chlamydomonas.


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    • Non-motile unicells (without flagella -organ for locomotion):- Many unicellular algae do not possess any outgrowth for locomotion. Example - Chlorella


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    The individual cells also exhibit a wide variety of different coverings. Some have the typical rigid cell wall found in plants. Some have only a cell membrane, while others are covered with a variety of plates, scales and vase like loricae.
  2. Colonial algae:- Colonies comprise single cells which typically exists as clumps. The key point about colonies is that there is no division of labour and each cell can survive on its own.


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    Some colonial algae possess flagella for motility. Oocystis is an example of a colonial green alga.
  3. Coenobia algae:-These organisms are also found primarily in the aquatic environment. The coenobium (plural coenobia) is a colony with a fixed number of cells. The cells are often embedded in a mucilaginous matrix. Colonies are typically aggregations of cells , with 4 cells as in Gonium, 16 cells in Pandorina, 32 cells in Eudorina. Volvox is a colonial organism composed of thousands of cells that very closely resemble Chlamydomonas. So, each cell would be capable of independent life, but they are arranged to work in a coordinated fashion. Morphological variation is due to differences in number and plane of cell division only. Division in definite and consistent planes results in formation of a regular colony while division in random planes results in the formation of irregular colonies. Majority of the cells are vegetative and only a few are reproductive. The main point about colonies is that there is no division of labour and each cell can survive on its own.


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    Both motile (possess flagella) and non-motile coenobia are found among algae colonial algae .

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  5. Filamentous algae (floating or attached) :- While the colonial body form appears to be a dead end from an evolutionary viewpoint, the filamentous algae apparently had the morphological flexibility to develop into more complex aquatic and terrestrial plants. The simplest filamentous algae consist of a thallus (body) of a single chain of cells. This is the result of cell division in one plane only.The filaments may be :-
    • Unbrached filamentous forms :-Such type of thalli are found in many algae and consist of a straight row of cells.


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      As a free living e.g. Spirogyra or attached e.g. Ulothrix and Oedogonium.
    • Simple branched filamentous forms :- When some cells in a filament show lateral outgrowth. Branching filaments occur when there is periodic division in a second plane.e.g Cladophora.
    • Heterotrichous forms (heteros =different): -Some cells in the filaments divide several times in different planes resulting in two parts.


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      Finally, some filamentous algae began to show some cellular differentiation. Where there are basal, prostrate filaments for attachment and erect branches for photosynthesis, this is said to be a heterotrichous filament e.g. Fritschiella sp.
    • These morphological features are an example of a parallel evolutionary adaptation to terrestrial life with the land plants. The flagellated reproductive cells show that Fritschiella is in fact closely related to the chlorophyte green algae, rather than to the charophyte green algae that gave rise to land plants.
    • Siphonous algae :-actually giant unicells. The plant body undergoes repeated nuclear division without the accompanying formation of cell walls.


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      As a result tubular structure with the multinuclear cytoplasm lining is formed known as coenocytic as in siphonales e.g. Vaucheria, Caulerpa
    • Uniaxial type :-The outer sheathing layer of the axis is always one celled in thickness eg. Chara.Plant body of Chara consists of a series of nodes and internodes , a structure shared with other plant groups. The nodes are points where branchings occur and the internodes are the stem-like segments in between. These organisms differ from the green algae in having tissue types, including the differentiated reproductive bodies, but they do not have vascular tissues characteristic of many land plants.
    • Parenchymatous algae :-Seaweeds made up of "boxy" cells like those of higher plants are termed parenchymatous. They may be membranous like Ulva, the sea lettuce. Some even have tissues and organs that resemble those of the higher land plants. However, these seaweeds are more closely related to the unicellular algae then they are to the land plants, and their anatomical complexity evolved independently. The term thallus (thalli pl.) is used to describe the seaweed body form. A typical seaweed has a root like holdfast which anchors the plant to the substrate, a stem like stipe, and a leaf like blade. The blades provide most of the photosynthetic surface for the algae.

Reproduction

Most green algae reproduce both asexually (by mitosis) and sexually. The green algae also reproduce by vegetative method. Vegetative reproduction is by fragmentation.


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The volvocine series is a group of algae that together seem to represent an evolutionary movement from simple, single-celled algae to colonies of increasing size, internal differentiation, and tendency towards heterogamous sexual reproduction. Thus, Volvox colony shows a marked division of labor; only a few of the cells are active in producing gametes, the remainder are permanently vegetative. This differentiation of cells into those capable of specialized functions, is characteristic of multicellular organisms; and Volvox begins to demonstrate the evolutionary transition from colonial life forms to multicellular organisms.

Reproduction in filamentous green algae :-

Reproduction in multicellular Green Algae

Multicellular green algae have some division of labor, producing various reproductive cells and structures. The green algae Ulva so closely resembles a plant that its common name is sea lettuce. Ulva undergoes a true alternation of generations, in that, it spends equal time as a haploid and diploid organism. Biflagellate isogametes are formed by certain cells of the haploid, gametophyte plant. These are liberated and fuse in pairs to form a diploid zygote(2n) which germinates to form a separate diploid plant called the sporophyte(2n). This resembles the haploid (n) gametophyte plant in outward appearance. Certain cells of the sporophyte(2n) undergo meiosis and form zoospores in sporangia; these zoospores are quite different to the gametes in that they form quadriflagellate zoospores (with 4 flagella). These are released, swim around for a time, settle and germinate to form the haploid (n) gametophyte thallus. The adult stages, sporophyte and gametophyte, are the stages that resemble lettuce, and the leafy structure is called a thallus.

Red Algae (Division- Rodophyta) :- Most of the known red algae species are small to medium-sized multicellular and live in the ocean (marine) with approximately 6000 species. Red algae contain chlorophyll as well as phycobilins, red and blue pigments involved in photosynthesis. Red algae are red because they contain pigments phycoerythrins and the blue pigment is called phycocyanin. Phycoerythrins which absorbs blue light and reflects red light. This makes it possible for red algae to carry out photosynthesis in comparatively deep waters, because blue light penetrates deeper than lights of longer wavelengths, such as red light. The thallus(plant body) may be filametous,and of different shapes platelike, coralline, crustlike, leathery, and featherlike forms. They live attached to rocks by a structure called a holdfast. The cells may be uninucleate or multinucleate with one or more plastids that may be with or without pyrenoids.Their cell walls contain cellulose and pectic material together with thick polysaccharides called phycocolloids, some of which contain sulphur e.g. Agar.Some species secrete calcium carbonate over their walls and form coralline structures and contribute much of the lime in coral reef deposits. Fossils of red algae have been found in rocks 500 million years old.

Reproduction in red algae

Brown algae (Division Phaeophyta) :-

Most brown algae have an alternation of haploid and diploid generations :- This form of reproduction may be called “Alternation of Generations” because there is an alternation between a gametophyte and sporophyte generation in the life cycle.But species such as Fucus reproduce via a diplontic life cycle, meaning the adult is the sporophyte and diploid.


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