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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.

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.

  1. Non-motile unicells (without flagella -organ for locomotion):- Many unicellular algae do not possess any outgrowth for locomotion. Example - Chlorella  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.
  1. 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. Some colonial algae possess flagella for motility. Oocystis is an example of a colonial green alga.

 

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

 

  1. 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. 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. 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. 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.

  • Asexual reproduction -In asexual reproduction only one parent is involved. On the basis of the types of spores produced, asexual reproduction is of different types.The spores may be motile i.e. zoospores or nonmotile i.e. aplanospores.The spores on germination gives rise to new plants.
  • Sexual reproduction-Sexual reproduction takes place through fusion of two gametes

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.

  • Chlamydomonas :-The vegetative stage is haploid; i.e., it only possesses one set of chromosomes.
  1. Asexual reproduction :- It reproduces in favorable condition asexually by the production of haploid zoospores by mitosis.

 

  1. Sexual reproduction (isogamy) :- When growth conditions are unfavorable, vegetative cells develop into haploid isogametes.Gametes from two different mating types are able to fuse,forming zygospore, and it will remain a zygospore in a state of dormancy until conditions become more favourable again. This is the only diploid stage in the life cycle. The zygospore undergoes meiosis, again producing haploid vegetative cells.
  • Gonium :- Sexual reproduction :-Isogamy- Each cell of a Gonium colony can divide to form new cells capable of establishing a new colony.
  • Pandorina :-
  1. Asexual reproduction :- After the colony reaches its maximum size, each cell divides to form a new colony within the parent, and the parent colony's matrix breaks open to release the newly formed colonies.
  2. Sexual reproduction (Isogamy) :- Sexual reproduction by means of isogametes, the zygote then undergoes meiosis to produce meiospores, each one of which is capable of forming a new colony.
  • Eudorina :-
  1. Asexual reproduction with plakeal stage and inversion to form daughter colonies within transparent vesicles in the parental gelatinous matrix, smaller cells often not undergoing division if colony contains different sizes of cells.
  2. Sexual reproduction anisogamous, homothallic (gametes from same thallus take part in fusion) or heterothallic. With large, biflagellate female gametes (eggs) and packets of small, biflagellate male gametes (sperm). Male gametes with a slender cytoplasmic protrusion at base of flagella after fusion- zygotes spherical, smooth-walled, often with reddish contents; on germination, one or two biflagellate gone cells escaping from zygote wall and dividing to form a colony.
  • Volvox :-
  1. Asexual reproduction in Volvox involves the formation of daughter colonies within the parent spherical colony. Each daughter colony develops mitotically from a specialized vegetative cell, and remains within the parent colony until the parent dies and breaks apart, releasing the daughters within.
  2. Sexual reproduction -Volvox colony contains specific differentiated cells which develop into eggs or sperm (oogamous). The sperm are released and swim to find and fertilize eggs.After fertilisation- the zygote develops into a zygospore with heavy, spiny walls, able to survive adverse conditions. Germination of the zygospore by meiosis occurs in the spring and results in a new colony (formed by mitotic division of those haploid cells).

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 :-

  • Spirogyra
    • Asexual reproduction :-Spirogyra can reproduce asexually, if the strand simply breaks then both strands will grow as new algae.
    • Sexual reproduction:- In sexual reproduction the contents of one strand will empty into another, so they are no gametes involved. As with Chlamydomonas, sexual reproduction produces a diploid zygote that is more resistant to unfavourable conditions. So, Spirogyra will often spend the winter as a diploid zygote, and then it will undergo meiosis in the spring when the weather warms again producing the haploid filament.

    Oedogonium

  • Asexual reproduction by zoospore-Under favourable conditions all cells except holdfast are capable of producing zoospores. In Oedogonium wallless zoospores (mitospores) with an eyespot, contractile vacuoles, and a ring of flagella are formed. The motile zoospores are released from the zoosporangium which is located in the parent algae and move to a suitable substrate. The flagella break down and secure the developing zoospore so that a new filament is able to develop by cell division.
  • Sexual reproduction:- Oedogonium reproduces sexually by producing oogonia, which each contain a single large nonflagellate egg cell, and antheridia, which contain small sperm with multiple flagella. Some species are monoecious and can self- fertilize. Dioecious species produce either the egg or the sperm and thus require two filaments for fertilization to occur, which increases genetic diversity.

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

  • Vegetative reproduction :-Vegetative reproduction by fragmentation is not common, though some red algae are able to regenerate the full plant from severed holdfast.
  • Asexual reproduction :- Asexual reproduction takes place by non motile spore.
  • Sexual reproduction :-
    1. Red algae are unique among the algae in that flagellated cells are not formed during the life cycle.
    2. The sexual reproduction is oogamous.
    3. Sexual plants (gametophytes) produce either male sex organs called spermatangia or female sex organs called carpogonia.
      • Carpogonia(singular=Carpogonium):- Carpogonium larger, non-flagellate female gamete produced in carpogonia. Carpogonia are produced at the tip of special branches (carpogonial branches), typically flask-shaped with long, thin neck called trichogyne.
      • Spermatangia(singular = spermatangium) Spermatium(plural = spermatia) non-flagellate male gamete produced in spermatangium, spermatia are carried by water currents to carpogonia.
      • Fertilization: spermatium fused with tip of trichogyne; a channel is enzymatically opened to allow the spermatium‘s nucleus to enter and cystocarps are formed.Cystocarp is the single carposporophyte plus the gametophyte tissues surrounding and protecting it.
      • Fertilized carpogonium produces diploid carposporophyte - triphasic life-cycle e.g. Polysiphonia
      • Carposporophyte produces and releases carpospores. Carposporophytes always live on the female gametophyte and receive nutrients from the gametophyte .
      • On germination a carpospore produces a second, multicellular, diploid generation, the tetrasporophyte .
      • Tetrasporophyte produces haploid tetraspores by meiosis in tetrasporangia. On germination tetraspores produce haploid gametophytes again. Life cycle may involve alternation of a haploid gametophytic generation with a diploid sporophytic generation.

Brown algae (Division Phaeophyta) :-

  • Brown algae belong to the class Phaeophyceae. There are approximately 1,500 known species of brown algae , almost all are marine. The brown colour of these algae are due to presence of xanthophyll pigment fucoxanthin, which masks the other pigments, Chlorophyll a and c (no Chlorophyll b), beta-carotene and other xanthophylls.
  • Shape and size :- All are multicellular, there are no known unicellular or colonial representatives; the simplest plant form is a branched, filamentous thallus.They include the largest of the seaweeds and the kelps.Except a few members, most brown algae are of giant (large) forms.Some of these are of massive size (due to massive size known as kelps) such as Laminaria(2-9 metres in length), Nereocystis (45 metres in length) Macrocystis more than 60 meters in length.
  • External morphology :-The body of a typical brown algae is divided into three parts :-
    1. Holdfast :- This is a basal root like structure by which algae is attached strongly to the rocks.At the base of the stalk is the holdfast.
    2. Stipe or Stalk :- The long or short stem-like structure is called stipe
    3. Lamina or Blade :-One or more large flat photosynthetic leaf-like structures arises from the stipe, or stalk are called lamina.In some large brown algae, certain cells are modified into long filaments called trumpet hyphae.Thes act as food-conducting tubes,like phloem in vascular plants carrying food from lamina to the holdfast.
  • Internal structure :- Some have parenchymatous growth, in which divisions of cells may occur in multiple directions. The kelps are the only algae known to have internal tissue differentiation into conducting tissue; there is, however, no true xylem tissue as found in the 'higher' plants.
  • Reproduction :-
    1. Several species of this group reproduce vegetatively by fragmentation.
    2. Asexual reproduction :-Asexual reproduction occurs by biflagellate zoospores formed inside reproductive organs called sporangia which may be unilocular (one celled) or many celled multilocular(many celled).
    3. Sexual reproduction :- sexual reproduction takes place by the formation of flagellate gametes that are formed inside gametangia. The haploid thalli form isogamous (both male and female gametes exactly similar), anisogamous (female gamete larger than male) oogamous gametes (small flagellated male and large non-flagellated female gametes ).The sexual reproduction is through union of flagellated male and female gametes or union of flagellated male and large non-flagellated female gametes. The haploid (gametangial) and diploid (sporangial) thalli may be similar (isomorphic) or different (heteromorphic) in appearance, or the gametangial generation may be extremely reduced (Fucales).

Most brown algae have an alternation of haploid and diploid generations :-

  • Haploid gametophytes (n) produce haploid gametes by mitosis.
  • Union of male and female gametes (n) forms a zygote (2n) that grows into a diploid sporophyte
  • The sporophyte (2n) produces meiospores (n) by meiosis.
  • A meiospore germinates to become a haploid gametophyte

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.