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Uploaded byDr. Renu Jangid
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Algae.pptx

Algae have diverse cell structures and life cycles. Prokaryotic algal cells have no nucleus but have a plasma membrane and cytoplasm divided into a peripheral, pigmented region and central non-pigmented region. Eukaryotic algal cells contain membrane-bound organelles and a single nucleus. Reproduction occurs through fragmentation, fission, budding or the formation of spores like zoospores, akinetes or tetraspores. Life cycles include haplontic with a dominant haploid generation, diplontic with a diploid dominant generation, and diplohaplontic with alternating haploid and diploid generations that may be isomorphic or heteromorphic.

Science◦
Algae Cell Ultrastructure and reproduction Dr. Renu Jangid
ALGAE • The term algae was first introduced by Linnaeus in 1753 but he had used this term for the plants we know as bryophytes now a days. It was A. L. de Jusssieu (1789) who delimited the term for the algae only known to us at present. • Algae are chlorophyll bearing thalloid plants with no differentiation into tissue or tissue system; however some algae have advanced complex thalli with slight differentiation of true tissues (Ulva, Sargassum, etc.). • In India, literature provided the evidence of phycology since 18th century where major interests were on macroscopic forms of algae. F. E. Fritsch (1907) published a marvelous work on subaerial and fresh water algae from Ceylon. He published the classification of algae in his book ‘Structure and Reproduction of the algae’.
CELL ULTRASTRUCTURE • Algal cells are:- Prokaryotic, Eukaryotic and Mesokaryotic. • Ultrastructure of Prokaryotic :- The prokaryotic algal cell can be divided into two parts- Outer cellular covering and cytoplasm. • (A) Outer cellular covering: a) Slime layer or mucilaginous sheath- Is is a characteristic feature of all cyanobacteria (blue-green algae). In the sheath, fibrils of peptic acid and mucopolysachharides are arranged reticulately so that the sheath appears to be homogenous. Its main function is to retain absorbed water to protect the cell from desiccation. • B)Cell wall- The cell wall is rigid and made up of mucopeptide. It consists of four layers which are named as L1, L2, L3 and L4. • c) Plasma membrane- It is made up of lipid bilayer.
(B) Cytoplasm:- It is differentiated into chromoplasm and centroplasm. a) Chromoplasm- It is the outer and peripheral pigmented region. It consists of parallel photosynthetic lamellae or thylakoids. These lamellae contain chlorophyll a, carotenoids and phycobilins. The membrane bound organelles are not found in chromoplasm. However, 70s ribosomes, αgranules, β-granules, structural granules, polyhedral bodies, gas vacuoles etc are found in chromoplasm. Gas vacuoles are made up of vesicles. These vacuoles provide buoyancy to the cell. Centroplasm- It is the central colourless region which consists of chromatin material or DNA material that is not bounded with histone proteins. Hence no organized nucleus is found.
Ultrastructure of Eukaryotic algal cell • a) Cell wall- It made up of cellulose. In some brown algae, alginic acid is present in their cell wall. Certain algae, particularly the diatoms possess silicified cell wall. Xylan, agar and carrageenin are present in cell wall of red algae. • b) Plasma membrane- It is made up of protein lipid bilayer. • c) Cytoplasm- Inside the plasma membrane dense cytoplasm is present. In cytoplasm, membrane bound cell organelles are present. Ribosomes are of 80s type. Cells of most algae contain one chloroplast per cell with the exception of few species whose cells have more than one chloroplast. single nucleus is present in most of the algae, but multinucleate eukaryotic algal cell are also found in considerable number. DNA is bounded with the histone proteins. • e) Flagella- In motile algal cell, thallus bears flagella which originates from the basal granules or blepharoplast. It shows a typical 9+2 arrangement.
3. Mesokaryotic cell The mesokaryotic cell is the intermediate cell of the Prokaryotic cell and the Eukaryotic cell. Dodge has first used the term mesokaryotic in 1966 for the cell that contains an intermediate nucleus (both eukaryotic and prokaryotic characters are present). Mesokaryotic cell contains a membrane-bound organized nucleus. The cells are medium-sized and have membrane-bound cell organelles like – mitochondria, plastids, endoplasmic reticulum, etc. The organisms having mesokaryotic cells are called mesokaryotes.
Plastids and Photosynthetic Pigments • The most prominent feature of an algal cell is the plastid, which makes an important characteristic of an algal cell for classification. Plastids which consist of chlorophyll a and chlorophyll b are called CHLOROPLASTS and the one which lacks chlorophyll b are called CHROMATOPHORES. • (i) Cup shaped: Chlamydomonas , Volvox • (ii) Discoid: Chara , Vaucheria , Dinophyceae, Bryopsidophyceae and many diatoms • (iii) Girdle or C shaped: Ulothrix • (iv) Ribbed: Volvocales • (v) Reticulate: Oedogonium , Hydrodictyon and Cladophora • (vi) Spiral or ribbon shaped: Spirogyra • (vii) Stellate: Zygnema
REPRODUCTION • Reproduction in algae takes place by vegetative, asexual and sexual methods. • A. Vegetative Reproduction • a) Fragmentation- In this process, filament breaks into fragments and each fragment give rise to a new filamentous thallus. The common examples are Ulothrix, Spirogyra, Oedogonium, Zygnema, Oscillatoria, Nostoc etc. • b) Fission- This process is common in desmids, diatoms, and other unicellular algae. The cell divides into two by mitotic division and then separation occurs through septum formation. • c) Adventitious branches- Protonema develops in certain algae like Chara and give rise to new thalli when detached from parent thallus. These adventitious branches develop mainly on the rhizoids. Other examples include Dictyota and Fucus. • d) Tubers- Tubers are spherical or globular bodies which are found on lower nodes or rhizoids of Chara. These tubers when detach from parent plant can give rise to new thalli.
f) Budding- In some algae like Protosiphon, budding takes place which results in new individuals. e) Amylum stars- In Chara, star shaped bodies filled with amylum starch are formed that give rise to new individual after detaching from the parent plant. g) Hormogonia- In some cyanobacteria like Nostoc, Cylindrospermum hormogonia develop that may give rise to new thalli. These hormogonia are of varying lenths and may develop at the place of heterocysts in the thallus. These hormogones are produced by breakage of filament into two or more cells. h) Hormospores or hormocysts- Hormospores are thick walled hormogones which are produced in drier conditions. Budding Fission Fragmentation
B. Asexual Reproduction • a. Zoospores- These are flagellated asexual spores which are formed in zoosporangium or directly from the vegetative cells. The zoospores may be bi, quadric or multiflagellate. e.g., Chlamydomonas (biflagellate), Ulothrix, Cladophora (quadriflagellate), Vaucheria, Oedogonium (multiflagellate). • b. Hypnospores- Hypnospores are thick walled, non flagellated spores with plenty of food reserves. They are produced under unfavourable conditions by some green algae. They germinate into new plants with return of favourable environmental conditions. e.g., Chlamydomonas, Protosiphon. • c. Akinetes- In filamentous forms, certain vegetative cells become thick walled elongated structures called as akinetes. It can survive under unfavourable conditions and can give rise to new individual on occurrence of favourable conditions. e.g., Anabaena Multiflagellate zoospore
d. Aplanospores- These are non flagellated thin walled asexual spores that are formed in majority of aquatic algae by the failure of flagella formation due to some unfavourable conditions. e. Tetraspores- Tetraspores are non motile asexual spores that are formed in some members of Rhodophyceae and Phaeophyceae. f. Monospores- Single spore formed in the sporangia is called monospore. Eg. Rhodophyceae. g. Autospores- These are actually aplanospores which appear identical to the parent cell. Hence referred as autospores. h. Heterocysts- According to some phycologists, heterocysts are sometimes able to reproduce asexually. These structures are found in blue green algae and depending upon the position in thallus they may be terminal or interstitial. i. Auxospores- auxospores are produced in the member of Bacillariophyceae. j. Carpospores- Carpospores are produced in carposporophyte of red algae. k. Paraspores- In some members of Rhodophyceae, paraspores are formed that give rise to new individual.
l. Statospores- Statospores are produced in the members of Bacillariophyceae and Xanthophyceae. Statospores formed by diatoms are thick walled. m. Neutral spores- In some alga, the protoplast of vegetative cells directly functions as spores and these are called neutral spores. e.g., Asterocystis, Ectocarpus. n. Nannocytes- In the members of chroococcales, the cell content divide repeatedly to produce numerous very small spores. The name nannocytes to these very small spores was given by Geitler. E.g., Macrocystis, Gloeocapsa. o. Gongrosira stage of Vaucheria- In Vaucheria, the protoplast divides into several cysts like structures or hypnospores. This stage looks like an algal form ‘Gongrosira’. Each hypnospore or cyst may give rise to new thallus.
C. Sexual Reproduction- • a) Isogamous- When fusing gametes are morphologically similar and physiologically different (+ and -) then the sexual reproduction is called as isogamous. E.g., Chlamydomonas, Ulothrix, Zygnema, Spirogyra. • b) Anisogamous- In anisogamous sexual reproduction fusing gametes are morphologically as well as physiologically different. The gametes are produced in different gametangia. The microgametes are male gametes while macrogametes are female gametes. e.g., Chlamydomonas. • c) Oogamous- Oogamy is the most advanced type of sexual reproduction in which microgamete or male gamete fuses with a large female gamete or eg. Male gametes are produced in antheridium while female gamete or egg is produced within a structure called as oogonium. During fertilization male gamete reaches the oogonium to fertilize the egg and a diploid zygote is formed. e.g., Chlamydomonas.
LIFE CYCLES IN ALGAE • 1. Haplontic Life Cycle- In this type of life cycle the main plant body is gametophytic (haploid). Eg. Green algae.
• 2. Diplontic Life Cycle- The dominant plant thallus is diploid. The thallus reproduces sexually by gametes that are formed by meiosis in sex organs. Eg. Diatoms
3. Diplohaplontic Life Cycle- In this type of life cycle two different generations alternate each other. a. Isomorphic- In this life cycle, alternating sporophyte and gametophyte are morphologically similar. E.g. Ectocarpus, Ulva, Cladophora, Dictyota etc. b. Heteromorphic- In this life cycle, alternating generations are morphologically dissimilar. E.g., Laminaria, Desmarestia etc.
4. Haplobiontic Life Cycle- This is either diphasic or triphasic life cycle. In Nemalion a red alga exhibits two haploid phases (gametophyte, carposporophyte) and a diploid zygote. Batrachospermum (red alga) do exhibit haplobiontic life cycle but it is triphasic as it consists of three prominent haploid phases (main gametophyte, carposporophyte and chatransia phase). 5. Diplobiontic Life Cycle- This life cycle consists three phases of which two phases are diploid and one is haploid. Tetraspores eventually develop into main gametophytic plant thallus. This type of life cycle is exhibited by some members of red algae such as Polysiphonia.
Algae.pptx