General Characters of Phaeophyceae & Life Cycle of Sargassum SMG

A brief account of the general characters of Phaeophyceae & Life Cycle of Sargassum

1. General Characters of Phaeophyceae & Life Cycle of Sargassum Dr. Saji Mariam George Associate Professor Assumption College Autonomous Changanacherry

2. Phaeophyceae (Brown Algae) General Characters 250 genera and 1500 species. i ) Habitat Most of them are marine . e.g. Sargassum , Dictyota . A few are fresh water forms e.g. Pleurocladia , Bodanella , Heribandiella etc.

3. Some species occur both in fresh and salt water . e.g. Pleurocladia lacustris Benthonic Lithophytes - growing attached to rocks, stones or other solid objects with the help of holdfast. They develop air bladders which help them to float on the surface of water. Algin coating of the thallus protect from desiccation when exposed to air.

4. Sargassum https://en.wikipedia.org

5. Dictyota

6. Bodanella Photograph by © by David Patterson and Bob Andersen posted  online http://cfb.unh.edu

7. Pleurocladia Photograph by ©  Wehr  et al. (2013) posted  online Wehr , J.D., R.  Stancheva , K.  Truhn  and R.G. Sheath 2013. http://cfb.unh.edu

8. Laminaria Image:https :// www.pinterest.fr

9. Macrocystis Image: https://www.flickr.com

10. ii) Habit / Thallus organization / Range of Thallus structure : Some brown algae are primitive and simple having a heterotrichous habit – the plant body has a creeping prostrate system and an erect system – both systems may be equally developed, erect system more prominent or extremely reduced in some species. e.g. Ectocarpus

11. Ectocarpus Photograph from Kobe University Macro-Algal Culture Collection (KU-MACC), Japan, posted  online

12. In higher Brown Algae , heterotrichous condition is absent. They have a pseudoparenchymatous ( Haplostichous ) or parenchymatous ( Polystichous ) organization. In some higher members , the thallus is differentiated into a cylindrical lower portion fixed to the substratum with the help of holdfast and a flat and dichotomously branched upper portion .

13. In most highly evolved brown algae, the thallus is extensively developed , large - sized thallus , commonly called as ‘Giant Kelps’ ( e.g Sargassum , Laminaria , Macrocystis ) . In them , the thallus has a cylindrical lower part called stipe or stalk and a flat , band – shaped, fan – shaped or forked upper part called blade or lamina . Stipe has a well developed attaching disc. In many higher forms, there are three well differentiated cell layers viz. epidermis, cortex and medulla.

14. Some Brown Algae are like miniature trees . The tip of its thick stipe divide many times forming blade – like branches. ( e.g. Lessonia flavicans ) Lessonia flavicans Image: https://macroalgae.org

15. Some have palm like appearance e.g. Postelsia palmaeformis . Hence is commonly called as ‘Sea Palm’. Its thallus consists of a short , thick and flexible stipe or axis which is anchored to the substratum by a much branched Holdfast and an apical crown of blades or laminae . Postelsia palmaeformis Image:  idaholyoaks4.blogspot.com https://www.pinterest.com

16. iii) Cell wall Two layered, inner cellulose and outer gelatinous layer of Pectin, Algin , Fucoidan and Fucin and in some members Callose . The gelatinous substances absorb and retain water and protect from desiccation.

17. iv)Protoplast Plasma membrane and Cytoplasm - contains nucleus, cell organelles, vacuoles etc. Major photosynthetic pigments include Fucoxanthin , Chlorophylls a, b, ; beta carotene and xanthophylls and violaxanthin . Minor pigments – diatoxanthin , diadinoxanthin . Pyrenoids are absent or single (solitary) , stalked. Fucosan vesicles

18. v) Reserve foods Laminarin and Mannitol . In some cases, chlorides, iodides, glycerol, sucrose etc. are also present. vi) Reproduction Vegetative Asexual Sexual

19. i ). Vegetative Reproduction i ) Fragmentation – the thallus accidentally breaks into small pieces or fragments. Each fragment can develop into a new thallus . ii) Propagules - Stalked vegetative branches or outgrowths from the main thallus . – later develops into new thallus .

20. ii). Asexual Reproduction – By spores produced in sporangia (Singular- sporangium). a) Unilocular sporangia Single chambered, sac – like sporangia. Develop only in diploid sporophytic plants. Haploid, biflagellate zoospores ( meiospores ) are produced by meiosis. The zoospores on germination, form haploid plants . In some cases, non –motile, aplanospores are produced.

21. Unilocular Sporangia Image:http :// www.biologydiscussion.com

22. Ectocarpus :Unilocular Sporangia Image:https :// pediaa.com

23. b) Plurilocular sporangia (Neutral sporangia) These are multi locular (multi chambered). Diploid, flagellated, motile zoospores ( Mitospores –formed by mitosis). The diploid zoospores on germination form diploid plants.

24. Ectocarpus : Plurilocular sporangia Image:http :// www.biologydiscussion.com

25. Ectocarpus : Plurilocular sporangia Image: http://botanystudies.com

26. Ectocarpus : Plurilocular sporangia Photograph by © Kobe, posted at u.ac.jp http://cfb.unh.edu

27. iii). Sexual reproduction Occurs in haploid Brown algae. Three types Isogamy Anisogamy Oogamy

28. i ) Isogamy : Fusion of morphologically similar motile gametes and form diploid zygote – transform into zygospore – on germination form a diploid plant. Common in Ectocarpus (Order Ectocarpales - e.g. Ectocarpus siliculosus ) Image:http :// sciencedoing.blogspot.com

29. ii) Anisogamy Fusion of morphologically dissimilar motile gametes to form the zygote which develops into a diploid plant. One of the fusing gametes is smaller than the other, biflagellate, called microgamete - is active and motile and behaves as the male gamete. The other gamete is larger one , passive and behaves like female gamete. e.g . Ectocarpus   secundus Image:http :// sciencedoing.blogspot.com

30. Physiological Anisogamy Fusing gametes are morphologically similar, but physiologically different. One of the fusing gametes is deflagellated , becomes non-motile and behaves as the female gamete. The other motile gamete behaves as the male gamete. The fusion of these two gametes results in the formation of diploid zygote which develop into zygospore – on germination develop into diploid plant.

31. iii) Oogamy Fusion of dissimilar gametes - one of the fusing gametes is small and motile, functioning as the male gamete ( antherozoid ) and the other is large and non – motile functioning as the female gamete (ovum ) to form zygote. Image:http :// sciencedoing.blogspot.com

32. The antherozoids are formed in antheridium ( Plural – antheridia – the male sex organs) Antheridia may be unicellular or multicellular . The mature antherozoids are released from antheridia.

33. The female gamete (ovum) is formed in oogonium (Plural – oogonia ) which is unicellular – Mature ovum is released from the oogonium . Thus fertilization in most cases is external resulting in the formation of diploid zygote which on germination gives rise to a diploid plant. e.g. Sargassum

34. Alternation Of Generations A distinct alternation of generations between the haploid , gametophytic generation and the diploid sporophytic generation is seen in the life cycle of Brown Algae. Most of the Brown algae exhibit isomorphic alternation of generations where the haploid , gametophytic generation and the diploid sporophytic generation are morphologically similar.

35. Some brown algae exhibit heteromorphic alternation of generations in which the haploid, gametophytic generation and the diploid sporophytic generation are morphologically dissimilar - In such cases, the sporophytic plant may be very small and the gametophytic plant very large or vice versa. In Fucales , no distinct alternation of generations as the gametophytic generation is represented only by the male and female gametes.

36. Sargassum SYSTEMATIC POSITION Division : Phaeophyta Class : Phaeophyceae Order : Fucales Family : Sargassaceae (Sp. Sargasso = Seaweed) marine alga - 150 sps . Examples Sargassum tennerimum S. carpophyllum S. dupliation S. chrisifolium S.myriocystum S. cinerium

37. Sargasso sea A region of North Atlantic Ocean with free floating seaweed, Sargassum .

38. Thallus Structure Diploid sporophyte Size varies from 10 to 50 cm in height Thallus has a basal holdfast and a cylindrical main axis. Primary branches from the main axis are called primary laterals or branches of unlimited growth or long shoots .

39. They bear secondary laterals called ‘ Leaves’ - simple, flat, broad, leaf-like structures with a mid rib. Their margin may be entire, serrate or dentate with acute apex. They bear many dots on their surface called ostioles – They are the openings of sterile conceptacles.

40. Spherical air bladders give buoyancy to the plant to float on the surface of the water. Long, branched, cylindrical or flattened structures called receptacles bear many flask-shaped structures called conceptacles which bear sex organs and hence are reproductive in function.

41. Sargassum Image:http :// cdn.biologydiscussion.com

42. Internal structure (Anatomy) of the Axis of Sargassum A transverse section (T.S ) of the axis shows three parts. i ) Epidermis ( Meristoderm ) Outermost layer covered by a thin cuticle. Composed of closely packed and thin-walled, parenchyma cells. Contain chromatophores and hence photosynthetic . ii) Cortex Made of thin –walled parenchyma cells with some intercellular space . Storage in function. iii ) Medulla At the center of the axis, composed of thick walled, narrow cells Function is conduction.

43. Internal structure of the Axis of Sargassum Image:http :// www.biologydiscussion.com

44. Internal structure (Anatomy) of the ‘ Leaf’of Sargassum A transverse section (T. S.) of the ‘Leaf’ shows the following features. i ) Epidermis ( Meristoderm ) Outermost layer covered by a thin cuticle. Composed of closely packed and thin-walled cells. ii ) Cortex Made of thin –walled parenchyma cells with some intercellular space . Storage in function.

45. Medulla It is present in the mid rib region. It is composed of thick walled, narrow cells .There are several sterile conceptacles called cryptoblasts on both surface of the leaves - These are flask - shaped structures with external openings called ostioles or cryptostomata . Ostioles appear as dots on the ‘leaf’ surface. Multicellular branched hairs called paraphyses arise from the wall of the sterile conceptacles and they come out through cryptostomata .

46. Internal structure (Anatomy) of the ‘ Leaf’of Sargassum Image :http://www.biologydiscussion.com

47. Internal structure (Anatomy) of the Air bladder of Sargassum Air bladder has A single layered meristoderm Multilayered cortex Medulla absent At the centre, a hollow cavity filled with air and gases. Growth – is apical , initiated by an apical cell. http://www.biologydiscussion.com

48. Reproduction in Sargassum Vegetative - Fragmentation e.g. S. natans Sexual - By oogamy Plant body is diploid Male sex organ – Antheridium Female sex organ – Oogonium Sex organs are produced in special flask- shaped cavities called conceptacles . Antheridia bearing conceptacles are called male conceptacles and the oogonia bearing conceptacles are called female conceptacles.

49. The conceptacles opens out through an ostiole , protected by hair – like periphyses . The floor of the conceptacle contains multicellular hairs called paraphyses . Some species are monoecious – i.e , male and female sex organs develop in the same plant on separate conceptacles on specialized fertile branches called receptacles. Some species are dioecious – male and female conceptacles are borne on receptacles on separate plants.

50. Antheridium Many antheridia are produced inside male conceptacle. Borne on branched paraphyses intermixed with sterile paraphyses . Each antheridium is small, ovoid, with a two – layered wall – an outer firm exochite and an inner gelatinous endochite .

51. When detached from the conceptacle , the antheridium comes out of the ostiole . Each antheridium contains about 64 antherozoids . Each antherozoid is a pear-shaped structure with two laterally inserted flagella . The antherozoids are liberated by the gelatinization of the antheridial wall.

52. Transverse section(T.S. ) of receptacle showing one male conceptacle of Sargassum Image:http :// www.biologydiscussion.com

53. Oogonium Produced on female conceptacle. Each oogonium is connected to the base of the conceptacle by a long gelatinous stalk. The oogonium is surrounded by a three layered wall – an outer, thin exochite , middle, thick mesochite and the inner, thin endochite .

54. At maturity, oogonia are discharged through the ostiole . Exochite ruptures at the time of the oogonial discharge and the mesochite elongates to form the stalk. Each oogonium contains a single egg or ovum.

55. Development of oogonium in Sargassum Image:http :// www.biologydiscussion.com

56. Fertilization The liberated antherozoids swim towards the female conceptacle and approach the egg. The male and female nuclei lie side by side for sometime and fuse to form the diploid zygote.

57. Germination of the Zygote Starts even when it is attached to the conceptacle. Zygote enlarges in size – liberated by the gelatinization of the oogonial wall. Rests on a solid medium – divides into upper and lower cells. Upper cell undergoes repeated division to form main axis . Lower cell undergoes repeated division to form hold fast.

58. Life Cycle of Sargassum Diplontic The plant body is a diploid sporophyte (2n) Haploid phase (n) is limited to the gametes. The sporophyte bears male and female conceptacles on the receptacles. The male conceptacle bears male sex organ antheridia (2n) and the female conceptacle bears female sex organ oogonia (2n).

59. Meiosis (Reduction division ) takes place during gamete formation. Antheridium produces antherozoid (n) Oogonium produces egg. Ferlilization – fusion of antherozoid and egg leads to the formation of zygote . The zygote develops into diploid sporophyte .

60. Life cycle of Sargassum http://www.biologydiscussion.com

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