General Characters of Bacillariophyceae & Pinnularia SMG

A brief account of the general characters of Bacillariophyceae & Pinnularia.

1. General Characters of Bacillariophyceae & Pinnularia Dr Saji Mariam George Associate Professor (Retired) Assumption College Autonomous Changanacherry

2. CLASS BACILLARIOPHYCEAE 200 genera 16,000 sps . Popularly known as Diatoms – ‘The Jewels of Plant Kingdom’ - Cell wall has silica deposits and are beautifully sculptured. Posted on  August 17, 2015  by  Katherine Faull https://www.sciengist.com

3. CLASS BACILLARIOPHYCEAE DIATOMS General Characters i ) Habitat Aquatic – freshwater, brackish water (water that has more salt than freshwater , but not as much as sea water )and marine. Many are planktonic . Some are benthic forms - attached to mud, sand, rocks.

4. Some are epiphytes attached by the mucilage covering of the frustule or by a mucilagenous stalk secreted by the Diatom . Some are epizoic . A few are symbionts in marine protozoans . Terrestrial – sub aerial – confined to moist places.

5. ii) Habit / Thallus organization / Range of Thallus structure : Microscopic Unicellular, diploid thallus . Isolated , pseudofilaments or colonies – several unicellular thalli may be held together by mucilage.

6. Variously coloured , diverse forms- like triangular, circular, semi-circular, disc –like, boat –like etc.

7. Diploneis  photo by Y. Tsukii   Protist Information Server http://protist.i.hosei.ac.jp

8. Navicula

9. Cymbella Saved from protist.i.hosei.ac.jp https://www.pinterest.com.au

10. Triceratium

11. Cells may be Radially symmetrical - Order Centrales (Centric diatoms) e.g. Cyclotella http://elmodernoprometeo.blogspot.com

12. Cells may be bilaterally symmetrical Order Pennales ( Pennate diatoms) – e.g. Pinnularia http://elmodernoprometeo.blogspot.com Protist Information Server http://protist.i.hosei.ac.jp

13. iii) Cell structure : Cell wall & Protoplast The Oceans, Their Physics, Chemistry, and General Biology.  New York:  Prentice-Hall,  c1942 1942. http://ark.cdlib.org/ark:/13030/kt167nb66r/ The gross structure of a simple diatom ( Coscinodiscus ).  a , valvular view;  b , girdle-view section of cell wall.

14. Coscinodiscus  sp. , valvular view Casco Bay off Falmouth, Maine USA 31 Oct 2007 http://cfb.unh.edu

15. Coscinodiscus  sp. , Girdle view Silicaceous spicules radiate outward in six planes . http://cfb.unh.edu

16. Cell wall is secreted by the protoplast. Inner pectin and outer silicified (made of Silica – Silicic acid) cell wall called shell or frustule or testa . The deposition of silicious material occurs in characteristic patterns or markings called striae .

17. Stria (Plural – striae ). A row of areolae on the valve. In Centric diatoms, usually oriented along radii. In Pennate diatoms, usually oriented more or less perpendicular to the apical axis. Features of the striae are important in Diatom taxonomy.

18. Striae Stephanodiscus ©  David G. Mann http://tolweb.org 

19.   Gomphosinica geitleri   showing the multiseriate striae . Image Credit: Marina Potapova https://diatoms.org

20. Frustule consists of two overlapping halves - One of these is older than the other – The older half fits closely over the younger half like the lid of a box - the upper epitheca ( epivalve ) and the lower hypotheca ( hypovalve ) . Each half or theca consists of two parts viz. valve - the main surface corresponding to the top or bottom and the connecting bands.

21. The two connecting bands are firmly united in the overlapping region called girdle. The overlapping upper portions of the connecting band is called epicingulum and the lower portions of the connecting band is called hypocingulum .

22. Image : http://wgbis.ces.iisc.ernet.in

23. The longitudinal groove along the central part of the valve is called Raphe – interrupted by thickening called nodules.

24. Pinnularia Image :https://biologyboom.com

25. In some cases, there may be outgrowths of the valves , projecting beyond the margin, called setae. Image : http://nordicmicroalgae.org

26. Diatoms are unique for their beautifully sculptured and ornamented cell wall - Hence they are often referred as the Jewels of the Plant kingdom.

27. Protoplast Living part of the thallus – Plasma membrane and cytoplasm. A large central vacuole. A single diploid nucleus. Cell organelles – Endoplasmic reticulum, Ribosomes , Mitochondria, Golgibodies etc.

28. Chromatophores enveloped by two membranes– number and shape vary with species. The matrix contain the lamellar system enclosing the pigments - chlorophyll a, c , fucoxanthin , diatomin – the principal colouring matter, diatoxanthin , carotenoids etc. Generally the diatoms are yellow brown, golden-yellow or olive -green.

29. The matrix also contain ribosomes , DNA, RNAs, enzymes etc. Pyrenoids devoid of starch sheath may be present . Reserve Food - Oil, leucosin or chrysolaminarin , volutin .

30. iv) Movement (Locomotion) : Centric diatoms are non motile. Pennate forms show gliding movements produced by cytoplasmic streaming through the grooves ( Raphe ) on the surface of the cell wall – direction of locomotion is opposite to the direction of cytoplasmic streaming.

31. v) Reproduction Vegetative - Cell division . Sexual - Isogamy in Pennales ; Oogamy in Centrales .

32. i )Vegetative Reproduction or Cell division By Fission Plane of division is at right angles to the main axis of the cell. Prior to cell division, the cell increases in size at right angles to the girdle. Karyokinesis – the nucleus divides mitotically .

33. The epitheca and hypotheca separate from each other. Cytokinesis along the median longitudinal axis - One of the resulting protoplast occupy the epitheca and the other will occupy the hypotheca – The missing theca will be soon secreted by each protoplast which becomes the hypotheca in each cell.

34. The daughter cell that has received the parent epitheca will attain almost the same size of the parent cell where as the daughter cell that has received the parent hypotheca will be slightly smaller in size. As a result of cell division, one cell maintains the size of the parent cell while the other cell becomes slightly smaller and thus during successive generations, the size of one of the daughter cells is progressively reduced.

35. ii) Sexual Reproduction Isogamous in Pennales and oogamous in Centrales . Influenced by temperature, light, nutrition etc. Most species are homothallic or monoecious .

36. In both Pennales and Centrales , the zygote formed by sexual reproduction transforms to a specialized spore called Auxospore (Growth spore). Auxospore formation is a ‘restorative process’ for restoring cell size, vitality and survival ability.

37. Auxospore formation is associated both with sexual reproduction and parthenogenesis. That is, auxospore is either a modified zygote or a modified parthenogenetic female gamete. Auxospore formation in Pennales

38. Auxospore formation by Syngamy Types : i ) Development of a single Auxospore by two conjugating cells. Two conjugating cells come in pair , get surrounded by a common mucilagenous sheath and undergo meiotic nuclear division. Of the four haploid nuclei formed in each cell, three degenerate and the remaining one transforms to a gamete.

39. The gametes are liberated – The gametes fuse forming a zygote. After some time the zygote elongates in a plane , parallel to the long axis of the parent cell and transforms to an auxospore . During this , it gets enclosed by a silicified pectic membrane called perizonium .

40. Image :http://bioexampreparation.blogspot.com

41. ii ) Development of two Auxospores by two conjugating cells. Common method of auxospore formation in Pennales . Two cells come in pair, get enveloped by a common mucilagenous sheath. Divide meiotically , producing four haploid nuclei in each cell – Two nuclei of each conjugant degenerate.

42. Cleavage of cytoplasm occurs resulting two uninucleate cells from each conjugant which transform into gametes. Cross fertilization occurs between the gametes of the two conjugants resulting in the formation of two zygotes which elongate and get encased by perizonium and transform into auxospores .

43. Image :http://bioexampreparation.blogspot.com

44. iii ) Development of Auxospores by Autogamy Less common in Pennales . The cell divide meiotically and form four haploid nuclei – two nuclei degenerate and the remaining two fuse to form a zygote. The zygote elongates and transforms into an auxospore .

45. Image: http://bioexampreparation.blogspot.com

46. 2. Auxospore formation by Parthenogenesis ( Apogamy ) The diploid nucleus undergoes one or more mitotic divisions and forms daughter nuclei. Only one nucleus survives, all other nuclei degenerate. The surviving nucleus get surrounded by cytoplasm and transforms into an auxospore .

47. Image :http://bioexampreparation.blogspot.com

48. Auxospore formation in Centrales Auxospore formation is associated only with sexual reproduction - That is, auxospore is a modified zygote. Development of Auxospores by Autogamy Similar to that in Pennales . The cell divide meiotically and form four haploid nuclei – two nuclei degenerate and the remaining two fuse to form a zygote. The zygote elongates and transforms into an auxospore .

49. 2. Development of Auxospores by Oogamy Vegetative cells transform into male sex organ – antheridium ( spermagonium ) and female sex organ – oogonium . Diploid cell of antheridium divides meiotically and form four haploid nuclei – Each one develop into uninucleate and uniflagellate antherozoid . ( spermatozoid) . On maturity, antherozoids are liberated.

50. In the oogonium , diploid cell divides meiotically and form four haploid nuclei. Of these, three degenerate and the remaining one transforms into an ovum. In some species, after the first meiotic division, one of the two haploid nuclei degenerates and the other nucleus divides into two, of which one degenerates and the other transforms into an ovum.

51. Image :http://bioexampreparation.blogspot.com

52. The motile antherozoids swim towards the ovum and one of them fertilizes it and form the zygote. The zygote elongates, secretes perizonium and transforms into an auxospore .

53. Germination of Auxospore Auxospore enlarges in size and restores the size of the parent cell. The diploid nucleus divide mitotically to form two daughter nuclei, of which one degenerates and the other develops into a diatom.

54. PINNULARIA Systematic Position Division : Bacillariophyta Class : Bacillariophyceae Order : Pennales Family : Naviculoideae Genus : Pinnularia

55. Typical Pennate Diatom. Mostly fresh water forms, a few are marine. Unicellular – e.g. Pinnularia viridis Colonial – e.g. P. socialis

56. Unicellular Pinnularia viridis Colonial P. socialis

57. Cells are elongate, elliptical with rounded ends. The cell wall has an inner pectic layer and an outer silica layer. The silicious cell wall or Frustule is made up of two halves or valves, one fitting into other like a box. The upper older valve is epitheca or epivalve and the lower, younger valve is hypotheca or hypovalve .

58. The two valves instead of overlapping each other directly, are attached to each other by incurved edges of the valves called girdles or connecting bands. (Each valve has two parts viz. a flat surface called valve face and its margin called mantle or girdle – The girdle of the two valves overlap to form the girdle band ) . The epicingulum   or upper girdle   is the side walls of the epitheca . Hypocingulum   or   lower girdle is the side walls of the hypotheca .

59. Image :https://istudy.pk

60. The view of the cell from the top is called valve view and the view from the side is called girdle view. Pinnularia -Valve view Pinnularia - Girdle View (Side View) © A.J. Silverside

61. The longitudinal groove along the central part of the valve is called raphe . Raphe are associated with gliding movements . Raphae are interrupted by thickening called nodules. Raphe has wider outer and inner ends and narrow middle portion.

62. Raphe has three round wall thickenings – nodules - The one situated in the centre called the central nodule and two located at the two poles called polar nodules . Stria (Plural – Striae ). A row of areolae (fine grooves) , or punctae   (tiny holes, singular: puncta )   on the valve – appear like transverse parallel ribs.   The areas between striae are thickened, rib-like, and termed  costae (singular: costa ).

63. Enlarged view of the Striae and rib-like Costae

64. Inside the cell wall is the protoplast . Cytoplasm is located in the periphery. The single nucleus is suspended in the center of the vacuole by a cytoplasmic bridge. A large central vacuole.

65. Two large, elongated, green or brown coloured chloroplast are parietal in position and contain chlorophyll a, chlorophyll c, beta-carotenes and xanthophylls including the fucoxanthin which impart the characteristic color to the thallus . One or two pyrenoids may be present. The chief food reserve is in the form of oil drops or chrysolaminarin .

66. PINNULARIA - Movement (Locomotion) The Pinnularia exhibit characteristic gliding movements which are brought about by circulation of streaming cytoplasm within the raphe and by the secretion of the mucilage through the raphe and its hydration.

67. PINNULARIA - Reproduction Reproduces vegetatively by cell division and sexually by the production of auxospores . i ) Vegetative Reproduction – Cell division This is the most common method of reproduction that results in the formation of two daughter cells of slightly different size.

68. The first indication of division, is expansion of the protoplast that causes a slight separation of overlapping epitheca and hypotheca . This is followed by mitotic division of the nucleus in a plane perpendicular to the valves.

69. The nuclear division is followed by duplication of cell organelles. Later, the protoplast divides in a plane parallel to the valves. One daughter protoplast lies within the epitheca and the other within the hypotheca . Each daughter protoplast secretes a new half next to its girdle and free face.

70. At this stage the parent connecting bands separate and each cell becomes an independent cell. The two frustules of each parent cell act as epitheca of the two daughter cells. Therefore, newly formed half wall is always hypotheca of daughter frustule .

71. The utilization of two old half walls as epithecae for the daughter cells results in one cell of the same size as the parent cell and the other being slightly smaller than the original parent cell. This progressive diminution in size result in a population with smaller cells, however this reduction in size does not continue indefinitely. It is checked by formation of auxospores which give rise to vegetative cells of maximum size for the species.

72. Pinnularia - Cell Division

73. ii ) Auxospore ( Renewal spores or Rejuvenescent cells )formation Gradual reduction in the size of cells due to continuous cell division is compensated by auxospore formation after sometimes and thus the normal size is restored. Auxospores are formed only when the individual cells are highly reduced in size. During auxospore formation, abundant mucilage is secreted by the cell. This results in pushing the valves apart and liberation of protoplast.

74. The protoplast grows to the maximum size of the particular species of Pinnularia . It then surrounded by a silicified pectin membrane called the perizonium . There is no nuclear division. The protoplast secretes a fresh epitheca and a hypotheca and thus a large new Pinnularia cell is formed.

75. Sexual Reproduction by Auxospore formation The sexual reproduction is isogamous and is influenced by various factors like temperature, light conditions and nutrition. The  Pinnularia  species are monoecious . In  Pinnularia two cells from common parent or different parents become enveloped in a common mucilaginous sheath (conjugation). The nuclei of both cells divide by meiosis. Out of the four haploid nuclei, three disintegrate and the remaining one transforms into a gamete.

76. The gametes are liberated from the parent frustules and fuse to form a zygote which transforms into an auxospore . The auxospore secretes new valves and form new Pinnularia cell . The auxospore expands to restore the original cell size of the Pinnularia species.

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