Introduction General Characters, Occurrence Morphology Cell Structure, Valve Morphology Protoplast, Raphe and Locomotion, Re

Home , Auxospore, Pennales

SEMESTER-I

MBOT CC-1 UNIT-1(Phycology, Mycology& Bryophytes)

TOPIC: BACILLARIOPHYTA (Diatoms)

RITA KUMARI , H.O.D

UNIVERSITY DEPARTMENT OF BOTANY, P.P.U. PATNA

Introduction General Characters,

Occurrence Morphology Cell structure,

Valve Morphology Protoplast, Raphe and locomotion,

Reproduction: Vegetative, Sexual, Classification

Economic importance Phylogeny, Summary

Exercise and practice References/ further reading

Introduction:

The Bacillariophyceae commonly known as Diatoms. It is special branch of phycology known as Diatomology-the science of diatoms. According to F. E. Fritch (1935) Bacillariophyceae is one of the 11 classes of algae on the basis of pigments and food reserve within the algal cell. Round (1965) and Bold (1967) placed Bacillariophyceae in phylum –Chrysophyta together with Xanthophyceae and Chrysophyceae. Kumar and Singh (1974) raised the class upto phylum and recognized- Bacillariophyta. Bacillariophyta has single family Bacillariophyceae which is characterized by the following features.

Habitat Aquatic and terrestrial

Pigments Chlorophyll a and c; B-carotene,

Xanthophylls (fucoxanthin)

Food reserves Fat, Leucosin

Cell wall component Hemicelluloses, Silica

Flagella one flimmer anterior (presented only in the male gametes of

in some members) .

Growth Forms Unicellular, colonial

Reproduction Vegetative and Sexual (anisogamous and oogamous )

General Characters,

1. s (class Bacillariophyta) are a type of mainly aquatic, photosynthetic algae. 2. Plant body is , mostly unicellular , but may be colonial, or filamentous. 3. The cell wall (frustules) is impregnated with silica and consists of two valves one of which overlaps the other like the lid on a box. 4. The frustules show beautiful sculpturing and ornamentations for this diatoms are described as “jewels of the sea" and "living opal”. 5. Because of such shells, diatoms have major economic importance in industry while also having a major role in biological and chemical processes. 2

6. Vegetative cells are diploid. 7. Reserve food material is in form of fats and chrysolaminarin besides volutin, leucosine and Butschin globules. 8. Chromatophore contains pigment chlorophyll –a, c, B-carotene, fucoxanthin, diatoxanthin, diadinoxanthin. These carotenoids dominate over the chlorophyll and hence the diatoms are golden brown in colour 9. Motile stage of diatoms show a single pantonematic ( tinsel )flagella. 10. Motile member show gliding movement. 11. Reduction in the size of cell occurring during vegetative multiplication which is compensated by the production of spores known as auxopores. 12. Sexual reproduction takes place by fusion, gametes or auxospores. 13. Members are either radially symmetrical( Centrales) or bilateral symmetrical (Pennales)

Occurrence

1. Diatoms are cosmopolitan and ubiquitous in distribution, present in freshwater and oceans. Diatoms can also be found in terrestrial environments in the soil 2. Grow epiphytically on other algae and higher plants. 3. Exist in two major forms-(a) benthic-which are non planktonic, attached to some substratum such as rocks, sand or mud or be epiphytic, epizoic or endozoic and (b) planktonic-free floating.

Morphology

1. The thalli of the diatoms are unicellular, colonial or filamentous. 2. Unicellular forms are oval, rounded (Cyclotella), elongated (Pinnularia), conical, rod shaped, disc –shaped &triangular in shape. 3. Colonial diatoms are organized into filaments (Melosira)or organized into fan like structure Licmophora flabellate.

Fig 1

Cell structure,

1. The cell wall (frustules) consists of two overlapping halves, the upper one is epitheca (lid) and lower one is hypotheca.

2. Each theca consist of two portions: (a)Valve — the main surface- upper flattened top and

(b) Connecting band or cingulum (pl. cingula) — Incurved margin

Fig 2 The connecting bands, where both the theca remains fitted together, is called girdle. 3. The diatoms cells therefore viewed from two directions-

a) the girdle view

b) the valve view

4. The diatoms cell walls are made up of crystalline silica which is deposited in vesicles covered over externally by a silicalemma.

5. Reimann et. al;1966 observed a coating of organic material which he called organic skin present over the outer surface of silicalemma.

6. The composition of organic skin is not known.

Valve Morphology 5

Hendey (1971) observed four basic types of secondary structures on the basis of electron microscopy. These are:

a. Punctae (small perforations on valve surface),

b. Canaliculi (tubelike narrow channels which run through the valve surface wall),

c. Areolae (large cavity like depressions) and

d. Costae (rib-like structures formed by silica deposition on the valve surface).

Hendey has also classified the diatom walls into two types based on their nature:

i. the laminar wall, a single silicified layer; and

ii. the locular wall, basically made up of two parallel wall layers with a number of loculi in between them.

Protoplast

1. The cytoplasm is bounded by a membrane and surrounds a central vacuole.

2. Cells are uninucleate- the nucleus may be suspended by the cytoplasmic strands in Pennales and it may be embedded in the peripheral cytoplasm in Centrales.

3. The chromatophore may be numerous and discoid –in centrales and one or two large plate- like frequently lobed chromatophore in Pennales

4. Naked pyrenoids occur in some diatoms.

5. The cytoplasm contains mitochondria, golgi bodies endoplasmic reticulum .

6. The photosynthetic pigments are chlorophyll a, c, β-carotene, fucoxanthin, diatoxanthin and diadinoxanthin (Bold and Wynne, 1978)

7. The reserve food of diatoms is chrysolaminarin and oil droplets .

Raphe and locomotion,

1. All diatoms with raphe are motile.

2. In some pinnate diatoms (Cybella Cistula, Pinnularia viridis etc.) an elongated slit is present on their valves, called raphe

3. Most of the members of the order Pennales contain raphe and perform gliding movement.

4. The gliding movement or cytoplasm streaming within the raphe by alternate contraction and expansion.

5. The locomotion is affected by temperature, light etc.

6. The raphe is interrupted at its midpoint by thickening of the wall called central nodule.

7. Similar thickening is also present at the ends called polar nodules.

Reproduction: Vegetative, Sexual,

Diatom reproduces by vegetative and sexual means.

Vegetative Reproduction: 1. Vegetative reproduction performs with the help of cell division. It takes place usually at midnight or in the early morning.

2. During cell division the protoplast of the cell enlarges slightly, thus the cell increases in volume and slightly separates both the theca (epitheca and hypotheca).

Fig 3

3. Then the protoplast undergoes mitotic division and gets separated along the longitudinal axis through the median line.

4. Thus one half of protoplast remains in epitheca and the other one in hypotheca. One side of the protoplast thus remains naked. So at the side where the hypotheca behaves as epitheca, the cell becomes reduced in size. Thus with continuous cell division some cells gradually become reduced in size.

5. New silicious valves are deposited towards the naked sides of the protoplast and always behave as hypotheca of the daughter cells.

2. Sexual Reproduction: 1. In both Pennales and Centrales the vegetative cells are diploid. 2. The pattern of sexual reproduction differs in both orders — Pennales and Centrales. In Pennales the sexual reproduction is isogamous but in Centrales it is oogamous. 3. A specialized spore called Auxospore is formed in both the groups. During cell division, those cells become reduced in size, are able to regain their normal size through the formation of auxospore, so it is a “restorative process” rather than multiplication.

A. Auxospore Formation in Pennales: It takes place by gametic union(mainly isogamous), autogamy and parthenogenetically.

1. Production of single auxospores by two conjugating cells.

a. Two conjugating cells come together , get surrounded by a mucilaginous sheath. The diploid nucleus of each cell undergoes meiosis.

b. Out of four haploid nuclei, three degenerate and only one survives. The surviving nucleus behaves as gamete (n). The gametes come out from the parent frustules and unite together, to form a zygote (2n).

c. After a short period of rest the zygote elongates considerably and functions as an auxospore. (Fig 4)

2. Production of Two Auxospores by Two Conjugating Cells:

a. This is a more common method of auxospore formation. In this process the conjugating cells come very close to each other and get enclosed by mucilage sheath. The nucleus (2n) of each cell undergoes meiotic division and forms four haploid nuclei.

b. Out of four nuclei, two degenerate, and remaining two nuclei in each cell remain functional. The cytoplasm of each conjugate then divides into two parts .each containing a single haploid functional nucleus.

c. These uninucleate parts metamorphose into gametes. Gametes are of equal and unequal size. In most species fusion is isogamous and the gametes moves towards each other.

Fig 4 d. Some species show anisogamy, in which one gamete is passive or non motile and the other is motile,

Fig- 5

e. After fusion two zygotes are formed. The zygotes elongate and function as auxospores. a The auxospores develop in the silicified perizonium .

3. Production of One Auxospore by Autogamy: During this process the diploid nucleus undergoes first meiotic division. Two haploid nuclei from a single cell fuse to form the auxospore.

4. Production of Auxospore by Parthenogenesis: The entire protoplast of a cell is transformed into an auxospore parthenogenetically.

Fig 6 B. Auxospore Formation in Centrales It takes place by autogamy and oogamy: 1. Auxospore Formation by Autogamy: After meiotic division four haploid nuclei are formed. Two of four nuceli degenerate and

the remaining two fuse to form a diploid nucleus and function as auxospore. 2. Auxospore Formation by Oogamy: Oogamy takes place by the fusion of egg and sperm developed inside the oogonium and antheridia respectively

Fig 8

Antheridium or Spermatogonia a. The four uniflagellate (tinsel type) spermatozoids are formed by the content of male sex organ, Antheridium or Spermatogonia. b. The vegetative cell either act directly as spermatogonium(Cyclotella tenustriata), or divide to form 4-8 spermatogonia (Melosira varians). c. The spermatogonia are dispersed passively through water and become attached by mucilage pad to oogonia.

Oogonium: Single vegetative cell increases in size and function as an oogonium. The protoplast of oogonium undergoes meiotic division and forms four nuclei. Of the four nuclei three degenerate and the remaining one functions as an egg.

Fertilisation: After fertilization only the spermatozoid nucleus injected into the egg cell and forms auxospores Auxospores in both pennales and centrales increases in size

3. Resting Spores: These spores are formed during unfavorable conditions. Some members reproduce by the formation of thick-walled resting spores, the cysts or statospores (Melosira.)

Classification

Hustedt(1930) divided the diatoms in two order, i.e Centrales and the Pennales. the same classification has been followed by Fritsch(1935) and most of the recent workers. It is based on ornamentation of the cell wall.

(i) Centrales- radially symmetrical, raphe and pseudoraphe absent.

(ii) Pennales- bilaterally symmetrical, raphe and pseudoraphe present.

Economic Importance (Diatoms – small cells with huge potential)

1. Diatomite: After the death of diatom cells the outer coverings i.e., the silicified walls become accumulated at the bottom of water. These deposits are called diatomaceous earth, diatomite or keiselghur ( Bold and Wynne, 1978). It is very suitable for use in different industries- as filter, as absorbent, as insulator, petroleum, abrasive and food.

Phylogeny,

Because of the siliceous nature of the diatom cell wall, well preserved fossils of Bacillariophyta are available. The presence of fucoxanthin and chlorophyll- a and –c link the Bacillariophyta with the Phaeophyta, and the characteristic food reserves relate the diatoms to the Xanthophyta and the chrysophyta. 13

Summary

The Bacillariophyta, popularly called diatoms, comprise a homogeneous assemblage of unicellular and colonial forms that differ generally from other algae in possessing highly sculptured and symmetrically ornamented cell walls. The main characters that collectively differentiate the Bacillariophyta from other phyla are: (1) the diploid nature of vegetative cells; (2) the presence of chlorophyll-c together with fucoxanthin, diatoxanthin and diadinoxanthin in chromatophores, the usual brown colouration being due to the preponderance of carotenoid pigments over chlorophylls; (3) the silicified nature of cell walls which consist of two highly perforated overlapping pieces sometimes bearing structures like spines and bristles; (4) the storage of oil and chrysolaminarin and not starch in food reserves; and (5) the reduction in size of cells occurring during vegetative multiplication compensated by the production of spores known as auxospores.

Exercise and practice

A. Long answer type

1. Give a general account of diatoms. 2. Give an account of auxospore formation

3.Describe Sexual reproduction in diatoms

B. Short answer type:

Write short notes on the following

1. Diatomaceous earth 2. Auxospores 3. Economic Importance of diatoms

4. Cell structure of diatoms 5. Locomotion in diatoms. 6. Cell division in diatoms/

References/ further reading

1 A TEXT BOOK OF ALGAE 1995 H.D. KUMAR AND H.N. SINGH (EWP)

2. A TEXT BOOK OF ALGAE 2008 O.P. SHARMA Tata McGraw-Hill/