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 1 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. 3 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 4 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 . 6 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). 7 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 epithe- ca, 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. 8 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. 9 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 10 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