Algae

Algae

Rain brings joy and life to various organisms on earth. Have you noticed some changes in and around you after the rain? Could you identify the reason for the slippery nature of the terrace and green patches on the wall of our home, green colour of puddles and ponds? Why should we clean our water tanks very often? The reason is algae. Algae are simple plants that lack true roots, true stems and true leaves. Two-third of our earth’s surface is covered by oceans and seas. The photosynthetic plants called algae are present here. More than half of the total primary productivity of the world depends on this plant group. Further, other aquatic organisms also depend upon them for their existence.

Algae are autotrophs, and grow in a wide range of habitats. Majority of them are aquatic, marine (Gracilaria, and Sargassum) and freshwater (Oedogonium, and Ulothrix) and also found in soils (Fritschiella, and Vaucheria ). Chlorella lead an endozoic life in hydra and sponges whereas Cladophora crispata grow on the shells of molluscs. Algae are adapted to thrive in harsh environment too. Dunaliella salina grows in salt pans (Halophytic alga) . Algae growing in snow are called Cryophytic algae. Chlamydomonas nivalis grow in snow covered mountains and impart red colour to the snow (Red snow). A few algae grow on the surface of aquatic plants and are called epiphytic algae ( Coleochaete, and Rhodymenia) . The study of algae is called algology or phycology. Some of the eminent algologists include F.E. Fritsch, F.E. Round, R.E. Lee, M.O.Parthasarathy Iyengar, M.S. Randhawa, Y. Bharadwaja, V.S. Sundaralingam and T.V.Desikachary.

1. General Characteristic features

The algae show a great diversity in size, shape and structure. A wide range of thallus organisation is found in algae. Unicellular motile (Chlamydomonas), unicellular non-motile (Chlorella), Colonial motile (Volvox), Colonial non motile (Hydrodictyon), siphonous (Vaucheria), unbranched filamentous (Spirogyra), branched filamentous (Cladophora), discoid (Coleochaete) heterotrichous ( Fritschiella), Foliaceous (Ulva) to Giant Kelps (Laminaria and Macrocystis). The thallus organization in algae is given in Figure 2.3.

Algae are Eukaryotes except blue green algae. The plant body does not show differentiation into tissue systems. The cell wall of algae is made up of cellulose and hemicellulose. Siliceous walls are present in diatoms. In Chara the thallus is encrusted with calcium carbonate. Some algae possess algin, polysulphate esters of polysaccharides which are the sources for the alginate,­ agar agar and Carrageenan. 

The cell has a membrane bound nucleus and cell organelles like chloroplast, mitochondria, endoplasmic reticulum, golgi bodies etc., Pyrenoids are present. They are proteinaceous bodies found in chromatophores and assist in the synthesis and storage of starch. The pigmentation, reserve food material and flagellation differ among the algal groups.

Algae reproduces by vegetative, asexual and sexual methods (Figure 2.4). Vegetative reproduction includes fission (In unicellular forms the cell divides mitotically to produce two daughter cells Example: Chlamydomonas); Fragmentation (fragments of parent thallus grow into new individual Example: Ulothrix ) Budding (A lateral bud is formed in some members like Protosiphon and helps in reproduction) Bulbils, (a wedge shaped modified branch develop in Sphacelaria) Akinetes (Thick walled spores meant for perennation and germinates with the advent of favourable condition Example: Pithophora). Tubers (Structures found on the rhizoids and the lower nodes of Chara which store food materials).

Asexual reproduction takesplace by the production of zoospores( Ulothrix, Oedogonium) aplanospore(thin walled non motile spores Example: Vaucheria);

Autospores (spores which look similar to parent cell Example: Chlorella ); Hypnospore (thick walled aplanospore – Example: Chlamydomonas nivalis); Tetraspores (Diploid thallus of Polysiphonia produce haploid spores after meiosis).

Sexual reproduction in algae are of three types 1. Isogamy (Fusion of morphologically and Physiologically similar gametes Example: Ulothrix) 2. Anisogamy (Fusion of either morphologically or physiologically dissimilar gametes Example: Pandorina) 3. Oogamy (Fusion of both morphologically and physiologically dissimilar gametes.

Example: Sargassum). The life cycle shows distinct alternation of generation.

2. Classification

F.E. Fritsch proposed a classification for algae­ based on pigmentation, types of ­flagella, ­reserve food materials, thallus­ structure and ­reproduction. He published his classification in the book “The structure and  reproduction  of  the  Algae”(1935).

He classified algae­ into 11 classes namely Chlorophyceae, Xanthophyceae,­ Chryso-phyceae, Bacillariophyceae, Cryptophyceae, Dinophyceae, Chloromonadineae uglenophyceae,­ Phaeophyceae, Rhodophy-ceae, Cyanophyceae (Table 2.2).

The salient features of Chlorophyceae, Phaeophyceae and Rhodophyceae are given below.

Chlorophyceae

The members are commonly called ‘Green algae’ . Most of the species are aquatic(Fresh water-Spirogyra, Marine -Ulva). A few are terrestrial(Trentipohlia). Variation among the shape of the chloroplast is found in members of algae. It is Cup shaped (Chlamydomonas), Discoid (Chara), Girdle shaped, (Ulothrix), reticulate (Oedogonium), spiral (Spirogyra), stellate(Zygnema), plate like(Mougeoutia). Chlorophyll ‘a’ and Chlorophyll ‘b’ are the major photosynthetic pigments. Storage bodies called pyrenoids are present in the chloroplast and store starch. They also contain proteins. The cell wall is made up of inner layer of cellulose and outer layer of Pectin. Vegetative reproduction takes place by means of fragmentation and asexual reproduction is by the production of zoospores, aplanospores and akinetes­. Sexual reproduction is present and may be isogamous, anisogamous or ­Oogamous. Examples for this group of algae includes Chlorella, Chlamydomonas, Volvox, Spirogyra, Ulothrix, Chara and Ulva.

Phaeophyceae

The members of this class are called ‘Brown algae’. Majority of the forms are found in marine habitats. Pleurocladia is a fresh water form. The thallus is filamentous (Ectocarpus) frond like (Dictyota)or may be giant kelps (Laminaria and Macrocystis). The thallus is differentiated into leaf like photosynthetic part called fronds, a stalk like structure called stipe and a holdfast which attach thallus to the substratum.

The Pigments include Chlorophyll a, c, carotenoids and Xanthophylls. A golden brown pigment called fucoxanthin is present and it gives shades of colour from olive green to brown to the algal members of this group. Mannitol and Laminarin are the reserve food materials. Motile reproductive structures are present. Two laterally inserted unequal flagella are present. Among these one is whiplash and another is tinsel. Although sexual reproduction ranges from isogamy to Oogamy, Most of the forms show Oogamous type. Alternation of generation is present (isomorphic, heteromorphic or diplontic). Examples for this group include Sargassum, Laminaria, Fucus and Dictyota.

Rhodophyceae

Members of this group include ‘Red algae’ and are mostly marine. The thallus is multicellular, macroscopic and diverse in form. Porphyridium is the unicellular form. Filamentous (Goniotrichum) ribbon like (Porphyra) are also present. Corallina­ and Lithothamnion are heavily impregnated with lime and form coral reefs. Apart from chlorophyll a, r-phycoerythrin and r-phycocyanin are the photosynthetic pigments. Asexual reproduction takes place by means of monospores, neutral spores and tetraspores.

The storage product is floridean starch. Sexual reproduction is Oogamous. Male sex organ is spermatangium which produces spermatium. Female sex organ is called carpogonium. The spermatium is carried by the water currents and fuse with egg nucleus to form zygote. The zygote develops into carpospores. Meiosis occurs during carpospore formation. Alternation of generation is present. Examples for this group of algae include Ceramium, Polysiphonia, Gelidium, Cryptonemia­ and Gigartina.

3. Economic Importance

The Economic importance of Algae is given in Table 2.3.

4. Oedogonium

Class – Chlorophyceae

Order - Oedogoniales

Family -Oedogoniaceae

Genus – Oedogonium

Oedogonium is a freshwater , filamentous alga and occurs in ponds, lakes and stagnant water. The filaments are attached to rocks. Some are epiphytic on aquatic plants. Oedogonium terrestre is a terrestrial form and grow in moist soils. The young filaments are attached but older ones are free floating.

 Thallus structure

The thallus is filamentous ,multicellular and unbranched. All the cells of the filament are cylindrical except the basal and apical cell. The basal cell is colourless and forms hold fast. The proximal end of the hold fast extends to produce finger like projections which help the filament to attach on the substratum. The apical cell is rounded or elongated in shape. Each vegetative cell is cylindrical and possesses a thick cell wall. The inner layer is cellulosic and the outer layer is made up of pectin. A thin layer of chitin is present above the pectin layer. Next to the cell wall a plasma membrane is present. A large vacuole is present. The protoplasm contains reticulate chloroplast and it extends from one end of the cell to the other. A single nucleus and many pyrenoids are present. The distal end of some cells possess ring like markings called apical caps. Such cells are called cap cells. The presence of cap cell is characteristic feature of Oedogonium (Figure 2.5).

Reproduction

Oedogonium reproduces by vegetative, asexual and sexual methods. Vegetative reproduction takes place by fragmentation and akinete formation. During asexual reproduction zoospores are formed. During favourable conditions, some of the vegetative cells function as zoosporangia. Usually a single zoospore is produced per zoosporangium. A ring of short flagella is found at the base of colourless, beak like anterior end of the zoospore. This kind of flagellation is called stephanokont. The zoospore is released from the zoosporangium and swims in water (Figure 2.6). If it reaches a suitable substratum, it divides into two cells. The lower cell forms holdfast. The green upper cell divides and produces the filament.

Figure 2.6:  Reproduction in Oedogonium

Sexual reproduction is Oogamous. The male gametangium is antheridium and female gametangium is called Oogonium. Based on the distribution of sex organs there are two types of species namely Macrandrous and Nannandrous.

Macrandrous monoecious – Antheridia and Oogonia occur on same filament – Oedogonium fragile.

Macrandrous dioecious – Antheridia and Oogonia occur on separate filaments – Oedogonium crassum

In nannadrous species antheridia are produced on reduced male filaments called dwarf male plants(O.cancatenatum).

In nannandrous species antheridia develop on specialised 2–4 celled filaments called dwarf males. The dwarf male is developed from androspores released from the androsporangium.. If the androsporangia and oogonia develop on same filament, it is called gynandrosporous (O. concatenatum ). If they are borne on different filaments it is called idioandrosporous (O. conferatum). The antheridium produces multiflagellate antherozoids. They are released by transverse splitting of the wall of antheridium. Antherozoids are attracted chemotactically towards the mature oogonium. A single antherozoid enters the oogonium through the opening present on the wall of the oogonium. The male nucleus fuses with the egg to form a diploid zygote. After fertilization the zygote separates from the oogonial wall and a thick wall is secreted around it. The diploid zygote undergoes meiosis to produce 4 haploid multiflagellate zoospores. The wall of the zygote ruptures to release the zoospores.The germination of the zoospore produces haploid filaments of Oedogonium (Figure 2.6).

In the life cycle of Oedogonium the diploid phase is short lived and is represented by zygote. The haploid phase is predominant and life cycle is of Haplontic type (Figure 2.7).

5. Chara

Class – Chlorophyceae

Order – Charales

Family – Characeae

Genus – Chara

Chara is commonly called as ‘ stone wort’ It is a submerged aquatic freshwater alga growing attached to the mud of the lakes and slow running streams. Chara baltica grows in saline water. The thallus is often encrusted with calcium and magnesium carbonate.

 Thallus structure

The plant body is multicellular, macroscopic and is differentiated into main axis and rhizoids. The rhizoids are thread-like, multicellular structures arise from the lower part of the thallus or peripheral cells of the lower node.They are characterised by the presence of oblique septa. The rhizoids fix the main axis on the substratum and helps in the absorption of salts and solutes (Figure 2.8).

 The main axis is branched, long and is differentiated into nodes and internodes. The internode is made up of an elongated cell in the centre called axial cell or internodal cell. The axial cell is surrounded by vertically elongated small cells which originate from the node. They are called cortical cells. In C. wallichii and C. corallina the cortical cells are absent. Three types of appendages arise from the node. They are  1. Branches of limited growth 2. Branches of unlimited growth 3. Stipuloides. The growth of the main axis and its branching takes place by the apical cell.

 The nodal cells are uninucleate with few ellipsoidal chloroplasts. The intermodal cells are elongated and possesses a large central vacuole, many nuclei and numerous discoidal chloroplasts.

The cytoplasm is divided into outer ectoplasm and inner endoplasm. The endoplasm shows cytoplasmic streaming.

Reproduction

Chara reproduces by vegetative and sexual methods. Vegetative reproduction takes place by Amylum stars, Root bulbils, Amorphous bulbils and secondary protonema.

Sexual reproduction - Sexual reproduction is Oogamous. Sex organs are macroscopic and are produced on the branches of limited growth. The male sex organ is called Antheridium or Globule and the female sex organ is called Oogonium or Nucule (Figure 2.9). The Nucule is located above the Globule. The antheridium is spherical, macroscopic and its wall is made up of eight cells called shield cells. The antheridium has spermatogenous filaments. These filaments produce antherozoids. The nucule is covered by five spirally twisted tube cells and five coronal cells are present at the top of the nucule (Figure 2.9). The centre of the nucule

possesses a single egg. At maturity the tube cells separate and a narrow slit is formed. The antherozoids penetrate the oogonium and one of them fuses with the egg to form a diploid oospore. The oospore secretes a thick wall around and germinates after the resting period. The nucleus of the oospore divides to form 4 haploid daughter nuclei of which, three degenerate. The oospore or zygote germinates to produce haploid protonema. The plant body of Chara is haploid and The oospore is the only diploid phase in the life cycle.  Therefore, the life cycle is of haplontic type. Alternation of generations is present (Figure 2.10).