Algae

Aug 09, 2022, 16:45 IST

CHARACTERISTICS OF ALGAE

Photosynthetic, simple, thalloid, mainly aquatic organisms, but also present on moist stones, soils and wood.They occur in association with fungi (Lichen) and animal (on Sloth bear). Form and size is highly variable, unicellular Chlamydomonas, colonial Volvox, filamentous Ulothrix and Spirogyra.

Reproduction by vegetative, asexual and sexual means.

Algae usually occur in a variety of habitats such as water, land as well as on the other plants and even animals. Some grow in marine water called seaweeds.

Vascular tissues are absent.

A mechanical tissue is absent.

A variety of pigments in algae provide different colours.

Sexual reproduction involves isogamy, anisogamy and oogamy. Sex organs are unicellular and non-jacketed. An embryo stage is absent.

Life cycle is various-haplontic, diplontic or diplohaplontic.

CLASSIFICATION OF ALGAE

Algae are usually differentiated on the basis of their pigments and storage products. Algae included under kingdom Plantae by Whittaker (1969) are of three types: red algae, brown algae and green algae.

CHLOROPHYCEAE: GREEN ALGAE

About 90% of the total species grow in fresh water habitats and 10% are marine. They are cosmopolitan in distribution. They may be

Terrestrial- growing on moist soil, walls and rocks, e.g., Fritschiella etc.

Epiphytes- growing on other plants, e.g., Trenteopohlia, Protococcus, etc.

Endophytes- growing inside the other plants, e.g., Coleoeheate nitellum inside the thaIlus of Nitella.

Epizoic- growing on the surface of animals, e.g., Cladophora and Charaeilum on mollusc shells and crustaceans, respectively.

Endozoic- living inside body of animals, e.g., Zoochlorella inside sponges, Chlorella in the body of Hydra.

Cryophytes- growing in the polar region on ice and snow, e.g., Chlamydomonas nivalis, (causing red snow ball).

Thermophilic- growing in hot springs, e.g., Chlorella sp.

Parasitic- growing as pathogens and causing diseases, e.g., Cephaleuros (causing red rust disease of tea and coffee).

Symbionts - as components of certain lichens.

Thallus is of various types: unicellular flagellate (e.g. Chlamydomonas), unicellular non flagellate (e.g., Chlorella, Characium, Acetabularia) or flagellate colonies, (e.g., Volvox), non-flagellate colonies (e.g., Hydrodictyon), coenocytic and siphonaceous (e.g., Vaucheria, Caulerpa), unbranched filament (e.g., Ulothrix), simple branched (e.g., Cladophora), heterotrichous with prostrate and vertical branches, (e.g., Draparnaldia), and parenchymatous (e.g., Ulva).

Cell wall contains cellulose with a few exceptions.

Photosynthetic pigments are similar to those of higher plants: chlorophyll a, chlorophyll b, carotenes and xanthopylls.

Food reserve is Starch.

Vegetative reproduction occurs by fragmentation, stolons and tubers.

Asexual reproduction takes place by mitospores. The common asexual spores are zoospores, aplanospores, hypnospores, akinetes, etc.Sexual reproduction is effected by isogamy, ani

sogamy and oogamy.

Three types of life cycle occur in green algae: haplontic, diplontic and diplohaplontic.

In haplontic life cycle the dominant phase is haploid. The diploid stage is present only in the form of zygote or zygospore. Meiosis occurs at the time of its germination of zygote (zygotic meiosis, e.g., Ulothrix, Spirogyra and Chlamydomonas).

In diplontic life cycle, the dominant phase of the alga is diploid. It gives rise to haploid gametes through meiosis (gametic meiosis, e.g., Caulerpa). The gametes fuse and the fusion product or zygote regenerates the diploid phase.

The haplodiplontic life cycle possesses well developed multicellular haploid and diploid structures. The haploid gametophyte gives rise to haploid gametes. The fusion product of gametes or diploid zygote grows directly into diploid sporophyte. The sporophyte produces haploid spores through meiosis (sporic meiosis, e.g., Ulva. Cladophora). Meiospores germinate into new gametophytes.

Grass-green due to presence of chlorophyll ‘a’ and ‘b’ pigments.

Chloroplast may be discoid, plate like, reticulate, cup-shaped, spiral or ribbon-shaped.

Pyrenoid is present in chloroplast, it is a protein core surrounded by starch.

Some green algae store food in the form of oil.

Zoospore is produced in zoosporangia.

In Spirogyra, sexual reproduction is isogamous and isogametes are non-motile.

E.g., Chlamydomonas, Volvox, Ulothrix, Spirogyra, Chara.

CHLAMYDOMONAS: LIFE CYCLE

HABITAT

Chlamydomonas is widely distributed fresh water unicellular alga, commonly occurring in standing or stagnant rainwater, ponds, pools, ditches and on moist soils. It grows, in abundance in water rich in ammonium compounds.

PLANT BODY

At the pointed anterior end of the cell, arise two flagella emerging through the same or separate canals. The flagella are acronematic (whiplash) and are of equal length. Each flagellum has a granule (blepharoplast) at the points of its origin. They are connected together by a transverse fibre called paradesmos, which is again connected with the intranuclear centrosome of the nucleus through a cytoplasmic strand, rhizoplast. The entire setup is well coordinated to perform sensory as well as locomotory functions and is known as neuromotor apparatus.Each cell typically possesses two contractile vacuoles located at the base of flagella in a plane at right angles to them.A tiny spot of an orange or reddish colour, known as stigma or eyespot, lies at the anterior end. It is a photoreceptive organ concerned with the direction of the movement.

Each individual has a single nucleus lying in the colourless cytoplasm filling the cup. The nucleus is typically of eukaryotic type.