Algae -Characteristics, History Classification and Life Cycle
Algae -Characteristics, History Classification and Life Cycle
Algae term is coined by Carl Linnaeus. Algae (L. alga = seaweed) are any organisms with chlorophyll a and a thallus-like plant body.
General Characteristics
They are haploid gametophytic, eukaryotic, sun-loving, autotrophic, chlorophyllous, cryptogamic, nonvascular thallophytic aquatic plants. Often called the children of the sea, they are primary producers of food, occupying three-fourths of the earth’s surface and accounting for about 90% of total photosynthesis. Marine algae are the main producers.
They are considered plants because they possess chlorophyll a, a cellulosic cell wall, starch as a reserve food, and absorb nutrients in liquid form. The plant body is a thallus that may be unicellular, filamentous, or colonial, and is formed of true parenchyma. Their reserve food is mainly starch.
Water is essential for fertilization. Asexual reproduction occurs by one-celled mitospores, while sex organs are unicellular, non-jacketed, and called gametangia. The zygote sheds and develops into a thick-walled, perennating zygospore, but it never develops into an embryo. There is progressive complexity in sex organs from lower to higher forms of algae. Meiosis is primarily zygotic, although in brown algae, it is characterized as sporic meiosis.
True alternation of generations is absent except in Ectocarpus, which shows isomorphic alternation of generations. The life cycle is mostly of the haplontic type. Algae do not require mechanical tissue because buoyancy holds them upright.
History of Algae
Morris is regarded as the Father of Algae and M.O.P. Iyenger is the Father of Indian Phycology. He discovered Fritschiella, a heterotrichous terrestrial alga. The study of algae is called algology or phycology (Gr. phykos = algae) and its expert is a phycologist. Narayana Rao worked on Indian fossil algae.
Algae have been designated in many ways, e.g.,
- Tsao by Chinese
- Limu by Hawaiians
- Phycos in Greek
- Fucus in Roman
The first reference to algae is found in the Chinese literature Canon of Poems and Materia Medica. There are about 2500 genera and 30,000 species of algae found in a variety of habitats, but mostly are aquatic.
The term algae was introduced by Linnaeus (1753), but he also included the Hepaticae (liverworts) and lichens also under algae. de Jussieu (1789) used the term algae for the plants which we now consider to be algae. F.E. Fritsch (1935) divided algae into eleven classes in his book Structure and Reproduction of the Algae, mainly on the basis of pigmentation, reserve food, flagellation, thallus structure, modes of reproduction and life cycle.
Whittaker (1969) has included only Chlorophyceae (green algae), Phaeophyceae (brown algae), and Rhodophyceae (red algae) in algae under the kingdom Plantae. In algae, β-carotene is the most abundant type of carotene, and Chl a is universal.
Classes of Algae (Fritsch, 1935)
Fritsch (1935) divided algae into different classes according on their colors, food-storage components, and flagella (movement-related structures).
- Chlorophyceae (Green algae) – They store food as starch, have two equal flagella at the front, and contain Chl a, b; α, β, γ-type carotene, xanthophyll.
- Xanthophyceae (yellow-green algae)- They store food as oil or leucosin, have two uneven flagella, and contain carotenoids and chlorophyll a, e, and, α- and β-carotene and xanthophyll.
- Chrysophyceae (Golden algae) – They have one or two uneven flagella and are high in fucoxanthin. They can also store food as leucosin or oil. Chl a, c; α- and β-carotene; fucoxanthin pigments are present.
- Bacillariophyceae (diatoms)- they store oil and contain golden-brown colors, Chl a, c; β- and α-carotene; diatoxanthin, fucoxanthin pigments are present. They are completely devoid of flagella.
- Cryptophyceae – They have two uneven flagella, store starch, and contain Chl a, c; α-carotene, xanthophyll.
- Dinophyceae (dinoflagellates)- they are special because they have mesokaryotic cells, store starch or oil, and have a mixture of pigments, including Chl a, c; β-carotene; xanthophyll, dinoxanthin, fucoxanthin and peridinin. They also move by using two lateral flagella.
- Chloromonadineae – they have two apical or subapical flagella, store oil, and contain chlorophyll a, e; xanthophyll.
- Euglenineae (Euglena) – have one to three apical flagella, store paramylum, a material that resembles starch, and contain Chl a, b; β-carotene, xanthophyll.
- Phaeophyceae (Brown algae) – they store food as mannitol and laminarin, have two uneven lateral flagella for movement, and contain Chl a, c; β-carotene, fucoxanthin.
- Rhodophyceae (red algae) – They store food as floridean starch, lack flagella, and contain Chl a, d; α, β-carotene, xanthophyll; phycobilins (phycocyanin, phycoerythrin).
- Cyanophyceae (blue-green algae or cyanobacteria or Myxophyceae)- they are prokaryotic (do not have a real nucleus), contain Chl a, β-carotene, xanthophyll, phycobilins (phycocyanin, allophycocyanin, phycoerythrin), store cyanophycean starch, and neither have flagella nor reproduce sexually.
Life Cycles in Algae
- Uniphasic or Monogenetic – There is only one visible phase, either haploid or diploid, e.g., Chlamydomonas, Ulothrix, Spirogyra, Fucus. Accordingly, life cycle is called haplontic (haplobiont, H, h type) or haplobiontic–diplont (H–d type).
- Biphasic – There are two distinct (haploid and diploid) visible phases in life cycle, e.g., Ectocarpus. This shows D, h+d type of life cycle.
- Triphasic – Three visible phases are present in the life cycle, e.g., Polysiphonia (a red alga). The three phases are gametophytic haploid, sporophytic diploid and tetrasporophytic diploid.
Here –
- H= Haplobiontic
- D = Diplobiontic
- h = Haplont
- d = Diplont
All sexually reproducing organisms have two phases in their life history viz., haploid (n) and diploid (2n). In most algae, the haploid phase is somatic, while the diploid phase is represented only by the zygote. In bryophytes, diploid is represented by the sporophyte. In thallophytes and bryophytes, the main dominant somatic/vegetative phase may be haploid (n) or diploid (2n).
The haploid phase is also known as the gametophyte, which produce gametes and gametes takes part in fertilization. After fertilization zygote develops into sporophyte. The diploid phase is the sporophyte that produces spore that develops into gametophytes
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