Algae are a group of autotrophic, non-vascular thalloid plants with unicellular or multicellular, non-jacketed sex organs without any formation of an embryo.
Characteristics of Algae
- Algae are thalloid plants that possess chlorophyll and lack differentiation into tissue or tissue systems, but some algae possess highly developed complex thalli with minimal differentiation of true tissues (Ulva, Sargassum, etc.).
- Sex organs are unicellular in general; when multicellular, every cell can reproduce and is never covered by a sterile jacket layer.
- No embryo is produced following gametic fusion.
- Sporophytic and gametophytic generations are independent when depicted in the life cycle.
- Algae reproduce by vegetative, asexual, and sexual modes.
Classes of Algae
Chlorophyceae- Green algae contain the chief pigments, chlorophyll a and b, xanthophylls, and carotenoids. Sexual reproduction varies from isogamous to oogamous.
Xanthophyceae- Most of the members are freshwater forms, and marine forms are very few. The members are yellow-green because there is an excess of xanthophylls. The major pigments are chlorophyll a and e, β-carotene, and xanthophylls. The reserve food material is oil. Sexual reproduction is absent, but if present, then it is isogamous.
Chrysophyceae- The majority of the members are marine and freshwater forms. The pigment of the algae is brown or orange because of the excess content of phycochrysin. Food reserve material is oil and leucosine. Sexual reproduction is uncommon.
Bacillariophyceae- The Diatoms are members of Bacillariophyceae. Members are freshwater and sea forms both. The characteristic pigment is diatomin and yellow or golden brown. Pigments include chlorophyll a and c, β-carotene, and xanthophylls. Reserve food material is oil, volutin, and leucosine.
Cryptophyceae- The members are both freshwater and marine. Cells contain two large parietal chloroplasts with pyrenoids. Starch is a reserve food material. Sexual reproduction is isogamous.
Dinophyceae- Members are mostly unicellular and motile. Motile cells are biflagellate. Chloroplasts are disc-shaped. Chlorophyll a and c are present. The pigment of the members is brown or dark yellow because the red phycopyrin, dark red peridinin, and yellow-green chlorophyllin are present. Reserve food material is starch and fat. Sexual reproduction is absent.
Chloromonadineae- Members are freshwater forms only. Members of this class are pale green because of having excess xanthophylls, and cells possess large discoid chromatophores.
Euglenophyceae (Euglenineae)- The class consists of unicellular flagellates. Important pigments are chlorophyll a and b, β-carotene, and xanthophylls. Reserve food substance is polysaccharide paramylon and lipids.
Phaeophyceae- These members are commonly referred to as brown algae. Pigments are chlorophyll a and c, β-carotene, and xanthophylls. The brown colouration is caused by an excess of fucoxanthin. Reserve food material is laminarin and mannitol in the form. Reproduction is both vegetative and sexual. Sexual reproduction varies from isogamy to oogamy.
Rhodophyceae- These are referred to as red algae. Major pigments are chlorophyll a and β- β-carotene, xanthophylls, and phycobilins- r-phycoerythrin, r-phycocyanin, and allophycocyanin. Reserve food material is a floridean starch. Sexual reproduction is specialized and oogamous.
Cyanophyceae or Myxophyceae- Members are referred to as blue-green algae. Members are prokaryotic. Thallus is unicellular, colonial, or multicellular. Key pigments include chlorophyll a, α-carotene, β- β-carotene, xanthophylls, and phycobilins- c-phycocyanin and c-phycoerythrin. Cyanophycean starch is a reserve food material. Mucopeptide forms the cell wall. The majority are embedded in a mucilaginous sheath.
Algal Cell
It may be prokaryotic or eukaryotic, which are as follows-
Ultrastructure of Prokaryotic Algal Cell
The prokaryotic algal cell is of two parts- the Outer cellular covering and cytoplasm.
Outer cellular covering– Slime layer or mucilaginous sheath- Outside the cell wall, there is a mucilaginous sheath, which is a diagnostic feature of all cyanobacteria (blue-green algae).
Cell wall– The cell wall is rigid and composed of mucopeptide. It has four layers, which are designated as L1, L2, L3, and L4.
Plasma membrane- The Plasma membrane lies beneath the cell wall and consists of a lipid bilayer. The plasma membrane and its invaginations are the loci of biochemical activities.
Cytoplasm- It is divided into chromoplasm and centroplasm.
Chromoplasm- It is composed of parallel photosynthetic lamellae or thylakoids. These lamellae are rich in chlorophyll a, carotenoids, and phycobilins. The membrane-bound organelles are found in the chromoplasm.
Centroplasm- It is the interior colourless area, which is made up of chromatin material or DNA material that is not histone protein-bound. Similar to bacteria, minute circular pieces of DNA exist, which are referred to as plasmids or transposons.
Ultrastructure of Eukaryotic Algal Cell
The ultrastructure of an Eukaryotic algal cell comprises the cells of all algae except blue-green algae. Various components of the eukaryotic algal cell may be described as follows-
Cell wall- The Cell is surrounded by a cellulose-containing cell wall. In most forms of algae, a pectose layer is also present outside the cellulose cell wall. In some brown algae, alginic acid is present in the cell wall. Some algae, especially the diatoms, have a silicified cell wall. Red algae have xylan, agar, and carrageenin in the cell wall.
Plasma membrane- It occurs just below the cell wall and consists of a protein lipid bilayer.
Within the plasma membrane, dense cytoplasm is found. In the cytoplasm, membrane-bound eukaryotic cell organelles are found. Cells of most algae consist of one chloroplast per cell, except some species whose cells carry more than one chloroplast. The chloroplasts can be cup-shaped, parietal, discoid, lobed, star-shaped, spiral, barrel-shaped, or girdle-shaped.
Nucleus- True nucleus with nuclear membrane and nuclear pores exists in eukaryotic algal cells, which is similar to the nucleus of higher plants. DNA is surrounded by the histone proteins.
Flagella- In a motile algal cell, the thallus has flagella, which are derived from basal granules or blepharoplast and emerge through a narrow canal in the cell wall. It has a characteristic 9+2 arrangement. Two central singlet fibrils are surrounded by nine doublet peripheral fibrils. Flagella are attached to the inner cytoplasm by tiny pores in the cell wall.
Algae Pigments
Three types of pigments occur in the plastids of algae. They are chlorophyll, carotenoids, and phycobilins.
Chlorophyll- There are five types of chlorophyll, i.e., chlorophyll a, b, c, d, and e, present in algae. Chlorophyll a is found in all the classes of algae. Chlorophyll b is found in Chlorophyceae and Euglenophyceae. Chlorophyll c is found in Phaeophyceae and Cryptophyceae. Chlorophyll d is present in Rhodophyceae. Chlorophyll e is found in Xanthophyceae.
Carotenoids- Carotenoids are yellow or orange coloured pigments. The different colors in algae are due to these pigments. There are six varieties of carotenes- α, β, γ, ε, flavicine, and lycopene. β-carotene occurs in all the classes of algae. α-carotene occurs in Rhodophyceae and Cryptophyceae. γ-carotene and lycopene occur in Charophyceae. ε-carotene occurs in Cryptophyceae and Bacillariophyceae. Flavicin occurs in Cyanophyceae.
Xanthophyll- Xanthophylls are oxygen derivatives of carotenes. There are around 20 kinds of xanthophylls that occur in algae.
Carotenoid acids- Carotenoid acids are similar to carotene and xanthophylls and are hydrocarbons, having a chain of carbon atoms.
Phycobilins or biliproteins- Phycobilins are water-soluble. There are three phycobilins- phycocyanin, phycoerythrin, and allophycocyanin. R-phycocyanin and r-phycoerythrin are limited to Rhodophyceae, whereas c-phycocyanin and c-phycoerythrin occur in cyanophyceae. Allophycocyanin occurs in Rhodophyceae.
Reproduction in Algae
Reproduction in algae occurs by vegetative, asexual, and sexual means.
Vegetative Reproduction
Vegetative reproduction in algae occurs in the following ways:
a) Fragmentation- In this process, the filament gets divided into fragments, and each fragment gives rise to a new filamentous thallus. The examples are Ulothrix, Spirogyra, Oedogonium, Oscillatoria, Nostoc, etc.
b) Fission- This is typical in desmids, diatoms, and other unicellular algae. The cell splits into two by mitotic division, followed by septum formation and separation.
c) Adventitious branches- Protonema is formed in some algae, such as Chara, and forms new thalli when separated from the parent thallus. These adventitious branches arise primarily on the rhizoids. Some other examples are Dictyota and Fucus.
d) Tubers- Tubers are globular or spherical bodies that are present on the lower nodes or rhizoids of Chara. These tubers, when they break off from the parent plant, can form new thalli.
e) Amylum stars- In Chara, amylum starch-filled star-shaped bodies are produced that give rise to new individuals after separating from the parent plant.
f) Budding- In certain algae, such as Protosiphon, budding occurs and forms new individuals.
g) Hormogonia- In certain cyanobacteria, such as Nostoc, hormogonia are produced, which might give rise to new thalli.
h) Hormospores or hormocysts- The hormospores are thick-walled hormogones formed under dry conditions.
Asexual Reproduction
In most of the algae, asexual reproductions occur through different types of spores and other organs. They can be motile or non-motile. Various forms of asexual spores and structures are as given below-
Akinetes- Akinetes are perennating bodies that can survive under unfavourable conditions and can give rise to new individuals in the presence of favourable conditions. e.g., Anabaena
Hypnospores- Hypnospores are thick-walled, non-flagellated spores with abundant food reserves. They are formed in unfavorable conditions by certain green algae. e.g., Chlamydomonas, Protosiphon.
Zoospores- They are flagellated asexual spores which develop in zoosporangia or directly from the vegetative cells. The zoospores can be bi, quadric, or multiflagellate. Example- Chlamydomonas (biflagellate), Ulothrix, Cladophora (quadriflagellate), Vaucheria, Oedogonium (multiflagellate).
Aplanospores- These are non-motile, thin-walled asexual spores produced in most of the aquatic algae by the non-formation of flagella due to some adverse conditions.
Tetraspores- Tetraspores are non-motile asexual spores which occur in certain members of Rhodophyceae and Phaeophyceae. Tetraspores are produced in tetrads in the tetrasporangia. e.g., Polysiphonia
Monospores- Monospores produced in the sporangia are known as monospores. Monospores occur as an asexual mode of reproduction in certain members of Rhodophyceae.
Autospores- These are essentially aplanospores that resemble the parent cell, and are thus called autospores.
Sexual Reproduction
Sexual reproduction occurs by gamete fusion. Any thallus vegetative cell can form gametes, and therefore, act like a gametangium, or a thallus-specific gametangium can be formed. The gametangia can be morphologically identical (isogametangia) or different (heterogametangia). The haploid gametes unite to form a diploid zygote, which forms the thallus. Depending on the morphological and physiological features of gametes, sexual reproduction may be of the following types-
Isogamous- When fusing gametes are morphologically alike and physiologically dissimilar (+ and -), then the sexual reproduction is referred to as isogamous. E.g., Chlamydomonas, Ulothrix, Zygnema, Spirogyra.
Anisogamous- In anisogamous sexual reproduction, fusing gametes are morphologically and physiologically different. The gametes are formed in different gametangia. Male gametes are microgametes, while female gametes are macrogametes. e.g., Chlamydomonas.
Oogamous- The most advanced form of sexual reproduction is oogamy, in which a motile male microgamete unites with a large, non-motile female gamete or egg. Antheridium produces male gametes while the female gamete or egg is formed inside a structure known as an oogonium. In fertilization, the male gamete swims to the oogonium to fertilize the egg and forms a diploid zygote. e.g., Chlamydomonas.
Ecological Importance of Algae
Algae are an important part of the ecosystem in aquatic and terrestrial environments. One of the most important functions they have is as primary producers within aquatic food webs.
Algae also function as oxygen generators, producing over 50% of the Earth’s atmospheric oxygen, which is essential for sustaining the lives of the majority of organisms. Moreover, they contribute to carbon sequestration as they take up carbon dioxide while undergoing photosynthesis and deposit it in seafloor sediments when dead and sink, thereby assisting in climate change mitigation. Macroalgae, including seaweeds (for example, kelp and Ulva) in coastal environments, provide refuge and breeding sites for marine animals such as fish, snails, and sea urchins.
In addition, cyanobacteria such as Anabaena and Nostoc fix nitrogen from the atmosphere in paddy fields, which makes the soil rich and conducive to rice farming.
Economic Importance of Algae
Importance in agriculture- Cyanobacteria play a very significant role in agriculture since they can fix nitrogen in the soil. Some of the significant soil cyanobacteria include Tolypothrix tenius, Aulosira fertilissima, Anabaenopsis, Oscillatoria, Anabaena, Nostoc, Spirulina, and Cylindrospermum.
Algae as food and fodder- Algae produce organic food material and are a major food source for fish and other aquatic organisms. Porphyra (rich in vitamin B and C) is employed on a large scale as a common food item in England, China, Japan, and South Korea. Rhodymenia palmata is eaten as food. Kombu, a Japanese delicacy, is made of Laminaria.
Medicinal role of algae- Chinese ancient literature showed the employment of Laminaria sp. in the treatment of goiter. Brown algae, which are rich in iodine, are utilized in the formulation of medicines for goiter. Laminariales members have been used as a surgical instrument and even during childbirth to dilate the cervix. An antibiotic, chlorellin, is derived from Chlorella.
Limestone formation- Several algae remove calcium from water, both fresh and salt, and precipitate it in the form of calcium carbonate in their cell walls.
Algae in sewage disposal- Chlamydomonas, Chlorella, Scenedesmus, and Euglena species are employed in sewage tanks for providing an efficient and inexpensive method of transforming the sewage into an odourless and useful fertilizer.
Role of Algae in Industries
Algae as a source of numerous commercial products are extremely beneficial for use in industries. The principal products extracted from algae include agar agar, carrageenan, algin, diatomite, and kelp. Industrial application of algae, especially seaweeds, is listed below-
a) Agar-agar- It is a mucilaginous substance recovered from red algae stored within their cell walls along with cellulose. Gelidium, Gracilaria, and Gigartina are the chief sources of agar-agar-producing algae.
b) Algin- The principal sources of algin are Ascophyllum, Laminaria, Lessonia, Ecklonia, Macrocystis, Sargassum, and Fucus.
c) Carrageenin- The primary source of carrageenin is the red alga Chondrus crispus (Irish moss). It serves as an emulsifying and stabilizing agent in food, textile, drug, leather, and brewing industries.
d) Diatomite- Diatomite is a rock-like deposit of unbreakable, siliceous walls of fossil diatoms, the dead bodies of which had accumulated over several million years at the bottom of seas as sediments.
Algae as indicators of industrial wastes- Different species of algae, such as Ulothrix zonata, Scendesmus bijuga, and Amphora ovalis, serve as valuable indicators of industrial wastes like paper mill wastes, oil wastes, distillery wastes, etc.
Algal Blooms
Abundant growth of algae on the water surface is referred to as a water or algal bloom. The water gets contaminated since the BOD value in these blooms rises. Algal bloom gives a fishy odor and an oily flavor to the potable water. Algal blooms are created in freshwater, lakes, and the oceans. Water blooms are toxic because some of the algae species contain neurotoxins.
But water blooms developed by cyanophycean algae are excellent fixers of nitrogen and thus can be employed as fertilizers. Algal blooms can be regulated by the use of copper sulphate or chlorine.
Interesting Algae Facts
- Algae generate more than 50% of the oxygen in the world.
- Though tiny, algae, particularly ocean phytoplankton, are responsible for producing more than half of Earth’s oxygen via photosynthesis, more than all the world’s forests together.
- Fossil evidence indicates that blue-green algae (cyanobacteria) lived more than 3.5 billion years ago and were responsible for creating Earth’s initial oxygen atmosphere via photosynthesis.
- Algae vary from small unicellular forms such as Chlorella and Diatoms to enormous kelp (Macrocystis pyrifera) up to 60 meters long, creating sea forests.
- Certain algae have developed in hot springs, snowfields, deserts, and within rocks. Dunaliella salina flourishes in high-salt conditions, imparting a pink color to salt lakes.
- Some of the algae are lipid-rich and can be transformed into biodiesel, which is a clean and environmentally friendly fuel source compared to fossil fuels.
- Spirulina and Chlorella are superfoods. These tiny algae are high in protein, vitamins, iron, and antioxidants and are marketed globally as human and animal nutritional supplements.
- Algae can absorb heavy metals, nitrogen, and phosphates from contaminated water. They are employed for bioremediation and the treatment of wastewater for purifying water naturally.
- Certain sea-dwelling dinoflagellates, such as Noctiluca, are bioluminescent, producing glowing waves or “sea sparkles” on nights through a chemical reaction within their cells.
- Extracts from algae such as alginate, carrageenan, and agar are employed in gels, face masks, toothpaste, and drug capsules because of their gelling and stabilizing effects.
- Certain algae, such as Microcystis and Karenia brevis, produce toxic blooms, referred to as HABs (Harmful Algal Blooms), which can kill aquatic life and contaminate drinking water.
Conclusion
Algae contribute importantly to supporting life on our planet through the generation of oxygen, as the foundation of aquatic food webs, and with diverse uses in food, industry, and medicine. Their economic and ecological value renders them essential to natural ecosystems and human civilization.
References
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