Algae: Characteristics, Types and Its Classification
Algae are the green slimy blanket which covers the rock surface or the top of the ponds or a poorly kept aquarium that have the ability to conduct photosynthesis. The algae belong to the subphylum Thallophyta of the kingdom Protista in modern classification of organisms. Latin ‘alga’ means seaweed. These are eukaryotic organisms, autotrophic in nature as have chlorophyll in their cells. During photosynthesis, they produce oxygen with help of light energy from the sun and generate carbohydrates.
They possess many types of life cycles and their size range from microscopic Micromonasspecies to giant kelps. In this case, kelps reach up to 60 meters (200 feet) in length.
More than 30000 species of algae have been identified. Most algae are aquatic but some grow in semi-aquatic and terrestrial environments. Many algae live as endophytes in plant or animal tissue and many grow on plant or animal as epiphytes. Some of them make asymbiotic relationship with fungi and exist as lichen. Green algae, brown algae, red algae, golden-yellow algae are main types of algae. The study of algae is known as Phycology.
Characteristics of Algae
Algae are unicellular, colonial or large multi-cellular organisms.
The multi-cellular algae develop specialized tissues but they lack the true stems, leaves, or roots.
Most algae are aquatic but some are semi-aquatic and terrestrial.
Cells contain photosynthetic chlorophyll and other pigments.
Algae cell wall composed of cellulose and pectin.
With a few exceptions, most algae are autotrophic; they do not have vascular tissues.
Most algae store carbohydrate as reserve food, few members contains alcohol, fat or oil as reserve food.
They reproduce through vegetative, asexual and sexual methods. Asexual reproduction occurs by fragmentation or producing spores. Sexual reproduction can be isogamous, anisogamous or oogamous types.
Gametangia (reproductive organ) always single celled, if multi-celled, do not cover with sterile cell layer.
Zygote develops by either mitosis or meiosis cell division. Zygote never form embryo.
Meiosis cell division is seen in different stages of life cycle.
Classification of Algae
Outline of Algal Classification
Division-1: Chlorophyta (Green Algae)
This group contains about 7000 species, among them, most occur in freshwater and some others in marine environment.
They contain pigments like chlorophyll a and chlorophyll b.
They also possess accessory pigments like carotenoids and xanthophylls.
Cell wall consists of cellulose, hemicellulose, and calcium carbonate in some species.
They are also known as stoneworts" and "brittleworts".
They are commonly found in freshwater environment.
They have unicellular, filamentous, colonial, or multicellular and plantlike bodies.
Many species bear flagellated cells.
They can store starch in characteristic plastids.
Examples: Stonewort (Chara), filamentous (Spirogyra) and desmids.
This class includes single order:
Order-1: Charales
Class-3: Pleurastrophyceae
They inhabit in both freshwater and marine habitats.
They include coccoid, sarcinoid, and filamentous algae.
They reproduce asexually by autospores or by biflagellate, flattened zoospores.
Sexual reproduction is unknown among the representatives of Pleurastrophyceae.
They possess a counter-clockwise orientation of the basal bodies and an unusual mitotic spindle.
Examples: Marine flagellate (Tetraselmis).
Class-4: Prasinophyceae (Micromonadophyceae)
They have both unicellular and colonial forms.
They possess one, two, four, or eight flagella, with or without cell walls, scales, thecae, or loricae.
They are mostly motile and photosynthetic algae containing pigments like chlorophyll a, and chlorophyll b.
They are mainly marine algae, but some are also found in brackish- and freshwater habitats.
A few algae inhabit benthic region, with both coccoid and colonial forms.
There are about 180 known species under 13 genera.
Examples:Micromonas, Ostreococcus, Pyramimonas, etc.
Class-5: Ulvophyceae
They are marine algae with a variety of shapes.
The body consists of a few cells with thin sheaths, long filaments.
They possess two or more apical flagella, if present.
They perform photosynthesis due to presence of chlorophyll in their body.
Alternation of generations occurs in their life cycle.
Examples: Sea lettuce (Ulva), Acetabularia, Caulerpa, Monostroma, etc.
Division-2: Chromophyta
They contain chlorophyll a and chlorophyll c.
They also contain accessory pigments such as Carentoids, xanthophylls.
They store foods as oils or carbohydrates laminarin.
If present, flagellum possesses hair like projection.
Mitochondria contain tubular cristae.
Mucous organelles are common.
Class-1: Bacillariophyceae (Diatoms)
They primarily inhabit in freshwater, marine, and soil environments.
There are about 12,000 to 15,000 living species.
The cell contains a silica cell wall which is known as frustules. In this case, frustule is made up of two valves called thecae.
They contain pigments like chlorophylls a and chlorophyll c with accessory pigments such as beta-carotene, fucoxanthin, diatoxanthin and diadinoxanthin.
Examples:Cyclotella, Thalassiosira , Bacillaria, Navicula, Nitzschia, etc.
Class-2: Chrysophyceae (Golden Algae)
They are commonly known as golden algae.
They inhabit in freshwater environment.
They are unicellular or colonial organisms.
The photosynthetic pigments are chlorophyll a, chlorophyll c and fucoxanthin.
They store energy as carbohydrate and oil droplets.
The cell contains silica deposition vesicles.
They possess apical flagella which is unequal in length.
They perform haplontic type life cycle.
Reproductions occur through binary fission, sporogenesis, etc.
They are heterotrophic or autotrophic (photosynthetic) organisms.
Photosynthetic forms contain chlorophyll a, and chlorophyll c with accessory pigments like peridinin or fucoxanthin.
The flagellum does not contain tubular hairs.
Mitochondria possess tubular cristae.
This phylum contains more than 1,500 described species.
Class-1: Dinophyceae
They are also is known as dark yellow to brown algae.
They store food materials as starch and oil.
The body contains characteristics nucleus with condensed and banded chromosomes.
They live as free living, symbiotic or parasitic forms.
Often, they are commonly known as sea water planktons.
The motile form possesses two different flagella.
Examples:Dinophysis, Alexandrium, Gonyaulax, Ceratium, Noctiluca, Gymnodinium, Polykrikos, Peridinium, etc.
This class includes the following orders:
Order-1: Haplozoonales
Order-2: Akashiwales
Order-3: Blastodiniales
Order-4: Apodiniales
Order-5: Dinotrichales
Order-6: Phytodiniales
Order-7: Brachidiniales
Order-8: Ptychodiscales
Order-9: Amphilothales
Order-10: Actiniscales
Order-11: Gymnodiniales
Order-12: Prorocentrales
Order-13: Nannoceratopsiales
Order-14: Dinophysales
Order-15: Gonyaulacales
Order-16: Thoracosphaerales
Order-17: Peridiniales
Division-6: Euglenophyta
This phylum contains about 800 species. Most of them inhabit in freshwater environment.
They lack a true cell wall, and the body is bounded by a proteinaceous cell covering which is known as a pellicle.
They possess one to three flagella for locomotion.
They store carbohydrate as paramylon.
They are both photosynthetic and heterotrophic organisms.
The primary photosynthetic pigments are chlorophyll a and chlorophyll b.
They also contain carotenoids and xanthophylls as accessory pigments.
They feed on organic material suspended in the water.
Class-1: Euglenophyceae
They are commonly known as pure green algae.
Most of them inhabit in freshwater environments.
The body is covered by flexible pellicle which is formed of protein.
The reserve food materials are carbohydrates or starch.
The body has two definite ends such as anterior and posterior ends.
The body possesses large and prominent nucleus and a contractile vacuole for performing osmo-regulation.
Autotrophic or heterotrophic nutrition occurs.
The body contains two apically or laterally placed flagella which lack tubular hairs.
Reproduction is isogamous type.
They contain single lobed chloroplast with central pyrenoids. In this case, photosynthetic pigments are chlorophylls a and chlorophyll b.
Mitochondria contain paddle-shaped cristae.
This class contains about 1000 known species.
Examples:Colacium, Euglena, Eutreptiella, Phacus, etc.
Types of Algae
Based on colors, algae are divided into the following major four groups:
Blue Green Algae
They belong to the class Cyanophyceae under the phylum Cyanophyta. They inhabit in freshwater or in a wide variety of moist soils of a terrestrial environment. They also form a symbiotic relationship with plants or lichen-forming fungi. They contain pigments like chlorophyll 'a', 'b', and phycobilins and they appear in blue green color. They are also known as Cynabacteria.
Green Algae
They may be either unicellular or multicellular algae which belong to the class Chlorophyceae under the phylum Chlorophyta. They contain pigments like chlorophyll a, chlorophyll b, carotenoids, and xanthophylls.
Examples:Chlamydomonas, Spirogyra, and Chara
Red Algae
Red algae belong to the class Rhodophyceae under the phylum Rhodophyta which is one of the largest phylum of algae. This phylum contains over 7000 recognized species. Among them, 6,793 species are found in the Class Florideophyceae. Most of the algae are multicellular and marine algae (seaweeds). Red algae contain pigments like chlorophyll a, chlorophyll d, carotenoids, xanthophylls, and phycobilins. About 5% of the red algae inhabit in freshwater environments. They are used as a stabilizer in milk products.
Examples:Porphyra, Gracilaria, and Gelidium.
Brown Algae
These types of algae belong to the class Phaeophyceae under the phylum Phaeophyta. They contain pigments like chlorophyll a, chlorophyll c, carotenoids and fucoxanthin. They form a large group of multicellular algae. Majority of them inhabit in marine environments. Over 1500 known species of brown algae are available worldwide.
Examples:Dictyota, Laminaria, Sargassum, etc.
Based on morphology, algae can be divided into several types. Some are filamentous forms having cells arranged in chains like strings of beads. Some filamentous are un-branched such as Spirogyra while others are branched and bushlike such as Stigeoclonium.
Algae are almost ever-present throughout the world. Ecologically, they can be grouped into the following types by their habitats.
Planktonic algae: They are microscopic and grow suspended in the water.
Neustonic algae: They grow on the water surface and can be either microscopic or macroscopic.
Cryophilic algae: They can occur in snow and ice.
Thermophilic algae: They can live in hot springs.
Edaphic algae: They can inhabit on or in soil.
Epizoic algae: They grow on the body of animals, such as turtles and sloths.
Epiphytic algae: They can grow on other algae, fungi, and land plants.
Corticolous algae: They can grow on the bark of trees.
Epilithic algae: They inhabit on rock surfaces.
Endolithic algae: They inhabit in porous rocks or coral.
Chasmolithic algae: They grow in rock fissures.
Endosymbionts algae: Some algae live inside other organisms, and this situation is known as endosymbionts. In this case, endophytic endosymbionts live in plants, fungi, or other algae.
Concluding Remarks
Algae are living organisms which are distributed throughout the world. They have different sizes, shapes, and colors. They can inhabit in freshwater and marine environments. They also grow on the body surfaces of other organisms such as turtles and polar bears, on rocks or in the soils, or under or inside porous rocks, such as limestone and sandstone. The algae have great important because they produce much oxygen on the earth for animals and human beings. Nutritionally, they contain several healthy elements such as proteins, fats, carbohydrates, and vitamins A, B, C, and E. They also contain a number of important minerals such as iron, potassium, calcium, magnesium, zinc and manganese.
